ELEMENTS OF
ENTOMOLOGY
By
DR. RAJENDRA SINGH
Aphid Biocontrol Laboratory
Reader in Zoology
DDU Gorakhpur University
GORAKHPUR
Foreword by
DR. G. C. SACHAN
PROFESSOR & HEAD
Department of Entomology
Govind Ballabh Pant University of
Agriculture and Tuchnology
PANTNAGAR
(UTTARANCHAL)
#] RASTOGI
[PUBLICATIONS
SHIVAJI ROAD,
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ELEMENTS OF
ENTOMOLOGY
ISBN No. : 978-81-7133-677-7
Elements of Entomology
ISBN
81-7133-677-9
© Singh, Rajendra
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TITLE CODE NO.
Z-57
2006-2007
FIRST EDillON
PUBLISHED
BY
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Contents
Chapter
1.
2.
Origin and Evolution of Insects
13-26
Classification and Identification of Economically
Important Insect Orders
4.
5.
6.
1-12
Elementary Knowledge of Collection and
Preservation of Insects
3.
Pages
Insect Integument
Segmentation and Body Regions
Digestive System
27-71
72-80
81-101
102-119
Contents
7. Circulatory System
8. Respiratory System
9. Excretory System
10. Reproductive System
11. Post-embryonic Development
12. Exocrine and Endocrine Glands
13. Nervous System
14. Sense Organs
15. Bioluminescence and Sound Production
120-131
16. Insects and The Abiotic Environment
17. Insect Population and Pest Outbreak
226-234
18. Insect-Plant Interaction
19. Locusts and Termites
20. Household Insects and Their Control
21. Insects Injurious to Man and Livestock
22. Insects Transmitting Diseases in Plants
246-263
(Aphids and White flies)
132-143
144-152
153-164
165-171
172-187
188-198
199-215
216-225
235-245
264-276
277-292
293-310
311-324
Contents
23. Insect
325-379
Injurious to Crops
Insect Pests of Crops
Pest of Maize
Chilo partellus (= C. zonellus)
Pests of Cotton
I . Aphis gossypii
2. Peciinophora gossypiella
3. Earias insulana and Earias
Fests of Vegetables
l. Aulacophora indica
vittella
4. Dysdercus cingulatus and
D. koenigii
5. Myllocerus undecimpustulatus
maculosus
6. Amrasca biguttula biguttula
2.
3.
4.
Pests of Sugarcane
5.
I. Scirpophaga (= Tryporyza)
nivella
2. Emmalocera depressella
3. Pyrilla perpusilla
4. Aleurolobus barodensis
6.
Pests of Oilseeds
I. Amsacta albistriga and
Pests of Paddy
A. moorei
2. Lipaphis erysimi
3. Athalia lugens proxima
4. Bagrada cruciferarum
(= B. picta)
l. Leptocorisa acuta
(= L varicoml)
2. Scirpophaga (= Tryporyza)
incertulas
3. Chilo suppressalis
4. Hieroglyphus banian
Pests of Fruit Trees
5. Dicaladispa (= Hispa)
I. Quadraspidiotus permiciosus
2. Eriosoma lanigerum
3. ldiocerus atkinsoni
4. Rhyncophorus ferrugineus
5. Oryctes rhinoceros
6. Papilio demoleus
armigera
6. Spodoptera mauritia
Pest of Wheat
Sesamia inferens
Pest of Pulses
Helicoverpa (= Heliothis)
arniigera
(= A. similis, A. testacea,
Raphidopalpa foveicollis,
R. bengalensis)
Leucinodes orbonalis
Bactrocera (= Dacus)
cucurbitae
Epilachna dodecastigma,
E. vigintioctopunctata
Phthorimaea
(= Gnorimoschema)
operculetta
Pieris brassicae
Pests of Castor
Achaea janata
24. Methods of Insect Pest Management
25. Beneficial Insects : Apiculture, Sericulture
and Lac Culture
26. Stored
455-503
Grain Pests and Their Management
(Safe Storage of Food Grains)
27. Ticks
380-454
and Mites of Economic Importance
504-521
522-532
-Glossary of Technical Terms
533-561
-Selected Readings
563-564
"This Page Is Intentionally Left Blank"
I
Origin and Evolution of Insects
Entomology (from Greek entomon = insect and logos = discourse) is a
branch of Zoology which deals with insects. In this branch we study the
origin and evolution of insects and their diversity and classification, body
organisation and functions, development, interactions with surroundings in
which they live, past history and their economic importance.
Insects belong to the class Insecta in the phylum Arthropoda, the
largest group in the Animal Kingdom. It includes about 80% of the
total described species of the entire animal kingdom numbering more
than a million. All arthropods are characterised by having segmented
body, bilateral symmetry, paired jointed appendages usually terminating
in claws, chitinous exoskeleton, ventral nervous system and dorsal heart.
The phylum includes besides the true insects, many other mandibulate
creatures (Subphylum : Mandibulata) such as crayfish, crabs, lobesters,
prawns, shrimps, barnacles, sowbugs (Class : Crustacea), centipedes
(Class : Chilopoda), millipedes (Class : Diplopoda), symphylans (Class :
Symphyla), pauropods (Class : Pauropoda) and chelicerates (Subphylum :
Chelicerata) such as scorpions, spiders, ticks and mites (Class :
Arachnida),
king-crabs
(Class : Merostoma),
pycnogonids
(Class :
Pycnogonida) and extinct form trilobites (Subphylum : Trilobita; Class :
Trilobitomorpha).
Chelicerates are free living, terrestrial and small-sized arthropods
whose body is regionated into prosoma (=cephalothorax, head+thorax)
and opisthosoma (abdomen). Prosoma bears one pair of clawed and
jointed chelicerae in place of mandibles, one pair of pedipalps, and four
pairs of walking legs. Antennae are absent and -abdomen usually does
not bear appendages. They breathe by book-gills (aquatic forms like
(Z-57)
Origin and Evolution of Insects
2 J
A
Fig. l.
Representatives of Subphylum Chelicerata. (A) King crab (Merostomata :
Xiphisura), (B) Mite (Arachnida: Acari), (C) Tick (Arachnida: Acari), (D) Pycnogonum
(Pycnogonida), (E) Spider (Arachnida: Araneida), (F) Pseudoscorpion (Arachnida :
Pseudoscorpionida), (G) Scorpion (Arachnida: Scorpionida).
·
king-crabs) or book-lungs (terrestrial forms like scorpions and spiders).
Excretion takes place through malpighian tubules or coxal glands.
Merostomans (e.g., Limulus, the king-crab, Fig. I-A) are marine,
benthic and abdomen bears 5-6 pairs of book-gills for respiration.· The
hind end of the abdomen forms a long telson.
Arachnids (Fig. 1-B, C, E, F, G) are more diversed group of
chelicerates and includes pseudoscorpions, scorpions, spiders (both
terrestrial), ticks and mites (mostly parasitic). The prosoma bears simple
eyes and six pairs of appendages (one pair each of chelicerae and
pedipalp, and four pairs of walking legs). Respiration takes place by
book-lungs (scorpion) or tracheae (ticks and mites). Many individuals
(Z-57)
Origin and Evolution of Insects
[ 3
have poison glands and poison fangs, jaws (spiders) or stings (scorpion).
Spiders spin silken threads (silk glands are situated in posterior part of
abdomen) for food capture, protection and locomotion.
Pycnogonids or pentapoda (Fig. 1-D) are small-sized marine sea
spiders. Cephalothorax consists three segments and forms major part of
the body. The abdomen is vestigial. Head usually bears four pairs of
appendages and two pairs of eyes. Respiratory and excretory organs are
absent.
Mandibulate arthropods are easily recognised from chelicerates by
having body divisible into cephalothorax (head+thorax) and abdomen or
head, thorax and abdomen or head and trunk. Head bears one or two
pairs of jointed antennae, one pair of mandibles and one or two pairs
of maxillae. Respiration takes place by gills or integument and excretion
by malpighian tubules or antennary glands. Life-cycle usually includes
larval forms.
Crustaceans are primarily aquatic in habit (Fig. 2-B, C, F) and have
five pairs of walking legs, paired jointed and biramous appendages on
the abdomen, two pairs of antennae, a pair of usually stalked compound
eyes and with two body regions, cephalothorax (head + thorax) and
abdomen. They breathe by gills or integument, excrete nitrogenous
wastes through antennary glands and develop indirectly through several
larval forms (e.g., nauplius, zoea, metazoea, alima, megalopa, mysis etc.).
Chilopods (hundred-legged-worms, Fig. 2-D) are the closest relatives
of the insects. They possess single pair of antennae, breathe by tracheae
and gonopores open at the posterior end of the body. Body is flattened
and divisible into head and many segmented trunk (15-180), each
bearing a pair of jointed and clawed legs, the first pair being poisonous
and are used to paralyse prey as they are carnivorous.
Diplopods (thousand-legged-worms, Fig. 2-E) resemble superficially
with centipedes but differ in following characters : body is cylindrical,
thorax four segmented, last three bear a pair of jointed clawed legs, all
abdominal segments (9-100) bear two pairs of similar legs, poisonous
claw absent and the gonopores opens forward close to the head. They
are herbivorous or detritivorous (feed on decaying vegetable matters).
Class Symphyla (Fig. 2-A) includes small terrestrial arthropods
(not more than 10 mm in length) like Scutigerella (the garden
centipede) whose body is divided into head and trunk like centipedes
but all trunk segments (15-22) do not bear jointed legs and the
gonopores open midventral between legs of fourth pair.
Pauropods are minute, soft and cylindrical worm-like terrestrial
arthropods whose body is divisible into head and trunk segments
(11-12), the latter are dorsally fused in pairs. Antennae are branched,
eyes absent, legs are 9-10 pairs and gonopores open ventral on third
trunk segment.
4 J
Origin and Evolution of Insects
(A) Scutigerella (Symphyla), (B) Cray
Fig. 2. Representatives of Subphylum Mandibulata
fish (Crustacea), (C) Sowbug (Crustacea), (D) Centipede (Chilopoda), (E) Millipede
(Diplopoda), (F) Shrimp (Crustacea).
Insects can be differentiated from the vast majority of other
arthropods by several following distinct characters (Fig. 3). Body is
divided into three distinct body regions : a head, a thorax and an
abdomen. Head bears a single pair of segmented antennae and
compound eyes and ocelli. The mouthparts are basically mandibulate but
are adapted for biting and chewing (e.g., cockroaches, grasshoppers,
beetles), piercing and sucking (mosquitoes, lice, bugs), sponging (house
flies), siphoning (butterflies, moths), lapping (honey bees) etc. The
thorax comprises three segments (pro-, meso- and metathorax), each
bearing a pair of legs, the tarsus of each leg is divided into 2-5
tarsomeres. Wings are always present on meso- and metathorax in
Origin and Evolution of Insects
[ 5
Fig. 3. Generalised winged insect.
winged insects. Abdomen consists of 11 segments (at least in the
embryo) with the gonopore on segment 8 or 9 ventrally and with cerci
on segment 11. Except some wingless insects, no pregenital appendage
1s present. Malpighian tubules are well developed. Embryonic
development takes place by superficial cleavage as eggs are
centrolecithal. They are epimorphic, i.e., born with full complements of
body segment. However, if we survey the insects as a whole, we find
exceptions to many of these characters, e.g., some insects are blind and tarsi
are undivided. However, these are the modifications acquired secondarily.
Ancestry, Origin and Evolution of Insects
[I] Ancestry of insects
Insects have more ancient lineage; trilobites and crustaceans being
abundant in the oceans as long as 500 million years ago. Trilobites are
extinct but crustaceans are still very much in the ocean and freshwater
bodies. The earlier occupancy of the water bodies by great numbers of
crustaceans may explain, in part, why insects have -not occupied the oceans
to any appreciable extent.
Insects are by no means the only arthropods occurring on land.
The land is also occupied by other major group of arthropods, the
6 J
Origin and Evolution of Insects
chelicerates that lack antennae and mouthparts consist of chelicerae,
which are not believed to be homologous to the mandibles of insects.
Respiration takes place by book-lungs. These four-paired legged
arthropods belong to an evolutionary line that diverged from the insect
lineage shortly after the arthropods first appeared on land, in early
palaeozoic era.
Centipedes and millipedes are one group of arthropods (Myriapoda)
that resemble insects in following characters : they bear a pair of
antennae, mandibles, maxillae as well as trach�al system. However, they
differ from insects by having only tagmata, head and trunk which is
composed of many segments each bearing legs. Myriapods also differ
in development. They born with only a few body segments and a few
pair of legs, as they grow and moult, additional segments and legs are
added, a condition termed as anamorphosis.
It is believed, that early in evolution of myriapod lineage certain
groups appeared in which segments and legs were not added at moults,
i.e., they were not anamorphic but epimorphic. Legs were retained on
the three segments behind the head and the remainder of the body
includes only 11 segments. The leg bearing segments tended to become
larger and more rigid, providing the leg musculature with space and
firm points of attachment. The posterior segments lacking with
locomotary appendages become specialised for reproduction and for
containing the major parts of the visceral organs. Thus, these arthropods
had three tagmata : head, thorax and abdomen.
At one time, all six legged arthropods were considered to have had
a common origin and were grouped with the insects, but this is now
questioned by most authorities. Springtails (Collembola), e.g., retain a
form of embryonic development different from insects and more like
myriapods, and they have only six abdominal segments. Another group
Protura, like myriapods, are anamorphic and unlike either myriapods or
insects, lack antennae. Finally, Dipleura, like Collembola, have the
segments of the antenna! flagellum individually musculated. All these
three groups (Collembola, Protura, Dipleura) are closely associated with
the soil and are blind, or nearly so and weekly sclerotised; all three
have unsegmented tarsi and much reduced malpighian tubules or none
at all. Their mouthparts are retracted into the head (Entognatha) unlike
insects (Ectognatha).
[II] Origin of insects
The class Insecta is generally considered to have evolved from a myriapod
or protomyriapod of some sort during the Devonian period. Based on
differences in mandibles and mandibular movement, Manton (1964)
Origin and Evolution of Insects
[ 7
<}121'1+1·1') af9fl �
prostomium
periproct
mouth
simple eye
A
mouth
appendages
antenna
mouth
jointed appendages
cercus
Fig. 4. Hypothetical stages in the evolution of the insect form.
concluded that insects are not direct descendants of myriapods and that
these groups are best looked upon as sharing a common ancestor.
A series of diagramms (Fig. 4-A-F) is helpful in a very simplified
way in visualising the hypothetical origin of the insects. The Figure 4-A
represents the segmented, legless, wormlike annelid or annelid like stage.
The undifferentiated body is composed of a series of sornites or
metameres capped anteriorly by the prostornium or acron and
posteriorly by the terminal body segment, the periproct or telson. The
mouth is located between the prostomium and the first body segment;
the anus opens in the periproct.
8 1
Origin and Evolution of Insects
The first
great step was the develop ment of a pair of ventral
appendages or legs on each body segments, aiding in locomotion. Figure
4-B represents this stage. Parallel to this, an improvement in the sense
organs of the head occurred; eyes and antennae were the ultimate result
of this. This level of organisation is met in Onychophora.
Next step represented by Figure 4-C was the development of
articulations in the legs i mproving the locomotion. The first pair of legs
are considered to be used in pushing food into the mouth at certain
geological period during evolution. Eyes and antennae are well
developed. There are no living forms of arthropoda as this, but the
fossil group Tilobita had essentially this sort of body organisation.
Figure 4-D can be looked upon as representing the myriapod level
of body organisation, bilateral appendages of segments 4, 5 and 6
becoming the typical primitive mandibulate mouthparts. Apparently the
appendages of the first body segment are never atrophied in many
groups. Append ages of second body segment ultimately become the
antennae, those of the fourth became the mandible. Appendages of fifth
and sixth segments became the first (maxillae) and second pair of
maxillae (labium). These three segments are termed as gnathal segments.
Figure 4-E represents the pauropod and chilopod level of
organisation. The gnathal segments became consolidated with the
prostomium, resulting in a head structure of compound origin much like
that in insects and their allies. This compound structure brought
together in one functional unit
all the organs ultimately connected
with feeding.
The insectan level of organisation is represented by Figure 4-F. The
body has been differentiated into the three tagmata, head, thorax and
abdomen characteristics of insects. The appendages of segment 7, 8 and
9 have been retained as locomotor structures, but the locomotor
appendages have disappeared from the remaining body segments. The
appendages of abdominal segment 8 and 9 have been modified as
external genitalia and the cerci, appendages of segment 11, have been
retained. The te1son has been lost, and the anal opening is now within
the 11th abdominal segment.
[ III] Evolution of insects
Carpenter (1977) recognises following four major stages in the evolution of
lnsecta :
1. Evolution of wingless insects. The first was the appearance of
pmrut1ve wingless insects, which probably resemble contemporary
bristle-tails and
silverfishes
(Order : Thysanura). These p rimitive
apterygotes are thought to have arisen during the devonian period. The
fossil record of apterygotes is poor because of less sclerotised
Origin and Evolution of Insects
[ 9
exoskeleton which was not amenable to fossilisation processes. In neither
group is there evidence of wings or evidence that they are derived from
ancestors that had wings. Thus they are grouped in a separate subclass
from the winged insects, called Apterygota (A = without, pteron
wing ; Greek) . The winged insects are grouped in subclass Pterygota.
Apterygote insects according to some taxonomists also include three
more orders : Protura, Dipleura and Collembola. But their affinity is
more with myriapod lineage rather than insect lineage.
2. Origin of wings. After the origin of insects (terrestrial air
breathing tracheate arthropod) the second landmark evolution in them
was the origin of wing which has thought to have occurred prior to the
lower carboniferous period.
The concentration of walking appendages in the midbody region the
thorax was undoubtedly an important preseqms1te for the later
development of wings, for this required the evolution of a boxlike, well
musculated thorax. The insects having wings and those that have Jost
their wings secondarily, are grouped in the subclass Pterygota.
The origin of wings is a matter of some dispute. Insect wings are
not modified appendages but are entirely new structures arising as
outgrowths of dorsal parts of the integument of the mesothorax and
metathorax. Following theories have been put forth to explain the origin
of wings :
(a) Flying-fish theory. Jannila Kukalova-Peck of Canad a has
hypothesised that ancestral pterygote insects were aquatic and evolved
movable musculated gill plates on most body segments, comparable to
those on the abdomen of immature mayflies today. Living on swampy
forests, they may have found it advantageous to climb out of the water
to feed on vegetation or escape enemies, and the gill plates could have
helped to break falls and to glide to other pools with a flapping motion.
Those gill plates toward the centre of balance eventually became
enlarged to serve as wings. This is sometimes called "flying fish" theory
of wing origin.
(b) Flying-squirrel theory. Followers of this theory believe that the
ancestral pterygote insects were terrestrial and arboreal. They have
developed lateral flangers of the thorax, which at first served for gliding
from tree to tree or to the ground. Later, they developed the hinges
and musculature necessary for true flight. It is noteworthy that some of
the machilids are able to jump and have lateral extensions of the sides
of the thorax. The cockroaches, which are earlier generalised insects
include many arboreal species having lateral thoracic flanges in the
immature stages and on the prothorax as adults.
(c) Solar-coll!!ctor theory. Matthew M. Douglas has suggested that the
lateral lobes that were precursors of wings may have served a role in
thermoregulation, i.e., they may have served as plates to absorb heat.
=
JO
]
Origin and Evolution of Insects
Thereafter, this structure is
insect
to
run,
to
seek
transferred to the leg muscles, enabling the
food
or
escape
from
enemies
at
lower
temperatures than might otherwise be possible.
Of course, these theories are not mutually exclusive. Thoracic lobes
serving originally for thermoregulation might also have served in sexual
display
(as
they
are
beautifully
coloured)
and
for
gliding
about
in
vegetation; then at a later stage, they may have developed sufficient size
and
an
adequate
hinge
mechanism
and
musculature
to
permit
true
flight. Even today insects do sometimes bask in the sun with their wings
spread, and some do use wing patterns as mating signals. Whatever, the
final answer,
there is no question t!iat the acquisition of wings was a
100 million years the insects were
major event in insect evolution. For
the
only
winged
animals,
and
today
they
remain
the
only
winged
invertebrates and by for the far most abundant of winged animals.
The primitive winged insects like modern dragonflies are believed to
have held the wings somewhat stiffly from the sides of the body. This is
suggested by the fact that many fossils from paleozoic rock in which
insects first made their appearance are preserved in a flattened position
with the wings out stretched. Some of these fossils are like dragonflies
and some are quite large, with wing-spans up to
are placed
into
order
Protodonata
and
were
75 cm. These insects
probably
ancestral
to
the
true dragonflies. Paleozoic rock also contains some insect fossils which
are
evolutionary
piercing
mouth
"dead-ends"
like
the
order
Palaeodictyoptera
having
parts.
After the origin of wing as side stretching of the thoracic body wall,
the second step in the evolution of insects was the evolution of capacity
to hold the wings vertically above the back in the manner of modern
mayflies [Order : E phemeroptera] and damselflies [Order : Odonata :
Zygoptera]. This undoubtedly involved a more fluttering type of flight.
The wing contains so many veins and cross veins but there were fewer
tendencies towards thickening of veins in anterior wing margins.
The
members
of
these
two
orders
(Ephemeroptera
and
Odonata)
lack the ability to fold the wings close to the body as they were only
held
extended
either
vertically
(Ephemeroptera,
Zygoptera)
or
laterally
(Odonata : Anisoptera). These insects have also several similarities, e.g.,
short bristle like antennae
believed
to
represent
a
and aquatic
similar
stage
larvae.
of
Nevertheless, they are
evolution,
when
insects
had
acquired the power of flight. But they retained wings with a complex
venation and no mechanism permitting them to be folded close to the
body. These two orders along with the
Palaeodictyoptera
extinct
in
orders,
are
therefore,
Palaeoptera (primitive
3.
Evolution
evolutionary
step
of
grouped
an
infraclass
and
other
called
the
winged insects).
wing
appears
flexing
to
have
mechanism.
The
occurred
during
third
major
the
lower
Origin and Evolution of Insects
l 11
carboniferous period. This was the development of the capacity to fold
the
wings
flat
or
rooflike
above
the
abdomen.
capacity are said to be neopterous (lnfraclass
By evolving
small
The
insects
with
this
: Neoptera; new wings).
this technique they were able to conceal themselves in
spaces
thus
becoming
occurred very early
Palaeoptera
in the
appeared
at
inconspicuous.
geological time;
the
same
time
This
advancement
also
indeed both Neoptera and
in
the
upper
carboniferous
period.
Wing
folding involved the d evelopment of a more complex hinge
mechanism, including a third axillary sclerite and associated musculature
as well as an appropriate folding or reduction of the hindwings so that
they would fit beneath the front wings (forewings). The forewings then
in some
when
cases become thickened,
folded
(Coleoptera,
serving
Orthoptera,
to protect the hindwings
Heteroptera).
Many
neopterans
exhibit a reduction in the number of wing veins and have a tendency
for concentration at the anterior margin.
thrust.
The
wings
flat
development
against
the
of
wing
body
It permits a stronger forward
flexion
was
-
the
undoubtedly
capacity
a
major
to
draw
step
in
the
insect
evolution.
of
The
earliest
their
wing
paleopterous
called
neopterans
pads
underwent
through
the
insects. Neoptera
Exopterygota
exopterygote
of
this
(development
insects
are
a
gradual,
immature
type
of
said
belong
wings
to
have
external
stages,
as
is
to
development
also
a
exterior
major
to
incomplete
true
of
series
body).
The
metamorphosis
(Hemimetabolous). All the neopterans of the upper carboniferous period
were Exopterygota, including some extinct orders as well as cockroaches
(Dictyoptera),
leaf
insects
(Phasmida),
termites
(Grylloblattodea), zorapters
(Zoraptera), and
These
mouthparts
orders
cockroaches
have
and
group of insects
way. A
order
to
as
shorter
other
second
biting
form
a
that has proved
major
H emiptera
group
(H emiptera),
(Siphunculata
basically
Orthopterodea.
like
those
This
difficult to classify
of E xopterygota
It
also
differs
simpler
wing
venation,
This
group
booklice
or
grylloblattids
earwigs (Dermaptera).
is
of
a
the
diverse
in a satisfactory
centres
around
the
having piercing and sucking mouthparts is referred
Hemipterodea.
antennae,
features.
superorder
(lsoptera),
also
from
Orthopterodea
fewer
includes
in
having
malpighian tubules
diverse
insects
like
and
bugs
(Psocoptera), thrips (Thysanoptera) and lice
Anopleura).
4. The evolution of pupal stage. The fourth and final step in insect
evolution
provided
environments.
They
insects
with
developed
further
the
opportunities
capacity
to
retain
to
exploit
their
wing
their
pads
internally, as imaginal discs, hence they are called Endopterygota. Since
wing development
is
suppressed
in the immature stage,
the
wings
Origin and Evolution of Insects
12 J
must developed rapidly
Thus,
an
additional,
larva
and
adult.
metamorphosis
prior to emergence of the adult,
sedentary
stage,
Endopterygote
(Holometabolous
the
pupa,
insects
are
is
winged
interposed
said
to
form.
between
have
complete
insect).
Among other major events that have taken place in the evolution of
insecta are: evolution of tracheal system, a relatively impermeable cuticle
and the fat body. These are considered as pre-adaptation for insects as
most of the terrestrial arthropods possess these characteristics.
While
the
more
generalised
members
of
this
group,
show
few
differences between larva and adult other than the presence or absence
of
wings,
the
remarkable
permitted
evolution
two
the
mouthparts,
fold
of
suppression
glands
complete -metamorphosis
pattern
of
not
etc.
evolution.
only
requiring
of
set
Presence
wings but
extensive
in
of
motion
a
pupal
stage
legs,
eyes,
even of
reorganisation
during
the
pupal stage. Larvae were able to develop special structures of their own
such
etc.
as
gills,
later
to
embryonic
prolegs
be
state.
on
replaced
Larvae
the
abdomen,
by
adult
became
specialised
structures
increasingly
mouthparts,
that
had
been
specialised
for
glands
held
in
exploiting
diverse source of food, while adults became specialists in dispersal and
reproduction. Although the pupa is relatively defenseless, insects evolved
a variety of mechanisms for its protection, i.e., concealed pupal cells,
cryptic form and colour, or cocoon. The major orders of endopterygotes
are
Megaloptera
(snakeflies),
Neuroptera
(lacewigs
and
antlions),
Coleoptera
(beetles),
Lepidoptera
gnats,
Mecoptera
(butterflies
midges),
and
(scorpionflies),
moths),
Siphonaptera
(fleas)
Trichoptera
Diptera
and
(true
(cadishflies),
flies,
Hymenoptera
mosquitoes,
(sawflies,
ants,
wasps, bees).
The success of each of the four major steps in insect evolution is
well
shown
by
the
number
of
orders
and
the
number
of
species
resulting from each step. Among the modern insects 1 % belong to the
group
developed
after
first
step
of
evolution.
The
second
step,
the
development of wings was of course, a most important one. Probably, at
least
80%
of
living
insect
species
have
complete
metamorphosis
(developed as fourth step). It is interesting to speculate what might have
happened had insects failed to develop wings or to develop the means
of flexing the wings or of retaining the developing wings internally in
the
immature
stages.
Probably they would
constitute
at
most
a
few
paragraphs in treatise on invertebrate zoology.
Important Questions
l.
2.
3.
Write an essay on origin and evolution of insects.
Give an account on the various theories dealing with the origin of wings in insects.
Write short notes on : (i) Significance of wing flexion (ii) Significance of pupal stage.
2
Elementary Knowledge of
C ollection
and
Preservation of Insects
Knowledge of the collection, preservation and storage of the insects and
the
techniques
understanding
of
of
their
the
culture
taxonomy
are
and
pre-requlSlte
biology
of
for
the
the
proper
insects.
The
main purpose of this chapter is to provide : (i) information about the
places
from
where
the
insects
should
be
collected,
collection and collecting equipments, and (iii)
(ii)
methods
of
preservation techniques of
the collected insects.
Collection
One of the best ways to learn about insects is to go out into the fields and
collect them from their habitat, handle them and manage the collections.
Insects
live
in
highly
diverse
habitats
and
can
be
found
everywhere and usually in considerable numbers. Many
practically
insects can be
observed at any hour of the day throughout the year. However, the best
period for collection is from early spring until late rainfall and the best
time for the collection of most of the species is during the daytime. Several
insects
are polyphagous
and therefore,
plants
provide
one
of
the
best
places for collecting them. Insects can be picked, shaken or swept off the
plant with a net. Different species feed on different kinds of plants, one
should,
therefore,
examine
all
sorts
of
plants:
grasses,
flowers,
weeds,
shrubs, and trees. Every part of the plant may harbour insects but the
14 J
Collection and Preservation of Insects
majorities are found on the foliage or flowers. Few insects are only found
on or in the stem, bark, wood, fruit or roots.
Detritivorous insects (feeding on decaying animals or plants) can be
collected from several types of debris. Some species can be found in the
leaf muld and litter on the surface of the soil, particularly in woods or
areas where the vegetation is dense; others can be found
under stones,
boards, bark, and similar objects; still others can be found in rotting or
decaying
material
of
all
sorts,
bodies of dead animals,
such
as
fungi,
decaying
rotting fruits, and dung.
Many
plants
or
the
insects can be
found in or around buildings, or on animals or human beings. Many use
buildings,
cavities
under
buildings,
crevices,
and
similar
places
as
a
shelter. Other house-hold insects may be observed feeding on clothing,
furniture,
from
grain,
various
food,
and
other
materials.
On warm
evenings,
insects
are attracted to lights and can be collected at
street, on the windows or screens of lighted rooms, or at lights put up
especially
sources
to attract them. In
fact,
this is one of the easiest ways of
collecting many types of insects.
Several
insects
are
aquatic
and
can
be
collected
from
the
ponds,
lakes, riverines, rivers etc. The adults of a great many species are best
obtained
by
collecting
their
immature
stages
and
rearing
them.
This
involves collecting cocoons, larvae or nymphs, and maintaining them in
some
sort
of
container
until
the
adults
appear.
This
method
provides
better adult specimens than the field collected adults.
[ I] Methods of collection
Since, insects live in diverse habitats, they may be collected by several ways
as follows : by handpicking, sweeping and beating; collecting with aerial
nets and aspirator ; trapping and using Berlese funnel and separator.
1. By hand picking. Small insects, specially the soft bodied ones
should be collected by hand either with the help of a fine camel hair
brush or by a forcep. The soft brush should be dipped in the medium
in
which
the
insects
have
to
be
preserved,
so
as
to
minimise
the
damage to soft skin. Forcep can be used carefully to avoid damage to
the insect as in the cases of ants and many insect larvae. Insects like
leaf-miners
insects
(Diptera),
living
under
aphids
stone
and
(Hemiptera),
vegetable
bark
inhabiting
(Dermaptera
and
beetles,
Coleoptera),
termites and ants etc. are collected by hand picking.
2. By sweeping. Sweeping with a proper net yield satisfactory result
while collecting insects from herbage. Sweeping nets must be of tough
cloth. A
50-60
cm long strong handle with
50
cm depth bag is quite
good. The disadvantage of sweeping method is that it does not provide
host plant data. Insects living in concealed places (e.g., within flowers,
leaves or near ground) are difficult to collect by this method.
Collection and Preservation of Insects
[ 15
3. By beating. Beating is usually employed to dislodge insects from
foliage or trees. A long stick is used to beat the plant part with
downward strokes and a tray or cloth is kept or spread over the ground
to get the falling insects. A net may also be kept on the ground to
prevent the insects from escape, after they fall to the ground.
4. By aerial netting. Aerial nets are most widely used to collect free
living flying insects, e.g., dragonflies, moth, butterflies, wasps, flies, bees
etc. The length of handle, diameter of ring, depth of the net may vary
on individual collectors' preference. But normally strong, light, easily
manageable handle with 30-40 cm diameter ring and strong, durable,
nylon bags with a depth of 50-70 cm are used (Fig. 2.1-A). After
netting the insects, the net should be turned to prevent the escape of
captured insects. Soft bodied insects like moth and butterflies may be
gently removed from the bottom of the bag.
5. By collecting with aspirator. Small active insects like leafhoppers,
whiteflies, other bugs, beetles etc. may be collected by a sucking tube
or aspirator, straight from the plant surface. It is a very simple device
(Fig. 2.1-B) and if used with little patience and caution may yield
desirable result. It is also useful to transfer insects from sweeping nets
or from rearing cages. All that one has to do is to suck the air by
rubber tubing which would draw the insect into the main tube through
the glass tube. A vacuum cleaner may also be used as aspirator to suck
the insects from herbage or from their hides.
6. By trapping. Traps are an easy and often very effective method
of collecting several types of insects. A trap is any device containing
something to which the insects are attracted and which is so arranged
that once the insects get into it they cannot get out. The attractive
materials used and the general form of the trap depends on the type of
insects one wish to collect. Some common types of traps are :
(a) Light trap. An artificial light (kerosene lamp, Petromax gas light,
electrical lamp) if placed adjacent to a white muslin cloth in field
attracts a number of insects like crickets, grasshoppers, moths, mantids,
beetles, etc. Most of the insects attracted to the light would rest on the
white cloth from where they may easily be picked up by hand or by
aspirator. Simplest form would be to suspend a light source over a
broad rimmed funnel which in tum may be fitted in a glass jar
containing poison vapour or other killing agent (Fig. 2.1-C). Light traps
may work all night and may also supply data indicating seasonal
incidence, peak period for a population etc.
(b) Bait traps. The odour of the particular kind, food or
pheromones (sex attractants) act as the principal agent in bait traps.
Baits may include over-ripe fruit, piece of meat or fish, rotten fungi,
animal excreta etc. and these may be put at proper place where the
(Z-57)
Collection and Preservation of insects
16 J
l
{r·
m
(1)
,..,
l ],/
(ti)
(iv)
(iii)
�
closed with
muslin cloth
glass vial
B
A
bulb
IP01s0cl
plaster of paris
saw dust
al coh ol
c
D
+
KCN
E
Fig. 2.1. (A) The handmade msect net (i) Grooves and holes are cut in the end of the
handle, (ii) the wire for the rim is bent, (iii) fitted into the holes and grooves and held
tighten with wire, (iv) finished net, (8) aspirator, (C) light trap, (D) Berlese funnel and
(E) killing bottle.
insects would gather. One of the simplest form is pit fall trap where a
jar containing bait is placed below the soil level to catch crawling
insects like cockroaches, ground beetles, ants, etc. Bait traps may also
be used for flying insects; a simple device is to put a metal funnel with
bait suspended at the top level, inside a killing bottle, which would
attract the flying insects . Pheromones when used in field-traps may
attract thousands of insects of different groups.
(c) Wind traps. The wind traps may range between a simple sock
attached to a pole in the direction of wind or may be a electrically
operated suction device. In India, not much of insect collections have
been made by wind traps. Yellow Pan Water trap is a simple device to
attract aphids to the preferable colour, in which the pan is painted. It is
a metal tray painted in yellow and half filled up with water; insects
being attracted to colour, fall into the water.
7. By using Berlese funnel. Soil insects or insects living in leaf litter
are collected along with part of the habitat and brought to the
laboratory where they are usually put in a funnel which acts as a
(Z-57)
·
Collection and Preservation of Insects
[ 17
separator. Several modifications of this device, known as "Berlese
funnel" are now available. The simplest form being a metal funnel with
sieve, inserted inside a can or collecting tube, the material (moss,
debris, litter) is put on the sieve which is subjected to continuous
heating by light bulb; the collecting tube contains preserving fluid like
alcohol and the lip of the funnel touches the fluid. Insects in order to
escape from the heat move down through the sieve and fall into the
preservative (Fig. 2.1-D).
[ II] Collecting equipments
Different types of insects living in different habitats are collected by using
several types of equipments and devices. Following are some of the widely
used equipments and devices :
1. Nets. A net essentially consists of a cloth bag or nylon net bag,
a metal ring which holds the mouth of the open-bag, and a handle to
which the metal ring is attached. According to the types of insects and
methods of collection following types of nets are commonly used :
(a) Aerial nets. Most winged insects, e.g., butterflies, moth, bees,
flies etc. that are active during the sunny days are captured with an
aerial net. It consists of a wire ring of about 35-40 cm diameter; the
ends of the ring should fit into a groove at the end of handle, the
detachable ring allows to change a dirty bag or a torn one. The depth
of the bag attached to the rim of the ring is usually 6q-70 cm or twice
the diameter of the ring. Muslin cloth or nylon net may be used for
the bag. The bottom of the bag should be rounded in order to prevent
the insects from becoming lodged and damaged at the tip. The handle
should be strong but light.
(b) Sweep net. Smaller insects infesting thick vegetation are collected
by sweeping method. The sweep net is similar to that of aerial net but
the bag used is of thick cloth instead of nylon net or muslin cloth and
the handle is fairly short and stouter to allow quick sweep over
vegetation. During use, the contents of the net should be frequently
observed and the specimens sorted and placed in proper killing bottles.
(c) Aquatic dip net. A dip net is used to collect the aquatic insects.
It is like an aerial net, but should be shallower (no deeper than the
diameter of the rim) and much stronger. The handle should be heavy
and the rim should be made of metal rod and securely fastened to the
handle. The part of the bag that is attached to the rim should be of
canvas, and it is desirable to have an apron of the same material
extending down over the front of the bag. The rim of dip nets has the
rim bent in the form of the letter D. Dip nets can be used to collect
free-swimming insects, insects on vegetation, and insects burrowing in
the sand.
(Z-57)
18 1
Collection and Preservation of Insects
J.. Brush, forcep, twigcutter and scissors.
Soft camel
hair drawing
brush of No. 0 or 1 is usually used for hand collection. A thin, light
weight forcep with bent end or straight end is used to pick insects from
the surface. A scissors or twigcutter may be used to cut plant-part or
twigs. harbouring scale insects, aphids and some other bugs.
3. Aspirator. This is a very useful device for collecting alive small
insects. Various forms of aspirators have been devised but one of the
simplest and easiest to handle
is
the vial type.
Sucking through the
mouthpiece will draw small insects into the vial and a cloth over the
inner
end
of
the
mouthpiece
tube
prevents
the
insects
from
being
sucked into the mouth. If one has a series of vials to fit the cork of
this type of aspirator, it is a simple matter to remove an insect-filled
vial and replace it with an empty one. The insects caught in the vial
may be killed by replacing the aspirator cork with a cork containing a
killing agent.
An aspirator may also be made from a piece of Pyrex glass tubing,
with the mouth-piece tube in one end and the intake tube in the other.
This type
which does not involve any bending of the glass tubing and
requires only one hole in each cork, is easier to make than the vial type
of aspirator; however, it is a little more difficult to transfer insects out
of this type of aspirator than out of the vial type.
4. Axe, knife and hammer. These are necessary tools for collecting
insects inhabiting soil, termite mound, under bark and rotten log. These
ate used to tear off loose bark or splitting wood or breaking open the
mound or digging out the borers and miners.
5. Killing bottle. If the insect is to be preserved after it is captured,
it must be killed in such a way that it is not injured or broken : for
this purpose, killing bottles of various sizes and shapes may be used,
depending on the type of insects involved, and various materials used as
the
killing
agent.
narrow-necked
Wide-mouthed
ones.
All
killing
bottles
bottles
or
jars
(Fig.
are
much better
2.1-E),
regardless
than
of
the
killing agent used, should be conspicuously labelled "POISON," and all
glass bottles
should be reinforced with tape to
reduce the hazards of
breakage. Usually glass/plastic jar with a layer of cyanide covered with
plaster of Paris, is used as a killing bottle.
may
make
often
some
small
other
specimens
liquid
brittle
a.nd
chemicals
Cyanide vapour, however,
even
like
change
chloroform,
its
colour.
benzene,
Very
ether,
carbon tetrachloride are used. Each one of these has some disadvantage
but in general their vapours serve the purpose of killing the insects. The
liquid may be poured over a layer of cotton and one or two filter paper
or
blotting
specimen
paper
from
could
coming
in
cover
direct
the
soaked
contact
with
cotton.
cotton.
It
prevents
Insect
must
the
be
handled carefully while they are put inside the bottle or taken out to
(Z-57)
Collection and Preservation of ln$ects
[ 19
prevent damage. A killing bottle with a layer of small chips and saw
dust soaked with a few drops of ethyl acetate also serves satisfactory for
a number of insects in killing and preserving the specimens which
remain flexible. The efficiency of a killing bottle depends to a large
extent on how it is used. It should never be left uncorked any longer
than is necessary to put insects in or take them out; the escaping gas
reduces its strength, and an uncorked bottle (particularly one made up
with cyanide) is a hazard. To make the bottle dry inside, few pieces of
absorbent material should be kept in side it. A killing bottle used for
moths and butterflies should never be used for other insects unless it is
first cleaned; insects put in a bottle with butterflies or moths will
become covered with scales and look dusty, and will not make good
museum specimens.
6. Collection vials. Small specimens, which are killed and preserved
in liquid, are to be kept in Homeopathic vial or similar other vials.
Vials with screw plastic caps are preferable. These may be numbered
beforehand and the corresponding number in the field notebook may
contain details about particular collection.
7. Hand lens. Strictly speaking, a hand lens is not a means of
collecting, but it is very useful for examining insects in the field. A lOX
hand lens (folding type) is useful to examine material in the field.
8. Paper packets and envelopes. Paper packets are used to keep
moths, butterflies, dragonflies and many other insects. As soon as they
are killed, the insects are transferred to these packets, made up of
oil-paper, for temporary storing and transportation. These may be
prepared in desirable sizes before one proceeds for field collection.
9. Chemicals and cotton. During field collection, preservative like
90% alcohol, killing agents like benzene or chloroform, or ethyl acetate
should be carried extra, to meet any emergency; cotton may be required
for packing after collection or to change the killing agent in the killing
jar and should be kept handy.
10. Traps. Pitfall trap or Yellow Pan Water traps have already
been discussed. The latter is a shallow pan trap painted yellow, which
should be filled up to the half before being placed in the field.
Petromax lamps are handy for light trapping and could be carried easily
and used specially where electricity is not available.
11. White tray and sieve tray. These may be useful to sort out
debris, litter and aquatic collection during preliminary examination. After
checking the material, the useless parts may be eliminated by sieving
and the samples may be put in Berlese funnels.
12. Haversack, boot and camera. The equipments could best be
carried in a haversack. However, a jacket which a collector may wear
may be tailor-made with specification to suit the need. A number of
20 1
Collection and Preservation of Insects
small pockets to keep the collecting instruments handy may be provided
in the jacket. One can get various kinds of other handy equipments
from shops of fishing equipments viz. fisherman jacket, bags, fishing nets
etc. A good quality hunting rubber soled boot would immensely help
while going to the field. A field camera with f. 1.4 or f. 1.8 lens and a
set of close-up lenses would be very useful for photographing ecological
conditions,
insect
community,
feeding
site,
habitat
and
other
observations.
13. Field notebook. A field notebook is most essential for keeping
all the data. Generally a numbered tag may be attached with the
collection and the same number in the field notebook may be used to
keep the following data: (i) Date of collection. (ii) Place of collection
indicating direction, approximate distance in kilometer from nearest
railway station, road head, aerodrome and altitude. (iii) Habitat.
(iv) Live colour. (v) Name of host plant or animal. (vi) Associated
insects or animals. (vii) Name of collector etc. A general note on the
collection locality would provide further information, say about a
Reserve Forest Area, its vegetation type etc.
14. Lunch packet. One should not forget to carry out a lunch
packet while going in field. Easy items to carry are : breads, biscuits,
sandwiches, fruits, water and tea or coffee in small flask.
Mounting and Preserving Insects
Collections, once made, are to be preserved in a manner which provides
scope to examine the specimens for identification and study and also
guarantees long period of storage, with proper care. Insects can be
mounted and preserved in various ways. Most specimens are pinned, and,
once dried will keep indefinitely. Specimens too small to pin can be
mounted on points on tiny minuten pins, or on microscope slides. Large
and showy insects, such as butterflies, moths, grasshoppers, dragonflies, or
damselflies, may be mounted in various types of glass-topped display cases.
Soft-bodied forms, such as nymphs and larvae and the adults of midges,
caddi�flies, mayflies, and stoneflies, should be preserved in fluids like
70% alcohol.
[ I] Relaxing
After the collection, the insects should be mounted as soon as possible as
after drying they become brittle and may be broken in the process of being
mounted. Specimens stored in envelopes for a long time must be relaxed
before being mounted. Any wide-mouthed jar that can be made airtight
can be used as a relaxing chamber, the bottom of which is covered with
wet sand with a little carbolic acid to prevent mould. After putting the
Collection and Preservation of Insects
[ 21
insects in the jar, the jar is tightly closed. Usually most of the insects are
relaxed to mount after a day or two in such a chamber.
[ II] Pinning
Pinning is the best way to preserve hard-bodied insects. Pinned specimens
keep well, retain their normal appearance, and are easy to handle and
study. To avoid colour fading, the specimens are dried rapidly. Insects are
pinned with entomological pins made of steel with chromium polish to
avoid rust. These pins are of various sizes (# 0-7) for different sizes of the
insects.
Insects are usually pinned vertically through the body as shown in
Figure 2.2. Place of pinning varies with the group of the insects, e.g.,
bees, wasps, flies, butterflies and moths are pinned through the thorax
c
D
E
Fig. 2.2. (A) Methods of pmmng insects. (B) grasshopper m lateral view. The black spots
m other msects show the location of the pm in the case of flies (C) bugs,
(D) grasshoppers and (E) beetles.
Collection and Preservation of Insects
22 J
between the bases of the front wings; with flies and wasps it is desirable
to insert the pin a little to the right of the midline. Bugs are pinned
through the scutellum, a little to the right of the midline if the
scutellum is large. Grasshoppers are pinned through the posterior part
of the pronotum, just to the right of the midline. Beetles should be
pinned through the ri�ht elytron, about halfway between the two ends of
the body. Dragonflies and damselflies are best pinned horizontally
through the thorax with the left side upper-most. All specimens should
be mounted at a uniform height on the pin-about 25 cm above the
point. If the abdomen hangs down when the insect is pinned, a piece of
stiff paper may be placed on the pin beneath the insect to support it
until it dries. A sagging abdomen may be supported by means of
crossed pins, with the abdomen resting in the angle of the cross.
[ III] Spreading insects
It does not greatly matter about the position of the legs or wings of most
insects when the specimen is pinned, as long as all parts can be easily seen
and studied, however, the wings of moths and butterflies and possibly some
other insects should be spread before the insect is put into the collection.
The method of spreading depends on the type of insect.
The wings of an insect is spread on a spreading board dorsal side
up, and the pin is left in the insect. There are certain standard
positions for the wings of _a spread insect. In the case of butterflies and
moths and mayflies, the rear margins of the forewings should be straight
across, at right angles to the body, and the hindwings should be far
enough forward that there is no large gap at the side between the fore
and hindwings. With grasshoppers, dragonflies. damselflies, and most
other insects, the front margins of the hindwings should be straight
across, with the front wings far enough forward that they just clear the
hindwings.
pin
paper
point
pin
A
paper
strip
B
Fig. 2.3 (A) Mounting of a minute bug on paper point, (B) Spreading of a butterfly upside
down on a flat surface.
Collection and Preservation of Insects
[ 23
The wings are held in position by strips of paper or other material
pinned to the board, the antennae and other structures are oriented and
held in position by means of pins as shown in Fig. 2.3. With specimens
mounted upside down, it is desirable to have the specimen securely held
down. Once the wings are in position, the antennae can be properly
oriented and held in place by crossed pins. With a specimen mounted
upside down on a flat surface, the final step is removing the pin from
the body of the insect, this is done by holding the body down with
forceps . and carefully withdrawing the pin.
[ IV] Mounting
Very small insects are mounted on a card point on a minuten pin, or on
a microscope slide, or they may be preserved in liquid. Most small
specimens are mounted on points. Points are elongated triangular pieces of
ivory paper or art paper, about 8 or 10 mm long and 3 or 4 mm wide at
the base; the point is pinned through the base, and the insect is glued to
the tip of the point that can be cut with scissors. One should use as. little
glue as possible to avoid imbedding of the insect body parts in it. The
specimen should be correctly oriented on the point. Standard positions of
an insect mounted on a point are shown in Figure 2.3-A. The point should
extend to the left of the pin, and if the specimen is mounted dorsal side
up, the head should be pointing forward; if it is mounted on its side, the
head should be directed to the left, with the left side of the insect
uppermost. Beetles mounted on points should always have the ventral side
of the body visible; flies, wasps and other insects in which the wings are
extended above the body are best mounted on their side. The glue used in
mounting insects on points should be quick-drying and should be quite
hard when it sets (e.g., Quickfix or Favifix).
Very small insects that can not be pinned (particularly soft-bodied
insects) and certain structures as genitalia, are mounted on microscope
slides. The procedure of mounting a specimen on a microscopic slide
depends on the group of the insects and on the type of mounting
medium used. Dark-coloured and thick-bodied insects or body parts
must be cleared before mounting in a 10- 1 5% cold or warm KOH
solution or saturated mixture of chloral hydrate and phenol (for aphids).
Four common types of mounting media are usually used to mount
the insects : Canada balsam, DPX, gum Arabic and PVA (polyvenyl
alcohol). Specimens mounted in balsam and DPX must be dehydrated
before they are put into the medium. Dehydration involves running the
specimen successively through increasing concentrations of alcohol (50%,
70%, and absolute), then through xylol (xylene), and finally into the
24 J
Collection and Preservation of Insects
balsam/DPX on the slide. The time left in each solution before moving
it to the next depends on the size of the insect and may vary from a
few minutes to an hour or two. Inside the balsam/DPX, the insect is
oriented and then the cover glass is put on. Once the cover glass is on,
the slide must be kept horizontal until the balsam hardens. Specimens
mounted in gum Arabic or PVA can be moved directly without
dehydration. This medium is par.icularly useful for mosquito larvae and
other insects that require httle or no clearing, the medium itself has
some clearing action. Gum Arabic requires a little longer to harden
than does balsam/DPX. .The gum Arabic medium is made up of the
following ingredients : Gum Arabic : 30-40 g, chloral hydrate : 50 g,
glycerin : 20 ml and distilled water : 50 ml.
[ V] Labelling
A specimen carries no meaning at all, if it is not properly labelled so far
as systematic studies are concerned. While labelling the following points
should be noted: (i) Name of the host (animal/plant) and location on host
for carnivorous and herbivorous insects, (ii) Locality (name, district, state
and if necessary, latitudes, longitudes and altitude), (iii) Date of collection,
and (iv) Name of the collector. It is also desirable to maintain a field
notebook to record all the necessary details in regard to host, colouration,
nature of damage to animals/plants if any, presence of preys/predators/
parasitoids/other insects. The label should be on fairly stiff white paper and
preferably not larger than l 0 x 25 mm in size and the ink used must be
water-proof. They should be at a uniform height on the pin. parallel to and
underneath the insect; more than one label may be used.
[ VI] Identification
Identification of insects always demands well-preserved specimens and
reference collection to compare with. Useful taxonomic literature along
with reference collection on the group can be said to be the keystone for
any short-term or long-term entomological work. In the beginning, one
has to build up a reference collection. normally getting the identification
done or getting the confirmation of tentative identification done by
specialist working in museum or institutes. In order to do so, one has to
assure proper packing and shipping of the specimens so as reach the
destination without damage. Packing and transporting or mailing insects
need special care.
Mounted insects should be firmly pinned in a container or mailing
box, with proper lining of cork or soft material like thermocole. Large
specimens must have additional pins set firmly on each side of the
specimens (cross-pinning) so as to prevent damage. Boxes containing
Collection and Preservation of Insects
[ 25
insect specimens, should be put in larger container with enough packing
material beneath, above, and on all sides; saw dust, paper strips, cotton,
etc. may be used for the purpose. The larger container should be closed
firmly, wrapped in thick paper or waterproof paper, labelled with postal
label, preferably written with a sketch-pen.
Vials containing specimens should be full of liquid, and each vial
should have a cotton plug at the bottom to avoid splashing and · should
be fitted with leak-proof cork. Each vial should be individually wrapped
with cotton or paper and before wrapping, the mouth with lid may be
dipped in sealing wax to prevent leakage. All vials after being wrapped
should be put in a box with cotton lining, and cotton should also be
put in between vials. The box containing vials may either be put in
large box, as before, or may be shipped directly. All mailing box must
be strong, made up of wood, thick card-board or thermocole, the last is
suitable for small packets to be sent by air-mail.
Slide containing specimen should be packed in small lot of 1 0 each,
being separated from the next by two thick cardboard piece, of the size
of slide labels, and the location then wrapped with a plane slide in
either side, by cellophane tape.
Live insects, specially young ones may be sent for rearing. Larvae
must be shipped with a part of food plant which should last till it
arrives at destination. Overloading of specimens or plant should be
avoided. Pupae are best sent when packed in moist moss. Adult insects
may be sent with a few stem of host plants and to avoid excess
moisture a few holes may be made in the container.
Following are the major Indian and foreign institute ' s where the
insects are identified :
1. Zoological Survey of India,
Division of Entomology, 34 Chittaranjan Avenue, Kolkata, 700 0 1 2,
West Bengal.
2. Indian Agricultural Research Institute,
Division of Entomology, New Delhi, 1 10 0 1 2 .
3 . Forest Research Institute,
Division of Entomology, Dehradun, Uttaranchal .
4. Tamil Nadu Agricultural University,
Department of Entomology, Coimbatore, Tamil Nadu.
5. British Museum, Natural History,
Deptartment of Entomology, Cromwell Road, London SW7 5BD,
England.
6.
Museum d'Histoire Naturelle,
45 me de Buffon, 75 005 Paris. France.
26 J
7.
8.
9.
Collection and Preservation of Insects
Institute fur Bodenforschung und Baugeologic,
Gregor-Mendel, Str. 33, A-1180 Wien (Austria).
National Museum of Natural History,
Department of Entomology, Praha, Kunratice 1, Czechs.
Museum d'Histoire Naturelle,
Route de Malagnov, 1211 Geneve, Switzerland.
Important Questions
I.
2.
3.
4.
Describe the various methods of collection o f insects.
Give an account of the collection kit used to collect insects.
How do you mount and preserve the insects ?
Write short notes on : (i) Insect traps; (ii) Killing bottle; (iii) Mounting of insects ant
(iv)
Aspirator.
3
Classification and
Identification of Economically
Important Insect Orders
Variation is the rule of nature. It causes diversity in animals through
evolution. In spite of its vastness and diversity, the animal kingdom has a
definite grading order in its diversity depending upon the similarities
among them. By grouping organisms based on degree of similarity, one can
arrive at a system of classification. Thus, classification is the process in
which the animals are grouped into a system according to their
resemblance and difference in morphological and biological characters or
attributes. A meaningful grouping of insects is the objective of classification
[French c/asse
akin to, group] . The data about the kinds of the
organisms determine their position in the systems and thereafter are
reflected by that position. Thus, the classification is the ordering of the
animals into groups on the basis of their relationships. Proper
characterisation (species concept) and identification of species and
assigning them proper scientific names (nomenclature) are pre-requisites
for classification.
Taxonomy [Greek words taxis
arrangement, and nomos
law] is
not a synonym of classification. Indeed, it is the theory and practice of
classification, and includes its bases, principles, procedures and rules. It
refers to the day-to-day practice of dealing with the kinds of organism
and includes handling and identification of specimens, the publication of
data, the study of literature and the analysis of variations shown by the
specimens.
=
=
=
28 l
Economically Important Insect Orders
Another word systematics [Greek .rystema] applied to the system of
classification developed by Linnaeus in his book Systema Naturae (1753)
is wrongly used to express the meaning of taxonomy and classification.
Systematics is a wider term and includes both taxonomy (also
classification and nomenclature) and evolution. It means that systmatics
not only concerns with the arrangement of the organisms into taxa and
naming them (taxonomy) but also with the causes and origins of these
arrangements (evolution).
The kind of organisms which constitute groups are arranged in
different levels of categories. When a category is defined, it is called
taxon (pleural, taxa), e.g., Phylum is a category and Arthropoda is a
taxon. Here Arthropoda means that the animals that belong to this
group possess following attri.butes: bilateral symmetry; segmented body,
the segments usually grouped into 2 or 3 rather distinct regions;
chitinous integument, paired jointed appendages; open circulatory
system; coelom reduced represented by haemocoel; excretion by means
of malpighian tubules, coxal glands or green glands. The insects belong
to the Phylum Arthropoda and Class Insecta. It implies that the insects
have the features of Arthropoda listed above and are further categorised
by having three body regions (head, thorax and abdomen), one pair of
antennae, three pairs of legs and usually one or two pairs of wings; the
attributes of the Class Insecta. Classes of animals are further divided
into Orders, the Orders into Families, the Families into Genera
(Singular, Genus) and Genera into Species. Thus, the main categories in
animal classification are Phylum, Class, Order, Family, Genus and
Species. However, for precise position of species, particularly when the
category is highly diverse, intermediate categories may be used by
prefixing Super-, Sub-, and Infra- words with main categories. A new
category Tribe (between Genus and Family) and Cohort (between Order
and Class) are also frequently used. The principal categories used in
classification, arranged in a systematic hierarchy (order of rank), also
known as Linnaean hierarchy. may be listed as follows in decreasing
rank : kingdom, phylum, subphylum, superclass, class, subclass,
intraclass, cohort, superorder, order, suborder, infraorder, superfamily,
family, subfamily, tribe, subtribe, genus. subgenus, species, subspecies.
Procedures in TaxonoIDy
( 1 ) Procurement of the specimens : from fields, by exchange, purchase, in
the museum.
(2) Procurement of literature : from library, on request, by booksellers,
agencies like INSDOC, IARI, CD-ROMs, internet.
(3) Study of the specimens : morphology and biology, every possible
aspects.
Economically Important Insect Orders
[ 29
(4) Study of literature : discussion and conclusion.
(5) Identification of the specimens : on the basis of literature and other
identified specimens in consultation with pioneer worker in the group.
(6) Publication of the results, description, key etc.
(7) Proposal of new name : based on characters, location, in honour of a
scientist etc.
(8) Classification of the species.
(9) Maintenance of the collection, study new methods and nomenclature.
Taxonomical Characters or Attributes
A taxonomic character is any possible trait an individual might possess.
This could be anything from the shape of a particular sclerite
(morphological character), to a particular kind of amino acid in the
haemolymph or cuticular hydrocarbon profile (biochemical characters), to
a particular mechanism for excretion (physiological character), to a specific
way of responding to a change in photoperiod (behavioural character), to
the nuileotide sequence of a particular piece of DNA (genetic character).
However, all characteristics of a given organism are not used in a
description of that organism. It follows, then, that any species description
plus all information gained since the discovery of that species is only a
partial picture and that a species description in this sense is never really
completed. In reality the vast majority of species descriptions are based on
selected morphological features, and although the modern trend is
definitely in the direction of including other sorts of data (particularly
physiological, biochemical, genetic, and behavioural), morphological
characters will likely continue to be central to most descriptions, at least of
insects because such characters are visible even in the dead insects.
There are a wide variety of characters to choose from in a
description. Following is the summary of the characters used m
classification :
1. Morphological characters
(i)
(ii)
(iii)
(iv)
(v)
General external morphology
Special structures (e.g., genitalia)
Internal morphology
Embryology
K.aryology (and other cytological differences)
2. Physiological characters
(i)
(ii)
(iii)
(iv)
Metabolic factors
Serological, protein, and other biochemical differences
Body secretions
Genie sterility factors
30 1
Economically Important Insect Orders
3. Ecological characters
(i)
(ii)
(iii)
(iv)
(v)
H abitats and hosts
Food
Seasonal variations
Parasites
H ost reactions
4. Ethological characters
(i)
(ii)
Courtship and other ethological isolating mechanisms
Other behaviour patterns
5. Geographical characters
(i)
(ii)
General biogeographical distribution patterns
Sympatric-allopatric relationship of populations
6. Molecular genetic characters
(i)
Isozymes
(ii) Nucleic acid sequences
(iii) Gene expression and regulation
No matter what type of character is used, it is extremely important
to know the extent to which that character varies within a species. For
characters to be of value in identification and classification they must be
reasonably constant or vary predictably. Failure to recognise variation
can lead to confusion. There are a number of characters that can vary
within a population of a given species, therefore, one should be cautious
while dealing with the classification. Summary of such characters are
given below:
1. Extrinsic or non-inherited variation
(i)
(ii)
(iii)
(iv)
(v)
Variation due to age
Seasonal variation
Castes in social insects
Variation due to habitat
Variation due to crowding
(vi) Climatically induced variation
(vii) H ost determined variation in parasitoids
(viii) Heterogonic or allometric variation
(ix) Traumatic variation
2.
Intrinsic or inherited variation
(i)
(ii)
Primary sex differences
Secondary sex differences
Economically Important Insect Orders
{ 31
(iii) Alternating generations
(iv) Gynandromorphs
(v) Intersexes
(vi) Mutations resulting in continuous variation
(vii) Mutations resulting in discontinuous variation
(viii) Genetic polymorphism
Nomenclature
Nomenclature means allocation of names to the taxa which is the first and
foremost task of every taxonomist. The scientific naming of animals follows
certain definite rules to avoid ambiguity and unstability. An animal name is
the key to its literature. The rules for zoological nomenclature are outlined
in the International Code of Zoological Nomenclature which was adopted
by the XV International Congress of Zoology, held in London, in July,
1 958 and published by International Trust for Zoological Nomenclature,
London in 196 1 .
The objective of the code is t o promote stability and universality
in the scientific names of the animals and to ensure that each name is
unique and distinct. Priority is the basic principle of zoological
nomenclature.
Taxa of rank above species group is uninominal. The name of a
species consists of the genus and species names, and that of subspecies
consists of the genus, species, and subspecies names; thus the scientific
name of a species is a binomial and that of a subspecies is a trinomial.
The name of subgenus is placed in parenthesis between generic and
specific names and is not counted as one of the word in binomial or
trinomial name of the species. The scientific name of the genera and
subgenera may be derived from any language but it should be in the
nominative singular; names of higher categories are Latinised nouns in
the nominative pleural. The generic, subgeneric, species and subspecies
are always printed in italics. In typewriting or handwriting it should be
underlined. The generic name must consists of a single word written
with capital initial letter. If genus is divided into subgenus the name of
typical subgenus must be the same as the name of the genus and the
subgenus is to be placed in parenthesis between the generic and specific
name. The author of a scientific name is that person who first descirbed
the species. When a species is transferred to another than the original
genus or the specific name is combined with any other generic name
than that which it was originally published, the name of the author of
the specific name is placed in parenthesis, e.g., Lysiphlebus delhiensis
(Subba Rao & Sharma). The species delhiensis was described by Subba
Rao & Sharma, who described it in some genus other than Lysiphlebus
(Z-57)
32 J
Economically Important Insect Orders
(they described the species under the genus Aphidius ), and this species
has since been transferred to the genus Lysiphlebus .
The names of some of the higher categories have standard, endings,
and hence can always be recognised as referring to a particular
category. These are as follows : Superfam ily names end in -oidea
(Aphidoidea, aphids), Family names end in -idae (Aphididae ), Subfamily
names end in -inae (Aphidinae ), Tribe names end in - ini (Aphidini ) , and
Subtribe names end in -ina (Aphidina ) . All the above-named categories
(taxa) have for nomenclatural purpose, a type genus by suffixing the
appropriate ending to the stem of the name of the type genus, e.g., for
the above taxa, the type genus is Aphis. The type of a species or
subspecies is a specimen (the holoype), the type of a genus or subgenus
is a species (the type-species), and the type of a family or subfamily is a
genus (the type-genus). If a species is divided into subspecies, the
particular subspecies that includes the holotype of the species has the
same subspecies name of the species name, e.g., Gryllotalpa gryllotalpa
gryllotalpa. Similary, if a genus is divided into subgenera, the subgenus
that contains the type-species has the same subgenus name as genus
name, e.g.. Formica (Formica) rufa. A species may have generic,
subgeneric, specific and subspecific names as Kerria (Kerria) lacca lacca
(lac insect).
Identification
The purpose of identification is to determine what kind of organism a
given specimen is. The meaning of "kind" depends largely upon one's
objectives. A student in a general zoology course may be happy to identify
a given organism as a fly, a wasp, or a cockroach, whereas a professional
entomologist may need to know the species of the fly, wasp or the
cockroach. Professional reasons for wanting to know the species of an
insect fall into two categories : the non-systematist's reasons and the
systematist' s reasons. The non-systematist needs to identify a specimen in
order to find pertinent literature and to have a name under which to
publish his or her findings. Thus the species name of an organism serves
as the "file category " under which any and all information pertinent to this
species is stored.
The systematist, in addition to using the species name as a key to
the literature and a category for data storage, needs to identify an
organism to species or at least to the point where it is realised that the
organism has not been previously described. This has been called
taxonomic discrimination. Identification of a specimen must necessarily
precede fitting this specimen into a scheme of classification.
(Z-57)
Economically Important Insect Orders
[ 33
[ I] Methods of identification
Ultimately, all methods used to identify the organisms are based on the
comparison of morphological, biological ethological or ecological
characters. There are 6 ways of identification as follows :
1. By having it identified by an expert. This method is simplest but
is not always available. For example, most of the Indian insects can be
identified at Zoological Survey of India, Division of Entomology, Kolkata
(mainly Coleoptera, Lepidoptera, Diptera, Hemiptera, Hymenoptera) and
its stations located in Dehradun, Shillong, Pune, Chennai, Patna,
Jabalpur, Solan, Berhampur, Hyderabad, Calicut and Port Blair; Indian
Agricultural Research Institute, Division of Entomology, New Delhi
(mainly insect of economic importance, Lepidoptera, Hymenoptera); and
Forest Research Institute, Division of Entomology, Dehradun (mainly
insects of forest areas). In addition to this method, one may contact the
specialist taxonomists personally. By having the address from literature,
a request letter along with the well mounted specimens may be sent to
them for identification. Other museums where identification service is
available is given in Chapter 2.
2. By comparing it with labelled specimens in a collection. One of
the best means for identifying a specimen is to compare it with
specimens that have previously been identified. In this method, one has
to visit a museum. By comparing the specimens-in-hand with those
available in the museum, the specimens can be identified at any level.
For most of the beginners this method may not always be available.
3. By comparing it with pictures. The specimens-in-hand may be
compared by using pictures of the insects in the fonn of colour plates,
black-and-white photos, or line drawings. These pictures may be of
entire organisms or parts of insect. Pictures are particularly useful when
an organism is highly patterned or characteristically coloured. However,
no book can illustrate all kinds of insects and still sell for a price a
student can afford to pay.
4. By comparing it with the descriptions. Original written
descriptions of species based on type specimens are the pennanent
records of the attributes of a given species. Such records are
particularly useful when the type specimens are available, and they
constitute the only original record if type specimens are destroyed, lost,
or unavailable.
5. By the use of an analytical key. One of the most common
methods of identification is the use of a key. A key is a printed
infonnation- retrieval system into which one puts infonnation regarding
a specimen-in-hand and from which one gets an identification of that
specimen to whatever level the key is designed to reach. Most keys are
dichotomous (double branching, Table 1 , 2). At the beginning of a key,
(Z-57)
34 J
Economically Important Insect Orders
Table 1. Key for identification of classes of Phylum Arthropoda.
1.
-
2.
-
2
Trunk divided horizontal.!l_ into 2 or more segments
Trunk divided by two longitudinal furrows into three lobes
Head fused with thorax (cephalothorax); head bears a pair
of chelicerae (mouth-parts) and a pair of pedipalps,
antennae absent and thoracic segments four and each bear
a_!!_air of walkin_g_ lceg§· _iSutmfil1um - Chelicerat&
Head distinct or fused with thorax (cephalothorax); head
bears mandibles and maxillae (mouth-parts) and one or
two pairs of antennae; thoracic segments and appendages
Subphylum - Trilobita
3
5
variable J_Subp_h_ylum - Mandibulat&
3.
-
4.
-
5.
-
6.
-
7.
-
8.
-
9.
-
Cephalothorax
Class - Pycnogonida
3-sc;g_mented
Cephalothorax more
than
3-segments
4
Cephalothorax with lateral compound eyes; abdomen with
5-6 pairs of gill-bearing appendages; respiration by
Class - Merostoma
bo...Q.k::&i!Js
Cephalothorax with lateral simple eyes; abdomen without
Class - Arachnida
bears
Class - Crustacea
aooendruzes· re�iration bY_ book-lull&.
Head fused with thorax forming cephalothorax and
antennae two _!!_airs· re�iration �ls
Head distinct and bears one pair of antennae; respiration
usual_!y_ bY_ trachea
Body divisible into three distinct tegmata into head
(6-segmented), thorax (3-segmented) and abdomen; each
thoracic segment bears a pair of legs; abdominal legs
absent
Body divisible into head and trunk (thoracic and
abdominal segments not distinct); thoracic as well as
6
Class - lnsecta
7
_g_ments bear lceg§
thoracic se
Head with eyes; each abdominal segments bears a pair of
8
l�
Head without eyes; few posterior abdominal segments do
9
not bear lceg§
Body cylindrical; each abdominal segment bears two pairs
Class - D iplopoda
of l�
Body flattend and each abdominal segments bears one
Class - Chilopoda
l.i!..air
of I�
Antennae
unbranched
Antennae
branched
Class - Symphyla
Class - Pauropoda
one is presented with two alternatives (a couplet), each of which leads
to another pair of alternatives. At each point the user of a key picks
the alternative that best describes some aspect of the specimen being
identified. Finally one reaches a terminal pair, one member of which
presents the identity of the organism. The simplest keys are designed
for the layman and may offer only superficial identification or deal only
(Z-57)
Economically Imponant Insect Orders
[ 35
Table 2. Key for identification of important fam ilies of the order Orthoptera
(Class : Insecta)
I
Antennae about as long or longer than body, many
segmented; tympanal organ when present, on fore-tibiae
2
(Suborder - Ensifera)
-
Antennae shorter, with less than 30 segments; tympanal
organ when present, at the base of abdomen (Suborder:
6
Caelifera)
2.
-
3.
-
4.
-
Tarsi 4-segmented, atieast
(Sl!Q_erfami.!Y_- Tettig_onioidea)
on
mid-
and
hind-legs
5
Tarsi-3 segmented (Superfamily : Grylloidea)
Second and third tarsal segments with large, mobile lateral
lobes, wi� when _lJfesent, coiled �ral.!Y_ at rest
Fore-wing without stridulatory organ; fore-tibiae without
!Y_ll!l?_anal o�ans
of
males
Schizodactylidae
4
Tarsi and wmgs otherwise
Fore-wings
3
usually
with
Gryllacrididae
stridulatory
organ;
Tettigoniidae
digitate
tibiae,
Gryllotalpidae
fore-tibiae with !Y_m_Q_anal o�ans
5.
-
6.
-
Fore-legs fossorial
ov_jp_ositor vest[gj_al
with
expanded
and
Fore-legs not fossorial; tibiae simple; ovipositor elongate
Gryllidae
Tarsi almost always 3-segment�d; antennae usually longer
(Sl!Q_erfami.!Y_- Acridoidea)
7
Tarsi 1 - or 2-segmented; antennae short with 12 or fewer
Tridactylidae
s�ments (S�rfami!Y_- Tridac!}'loidea)
7.
-
cover abdomen;
Pronotum
to
extended
backward
em_Q_odium absent, antennae lon�r than fore-femur
Pronotum normal, or if extended behind, then empodmm
Tetrigidae
8
�esent or antennae shorter than fore-femora
8.
Hind-legs not markedly different from fore- lll!d mid-legs,
femora not _great!Y_ enlar�d
-
Hind-legs markedly saltatorial with enlarged femora
Pneumondae
A crididae
with insects found in a specific habitat, such as a garden. More complex
keys are designed for the non-systematist professional biologist who
needs to identify an organism to a more specific level than the
layperson. These keys are more technically oriented and require some
training in morphology before they can be used effectively. Finally, there
are keys designed for the specialist, which are very technical and
require detailed knowledge of the morphology of a particular group of
organisms. These keys usually carry identification to the species level.
6. By a combination of two or more of these procedures.
Combinations of the above methods are usually used in identification.
Although the layperson or non-systematist professional biologist can
identify a specimen to family, or in some cases below the family level,
Economically Important Insect Orders
36 1
the best and safest way to obtain a species identification is to consult a
specialist. From experience and familiarity with pertinent literature, the
specialist is in the best position to use the highly specialised keys,
original descriptions, and type specimens. In view of the vast number of
insect species already described, a given individual is likely to specialise
in the systematics of only a very few families or even a single family.
[ II] Problems encountered in identification
Unfortunately, identification is not without problems. Since m key
preparation, less number of characters are used, therefore, it is possible
that such keys may not include the organism being identified, fail to
include the stage or sex of the unknown organism, or fail to take into
account any of a large number of possible variations in morphological or
other characters used. Pictures may mislead, especially if an unknown
organism resembles very closely with the one illustrated. Written
descriptions can be ambiguous, particularly when such traits as colour and
texture are described, and such descriptions commonly require knowledge
of specialised terminology to be of use. A collection of accurately identified
specimens is not always available. Type specimens may be very difficult or
impossible to obtain and may have been lost or destroyed. The major
difficulty with consulting a specialist on a given group of organisms is that
such a person may not be alive and even if alive, there is still the problem
of contacting this person. An additional problem is the existence of sibling
species. A sibling species group is an assemblage of morphologically
indistinguishable forms that actually represent two or more biologically
distinct species. Unfortunately, morphology is sometimes the only avenue
for identification, and sibling species remain undetected.
[ III] Description
The systematist, having completed an identification, will either put aside
the specimens for possible future study with other specimens of the same
species or will describe as a new species and placed in an appropriate
hierarchy. By using this description, the species can be identified by other
investigators. Ideally, specimens to be described are obtained from field
collection. Most of the insects, in past, were described on the basis of a
single individuals, however, since a species is a population, not an
individual, its description must be based on several individuals or types.
These types serve several purposes : (i) as base for description, (ii) as
standard for comparison, and (iii) as a source of data not shown by the
type. Following types are of utmost important :
1. Primary types. The single nomenclatural type : Holotype if a
named species was based on a single specimen, that specimen is the
holotype; Lectotype
one of several syntypes (see below) designated
-
-
Economically Important Insect Orders
[ 37
after the publication of a species group name as the type specimen of
the taxon bearing that name. If a nominal species has no holotype any
zoologist may designate one of the syntypes as lectotype ; Neotype a
specimen selected to replace the holotype when primary type material of
a species is lost or ruined.
2. Secondary types. The specimens from which the primary types
must be selected. Syntypes - every specimen in a type series of equal
rank upon which a species is based and no holotype has been
designated. Paralectotype - any one of the original syntype remaining
after the selection of a lectotype.
3. Tertiary types. Other specimens originally set aside as of special
taxonomic interest to supplement the primary types. Paratype
every
specimen in a type series other than the holotype; Allotype a paratype
of opposite sex of the holotype.
4. Other types. Topotype - a specimen obtained from the type
locality; Metatype - a specimen compared with the type (primary) and
believed to agree with its features; Homotype (homeotype) - a species
compared by its describer; Plesiotype (Hypotype) - a described, figured
or listed specimen ; Plastotype a cast of type.
-
-
-
-
Classification and Identification of
Economically Important Insect Orders
The original divisions of the class Insecta are a matter of dispute and it is
not easy to decide which divisions to use. Many different ordinal groupings
are recognised by different experts, and which one is ' 'best'' will no doubt
remain a problem. Because there are different opinions regarding the
category levels of various insect groups, especially between class and order,
categorical names, such as subclass and division, like most of the recent
treatises on Entomology, the class Insecta is directly divided into Orders
However, certain groupings, for example, Apterygota and Pterygota,
Exopterygota and Endopterygota are retained but these should simply be
viewed as groups within the Insecta that have certain characters in
common.
[ I] Characters of Apterygote insects
( 1 ) All primitively or primarily wingless (i.e., none of their ancestors
possessed wings).
(2) Frequently possess small, paired styli (fingerlike projections) on some
of the pregenital abdominal segments.
(3) No metamorphosis (development ametabolous), immatures and adults
alike except for size differences and the presence of genitalia.
Economically Important Insect Orders
38 1
Apterygote insects include four orders, viz.
Protura, Collembola,
Diplura, and Thysanura.
1. Order-Protura (Prot
first, ura
tail; soil insects, Fig. 1 ) :
Minute, elongate, whitish, entognathous and sucking . mouthparts; eyes,
ocelli and antennae absent; tarsi I -segmented with a simple claw;
abdomen short, bilateral styli on first three segments, 12 segments in
adult and 8- 1 1 segments in immatures, ccrci absent; progressive addition
of abdominal segments during development, termed anamorphosis
(1 segment per moult); tracheae usually absent; malpighian tubules
papillae-like. These are inhabitant of soil and leaf litter and require
moist conditions. They feed on decaying organic matter. Example :
-
-
Acerentulus.
2. Order-Diplura (Dipl- two, ura- tail; two-pronged bristle tails,
Fig. 2). Minute to medium, slender and elongated ; entognathous and
chewing mouthparts ; eyes and ocelli absent; antennae long and filiform
; tarsi 1 -segmented with two-claws; abdomen IO-segmented, cerci usually
either forcep-like or long caudal filaments, styli present on segment 1 -7.
They are found in damp situations in caves, under tree bark, i11 the soil.
They do not harm us. Example : Japyx.
3. Order-Collembola (Coll-glue, embol-a wedge; springtails, soil
insects, Fig. 3). Minute, somewhat tubular or globose; entognathous and
chewing mouthparts; eyes and dorsal ocelli absent, lateral ocelli present;
antennae short to long, usually 4-segmented; tarsi fused to tibia or
I -segmented; abdomen 6-segmented; lobelike organ on venter of first
segment, the ventral tube or collophore, which functions in water uptake
and as an adhesive organ; forked structure furcula present on the
venter of fourth segment, help in jumping species (springtails); abdomen
never more than 6 segments, even in immatures. This is the largest
order of apterygotes and widely distributed and are commonly found
several millions in a single hectare. They are common in soil, leaf litter,
and other decaying organic matter. Generally they are not pestiferous
but few may harm mushrooms, greenhouse, cereal crops, sugarcane etc.
Example : Sminthurus virdis (pest of alfalfa, pea).
4. Order - Thysanura (Thysan fringed,
ura-tail;
bristle tails,
silverfishes, Fig. 4). Small, 0.05 - 5.0 cm in length, delicate, elongated;
mouth-parts 'ectognathous. visible externally, adapted for biting; head
broadly sessile and very little movable; antennae long, filiform, many
segmented, pleriarticulated (only basal segment is provided with intrinsic
muscles); large compound eyes, sometimes vestigial/absent; tarsi 2-5,
commonly 2 claws ; abdomen 1 1 segmented tapering behind with
variable number of lateral styliform pre-genital appendages, the last
segment tends in a long segmented median process, with a pair of long
cerci on its side; tracheal system highly developed. Thysanura includes
Economically Important Insect Orders
[ 39
8
Fig. 1 to
Lepisma;
13.
1 . Proturan, Acerentulus;
2. Dipluran, Japyx; 3. Springtail;
4. Silverfish,
5. Damselfly; 6. Dragonfly; 7. Mayfly; 8. Stick insect; 9. Leaf insect; 10. Bush
katydid; 1 1 . Cricket, Gryllus;
Hieroglyphus
12. Mole cricket,
Gryllotalpa; 13.
Short horn grasshopper,
40 1
Economically Important Insect Orders
two large families : Lepismatidae (silverfishes : Lepisma saccharina,
firebrats : Thermobia domestica) and Machilidae (spring tails).
Body of silverfishes and firebrats depressed, eyes small, separate,
may be absent, ocelli wanting, mandibles with two articulation
(dicondylic), maxillary palpi 5-segmented, paraglossae simple, meso- and
metathoracic coxae without styles, gonangulum present, malpighian
tubules 4-8. Silverfishes are found in cool, damp locations whereas
firebrats prefer warmer locations (around steam pipes, furnaces). They
feed upon book bindings, starched clothing, decaying matters.
Body of spring tails compressed, eyes large, contiguous, ocelli
present, mandibles with single articulation (monocondylic), maxillary
palpi 7-segmented, paraglossae 3-lobed, meso- and metacoxae with
styles, gonangulum absent, malpighian tubules 12-20. Example : Machilis
polypoda.
They are found inside houses, libraries etc. as well as under tree
bark, dry leaves, stones, refuse etc. In houses it is common among
neglected books, papers, cardfiles, behind pictures hanging on the wall
and usually destroy them and sometimes need control measure.
[ II] Characters of Pterygote insects
( 1 ) Primarily winged insects (secondarily wing may lost).
(2) Mouthparts always ectognathous, and mandibles articulated at two
points (dicondylic).
(3) Metamorphosis
incomplete
(development
hemimetabolous),
or
complete (development holometabolous).
According to the development, the pterygotes are grouped into
Exopterygotes and Endopterygotes. In exopterygotes, the wing develops
externally and the immatures outwardly resemble the adults even in the
habitat with very few exceptions (dragonflies, mayflies) while in
endopterygotes, the wing develops internally and the immatures differ
morphologically with contrast.mg habitat and food habit from adults. A
pupal stage is met between, larval and adult instars. The exopterygote
insects includes 1 6 orders, important ones are : Odonata (dragonflies),
Ephemeroptera (mayflies), Phasmida (leaf/stick insects), Orthoptera
(grasshoppers, locusts, crickets), Dictyoptera (cockroaches, mantids),
Isoptera (termites), Hemiptera (bugs), Mallophaga (bird lice), Anoplura
(lice},
and Thysanoptera
(thrips).
Out of these orders,
only
grasshoppers, locusts, termites, bugs and thrips are of economnic
impoitance · as they are crop pests. Bird lice are ectoparasitic upon
poultry birds whereas lice suck the blood from human beings or cattle
and may spread diseases. The endopterygote insects include 9 orders,
however, important ones are
Coleoptera, Diptera, Siphonaptera,
Hymenoptera and Lepidoptera.
Economically Important Insect Orders
{ 41
1. Order-Odonata (Odon - a tooth; dragonflies and damselflies).
Medium to very large, elongated; chewing mouthparts, larva (naiad) with
prehensile labium; compound eyes large and 3 dorsal ocelli; antennae
short and bristle-like; wings 2 pair, membranous, wing vanation netlike
with pigmented cell (stigma) near the apex of each wing, wings can
not be flexed over abdomen at rest; legs adapted for prey capture, tarsi
3-segmented; abdomen elongated, I -segmented cerci in males serve as
claspers during copulation. Adults are good flier and predatory· upon
flying insects (e.g., mosquitoes and midges, moths, bees). They are able
to catch, hold, and devour prey in flight. Larvae are aquatic with closed
tracheal system and feed upon aquatic insects (e.g., mosquito's larvae).
Although these insects have been recorded as occasional pests in
apiaries, they are generally considered to be very beneficial. Odonata
includes
two
suborders,
Zygoptera
(damselflies)
and
Anisoptera
(dragonflies).
(a) Suborder-Zygoptera (Damselflies, Fig. 5). Weak fliers, forewings
and hindwings approximately the same size and shape and wing-bases
narrowing gradually. At rest, adults typically hold the wings together
over the body with the body itself held in a horizontal plane.
Compound eyes widely separated. Males have 4 terminal abdominal
appendages. They usually occur along clear fast moving streams.
Example : Lestes.
(b) Suborder-Anisoptera (Dragonflies, Fig. 6). Strong fliers,
forewings and hindwings unequal in size and hindwings basally broader
than forewings. At rest, wings laterally spread. Compound eyes in close
proximity. Males have 3 terminal abdominal appendages. They usually
occur along clear fast moving streams. Example : Anax.
2. Order-Ephemeroptera (Ephemero-for a day, ptera - wing;
mayflies, Fig. 7). Small to medium, fragile, soft-bodied insects;
mouthparts in nymph chewing type while in adults vestigial; compound
eyes and 3 dorsal ocelli present; antennae short, setaceous; wings
membranous with many cross-veins, held vertically at rest, forewings
larger than hindwings; few species with only forewings; tarsi 3-5
segmented; bilateral respiratory gills on the first 4 to 7 abdominal
segments in nymphs; a pair of long, filamentous cerci in adults; some
with additional long median caudal filament. Tracheal system is closed
in nymphs and open in adults. Nymphal stages are aquatic and adults
are terrestrial-aerial like odonates.
Mayflies are uniques among the insects in passing through a winged,
subadult stage that is essentially identical to the adult stage except that
it lacks functional genitalia. Larval period is fairly long, a year or more
but the subadults and adults do not take food and survived for a few
42 1
Economically Important Insect Orders
hours to a few days during which they copulate and oviposit in water.
Example: Ephemera.
3. Order-Phasmida (Phasm -phantom; stick and leaf insects). Large,
apterous or winged, frequently leaf like or elongated; head small,
prognathus, mouthparts biting and chewing; antennae small or large,
filiform; prothorax short, meso- and metathorax usually long; forewing
when present usually small with stridutatory costa; wingpad do not
undergo reversal during development; legs similar to each other, often
have leaf-like expansions, tarsi-5 segmented; ovipositor usually small
concealed with enlarged eight sternum; male 1 extemal genitalia variable asymmetrical, concealed by 9th abdominal segment; cerci short,
unsegmented; specialised stridulatory and auditory organs absent; eggs
deposited singly and need 1-2 years to hatch; metamorphosis slight;
parthenogenesis (both facultative as well as obligatory) is frequent;
protective resemblance to foliage and leaf twigs of vegetation. They are
phytophagous but no species is a serious pest. Mostly distributed in the
tropical rain forests and mimic by the cryptic body colouration,
markings and mimesis. It includes two families : Phasmatidae and
Phyllidae.
(a) Family-Phasmatidae (Stick insects, Fig. 8). Elongated, often
extremely so, never leaf like, tibiae without a triangular apical area.
Example : Carausius m oros':'s (Indian · stick insect), Megaphasma dentricus
(giant walking insect, 1 8 cm).
(b) Family-Phyllidae (Leaf insects, Fig. 9). Robust species, flattened
occasionally and leaf-like, tibiae with a small triangular area delimited
ventro-apically. Example : Phyllium spp.
4. Order - Orthoptera (Onho-straight, ptera- wing; grasshoppers,
locusts, crickets, katydids). Small to large, winged or brachypterous or
even apterous; head broadly articulated to the thorax with slight
freedom of movement having short or elongated filiform antennae;
mouthparts biting and chewing; compound eyes well developed, ocelli
1-3; forewing long and slender, thickened with many veins (tegmina) anJ
bindwings wider than forewings, wingpads of nymph undergoes reversal
during development; prothorax large, movable; hindlegs usually enlarged
and modified for jumping; tarsi 3 or 4 segmented, rarely 5 or 2; females
with ovipositor, not hidden by 7th and 8th segmental sclerites; male
external genitalia symmetrical concealed at rest by large 9th sternite
which may or may not bear a pair of styles; cerci usually short and
unsegmented; specialised auditory and stridulatory organs developed;
metamorphosis slight (paurometabolous).
Over 1 7 ,000 species are described, flourish best in tropical,
subtropical, oriental areas, mostly terrestrial, some strong flier, many
notorious crop pests. The vast majority of the species are capable of
Economically Important Insect Orders
I 43
jumping and are mostly ground living fonns. Many are arboreal and
some are subterranean and caverniculatous. Certain species migrate in
great swanns at intervals. Except some predatory species, almost all
members of the order are phytophagous. All species are oviparous,
parthenogenesis very rare. For oviposition, the females drill a hole with
the tip of the abdomen in the soil and lay 50-200 elongated eggs in a
cement like secretion (eggpod).
Orthoptera is divided into 2 suborders : Ensifera (Saltatoria or
Acridodea) and Caelifera (Gregaria, Tettigoniodea).
( a) Suborder --Ensifera (Long horn grasshoppers, katydids, bush
crickets, crickets, mole crickets). Antennae about as long as body length;
tympanal organ when present on foretibiae; stridulatory organ when
present usually tegminal; tarsi usually 4-segmented; ovipositor when
present more or less elongated, needle-like or sword shaped. Following
are the representative families :
(i) Family-Tettigoniidae (Long horn grasshoppers, bush crickets,
katydids, Fig. 10). Popularly called Locustidae; winged or wingless
fonns; when wings present, left tegmina overlap right one; in male
cubito-anal
region
of
tegmina
modified
for
stridulation;
tarsi
4-segmented; ovipositor very
long
even exceeding body length,
sword-shaped or sickle-shaped; carnivorous species are more followed by
omnivorous and phytophagous. Over 5000 species, mostly tropical.
Example : Holochlora indica (India), Phasgoneura viridissima (green
katydid of Palaearctic).
(ii) Family-Gryllidae (Crickets, Fig. 1 1 ). Tarsi- 3 segmented; male
forewings with stridulatory organ; foretibiae with tympanal organ;
ovipositor
needle-like,
forelegs
nonnal.
Mostly
omnivorous,
few
phytophagous. About 2,300 species are described, live in burrows, under
logs, among dry leaves, in houses. Example : Gryllus campestris (field
cricket), Acheta domesticus (house cricket).
(iii) Family-Gryllotalpidae (Mole crickets, Fig. 1 2). Tarsi-3
segmented; forelegs modified and greatly expanded and armed, with
strong teeth to assist digging (fussorial); eyes reduced� stridulatory and
auditory organs generally absent; ovipositor vestigial; hindwings are
extended backward like a process. Example, Gryllotalpa africana,
G. gryllotalpa. Adults are carnivorous but during making hole in soil,
destroys roots of crop.
(b) Suborder - Caelifera (Short horn grasshopper, locust, Fig. 1 3) .
Antennae shorter than body; auditory organ when present o n the base
of abdomen; stridulatory organ when present fernoro-alary; ovipositor
when present short and robust. One of the family Acrididae is of
economic importance which can be identified as : tarsi usually
3-segmeated; pretarsus usually with arolium; stridulation well developed;
44 J
Economically Important Insect Orders
ovipositor reduced, curved; eggs are laid in holes in ground or decaying
wood in eggpod; body colouration cryptic; mostly oriental and tropical;
destructive pests, phytophagous. Some of them form two distinct phases
: gregaria and solitaria. E xample, Locusta m igratoria (migratory locust),
Schistocerca gregaria (desert locust), Patanga succincta (Bombay locust),
Heiroglyphus banian (rice grasshopper).
A locust is a migratory grasshopper that swarms at regular intervals.
The periodicity of locust swarms is largely dependent on the
environmental factors. The migratory locust, L. migratoria is a
polymorphic species existing in following unstable phases : (i) phasis
solitaria (saltatory phase), (ii) phasis gregaria (gregarious phase) and
(iii) phasis transiens (transitory phase) differing in structure and habit.
Swarming takes place in gregarious phase. The eggs of the individuals of
this phase undergo diapause. Migration precedes sexual maturity with
colour changes. The solitary phase is characterised by isolated
individuals that show no tendency for migration. The eggs of these
individuals develop without diapause and sexual maturity is not
accompanied by colour changes. The actual stimuli which promote
migration are internal and are connected with the maturing of the
gonads. These peculiarities are observed also in S. gregaria.
5. Order-Dictyoptera (Dictio- net;
ptem - wing; cockroaches,
mantids). Head hypognathus, highly movable; mouthparts biting and
chewing; antennae longer, filiform, many segmented; compound eyes well
developed; pronotum large; legs similar to one another; forelegs
raptorial in mantids; coxae large, tarsi
5 segmented; forewing modified
more or less in thickened tegmina; wingpads of nymph never undergo
reversal during development; female
with
reduced
ovipositor,
concealed by enlarged 7th abdominal sternite; male genitalia complex,
asymmetrical and concealed by 9th abdominal sternite which bears a
pair of styles; anal cerci many segmented; specialised stridulatory and
auditory organs absent; eggs are laid in oothecae; essentially terrestrial,
omnivorous/carnivorous/scavanger.
Medium to large insects, essentially a group of thermophilous insects
and is absent in extreme arctic regions but moderately abundant in the
temperate and rich in the tropical countries. Many species have become
cosmopolitan. Some cockroaches are pestiferous while mantids being
entomophagous helps in natural control of the pest insects. Two
suborders have been distinguished : Blattaria and Mantodea.
(a) Suborder -Blattaria (Cockroaches, Fig. 14). Nearly 3000 species,
distributed over 350 genera grouped in 24 families, have been described.
The important family is Blattidae, some species of which are household
pest. The family may be identified by the characters as : only pronotum
prolonged as a broad hood concealing head; ocelli-2 or absent; antennae
-
Economically Important Insect Orders
[ 45
extremely long, more than 100 segments; legs cursorial, similar with
large broad coxae, tibae spined; proventriculous with powerful gizzard;
scavanger or omnivorous. Example, Blatta orientalis (Asiatic cockroach),
B. germanica (German cockroach), Periplanata austrolasiae (Australian
cockroach),
P. am ericana (American cockroach), P. hum bertiana,
Polyphaga indica.
Cockroaches belong to one of the oldest living insects and are also
one of the most widely distributed groups. They are mostly found in
warm and damp places.
Most cockroaches
are brown,
grey
mahogany-red, or black but many of oriental species are brilliantly
coloured (green, yellow, red, orange). Cockroaches are nearly all
gregrarious; mostly nocturnal, but almost all of them frequently enjoy
diurnal habit. Life-history occupies a few months to 5 years. There are
5-7 moults but 1 1 nymphal instars greatly resembling the adult are
found in P. americana.
(b) Suborder - Mantodea (Praying mantids, Fig. 15). The Mantodea
comprises 450 genera of nearly 2000 species grouped in 13 families,
chiefly ethiopean but also frequently found in other regions except arctic
region. Most common family is Mantidae which may be identified by the
following charatcers: entire prothorax elongate; small triangular head
quite evident, movable with neck; ocelli 3 or absent; forelegs raptorial
adapted for predation having elongated coxae and spined femur and
tibiae, held raised forward in a characteristic praying attitude; walking
insects; carnivorous. The eggs laid in oothecae are glued to some plant.
Young nymphs are prone to cannibalism . . They moult 3- 12 times to
attain
adulthood
in
about
a
year.
Some
species
reproduce
parthenogenetically. Example, Mantis religiosa (praying mantid).
The mantids are largely arboreal but some apterous forms are
found on ground. They are entirely carnivorous both as young and adult
mostly feeding upon insects like flies, leatboppers, grasshoppers,
caterpillars, butterflies etc., thus, beneficial for us in naturally controlling
the population of pest insects.
6. Order-Isoptera (lso- equal, ptera-wing; termites, Fig. 1 6). Soft
bodied, social and polymorphic species living in large communities
composed of reproductive forms together with numerous apterous,
sterile soldiers and workers; head movably articulated, prognathous with
compound eyes in winged forms only (reproductive castes, frontal gland
exits through fontanelle) ; antennae short and moniliform; mouthparts
biting and chewing, mandibles large in soldiers, ligula 4-lobed; prothorax
freely movable, narrower than the head, meso- and metathorax wider
than fong; fore- and hindwings are similar in size, form and venation,
capable of being shed by means of basal fracture (humoral suture),
anterior veins more sclerotised, no cross veins; legs short and stout,
Economically Important Insect Orders
46 J
16
18
17
19
1 4 to 1 9. 1 4. Cockroach; 15. Praying mantid; 16. Termite soldier; 17. B ird lice;
1 8. Human head louse; 1 9. Thrips
Fig.
Economically Important Insect Orders
coxae
greatly
enlarged,
tarsi-4
['4 7
segmented;
cerci
genitalia rudimentary or absent in both
sexes;
absent.
or
Termites
communities
are
in
subterranean
mostly
underground
forms
earthworms
in
addition
wood,
to
tropical
nests
probably
that
they
aerate
termites
an
and
feed
or
a
very
dry
of
short;
slight or
in
large
wood.
role
nutriment
variety
live
in
ecological
add
on
or
subtropical,
or termitaria
play
short
metamorphosis
The
similar
to
the
to
soil.
In
cellulose-containing
materials, fungi and dried animal remains. The digestion of cellulose is
carried out by flagellate protozoans e .g.,
Trichonympha
or bacteria, which
are mutualistic inhabitants of the gut. Though all the families of termites
are
economically
important,
the
largest
family
following characters : wide range of food
worker
caste
well
developed;
hindwings
is
Termitidae
with
habit and colony structure ;
without
anal
lobes,
venation
reduced, fontenelle and ocelli present; pronotum of workers and soldiers
narrow with raised anterior lobe;
hairy,
anterior
wing
scale
Odontotermes banglorensis
(sugarcane
wings slightly reticulate, more or less
short.
Microtermes beesoni,
Termes, Trinervitermes heimi
Example
(sugarcane
pest),
pest).
7. Order - Mallophaga (Mallo-wool, phaga-eat; chewing lice, bird
lice, Fig.
broader
17).
Minute to small, dorsoventrally
than
thorax;
mouthparts
flattened,
chewing;
triangular head
compound
eyes
reduced,
ocelli absent; antennae 3-5 segmented, usually capitate or filiform; wings
absent;
tarsi
modified
for
grasping
hairs
in
some
species
infesting
mammals; cerci absent. Most Mallophaga are ectoparasites of birds but
considerable
number
of
species
have
mammalian
host.
They
feed
feathers, skin, epidermal secretions. Some species feed upon blood. Few
species
are
poultry
pest,
Menopon pallidum
e.g.,
(common
chicken
louse).
8. Order-Anoplura (Anopl -unarmed,
flattened,
secondarily
orthognathous
or
apterous,
prognathous,
antennae; mouthparts
conical
highly
ura-tail;
ectoparas1tlc
and
modified
sucking
on
3
bears
and
lice). B ody
mammals;
5
to
adapted
head
segmented
for
piercing
and sucking, with a labial rostrum, retracted within head when not in
use; eyes reduced or absent, ocelli absent; thoracic segments indistinct;
legs ·clinging type, tarsi ,unsegmented having a single powerful claw
(specialised for grasping hairs); cerci absent; metamorphosis slight. More
800 species of Anoplura are described, two species on man and
than
more
than
extreme.
however,
families
a
dozen
species
The
order
is
most
of the
Pediculidae
genera
and
arranged
domestic
in
into
animals.
about
of economic
a
Host
half
specificity
dozen
importance
belong
is
families,
to
the
Heamatopinidae.
(i) Family-Pediculidae
thinner setae,
on
classified
(Fig.
rows
1 8). Body less densely clothed with
and rarely modified
into
scales;
eyes
(Z-57)
48 1
Economically Important Insect Orders
large, convex and almost always distinctly pigmented; proboscis short;
tibia and tarsus without a distinct sclerite between them; abdominal
paratergal plates present on atleast one segment without freely
projecting apical margin; tergal and sternal plates absent; abdominal
cuticle unwrinkled, membranous, except for genital region. Parasite on
primates. Example, Pediculus, Phthirus.
Pediculus humanus is a notorious species infecting human head
(head louse) and body (body louse). Females are dark brown to black,
about 2.5 mm to 4.2 mm long, males are somewhat smaller than
females. Two subspecies are distinct : (i) head louse, P. humanus capitis
and (ii) body louse, P. humanus corporis. The head louse is slightly
smaller and darker than the body louse and has also somewhat stouter
antennae. A single female deposits about 200-300 eggs at the rate of
8- 12 eggs each day. The eggs are attached by their posterior ends by a
hard cementing secretion to the hairs. The young nymph hatches on the
6-8 day after deposition of the eggs. Three nymphal instars are observed
each having a distinct chaetotaxy. Sexual maturity is attained after 20-30
days of hatching. The two subspecies freely interbreed under
experimental conditions and remain fertile through several generations. It
transmit several diseases of which epidemic typhus, caused by Rickettsia
prowazeki (a micro-organism) (transmitted not by feeding but when
crushed on injured tissue of skin) is most important.
(ii) Family-Haematopinidae. Body less densely clothed with thinner
setae, arranged in rows and rarely modified into scales; eyes very
indistinct or absent; proboscis very long; legs with a sclerotised sclerite
in between tibia and tarsus; abdominal paratergal plates present on at
least one segment without freely projecting apical margin; tergal and
sternal plates absent; abdominal cuticle finely wrinkled, indistinct tergal
and sternal plates some time present. Example, Haematopinus suis (hog
louse), H. tuberr:ulatus (buffalo louse; it transmit Trypanosoma evansi,
the causative agent of the surra disease).
9. Order-Thysanoptera ( Thysano- a fringe, ptera-wing; thrips,
(Fig. 19). Small to minute insects (0.5 to 0.8 mm), generally found
feeding on flowers and foliage; body slender, antennae short, 6 to 10
segmented; mouthparts adapted for piercing and sucking the plant sap;
their component are asymmetrical; maxillary and labial palpi present;
prothrorax well developed and free, meso- and metathorax fused; tarsi 1
or 2 segmented, each with a terminal protrusible vesicle; wings when
present, very narrow with greatly reduced venation and fringed with
large marginal setae; cerci absent, metomorphosis is accompanied by 2-3
in active pupal like instars. It includes two suborders : Tubulifera
(ovipositor absent, 10th abdominal segment usually tubular in both sexes.
Forewing venation absent) and Terebrantia (a saw like ovipositor
(Z-57)
Economically Important Insect Orders
[ 49
present, apex of abdomen conical in female, blunt rounded in male,
forewing with at least one vein reaching to apex). Terebrantia includes
several economically important species. Largest family is Thripidae.
Example: Sc.irothrips citri (on citrus), S. dorsalis (on chilli), Thrips tabaci
(on onion, cotton, garlic), Anaphothrips sudanensis (on wheat), Thrips
oryzae (on rice).
10. Order-Hemiptera (Hem i - one-half, ptem wing; true bugs,
aphids, whiteflies, scale insects). Two pairs of wings usually present,
anterior pair most often of harder consistency than the posterior pair,
either uniformely so (suborder Homoptera) or with the apical portion
more membranous
than the remaining (suborder Heteroptera) ; head
hypognathous (Homoptera) or prognathous (Heteroptera) with a well
developed labial rostrum; mouthparts
piercing
and
sucking,
symmetrical, palps are atrophied, labium forming a dorsally grooved
sheath in which lie two pairs of bristle-like stylets which are modified
mandibles
and maxillae;
pronotum
small
(Homoptera) or large
(Heteroptera); tarsi usually I to 3 segmented ; metamorphosis usually
gradual, rarely complete in male Homoptera. Most of the bugs are pests
of agricultural crops, plant lice transmit viral diseases while few insects
are beneficial (lac insect, cochineal bug).
The order Hemiptera includes 2 Suborders: Homoptera and
Heteroptera which can be identified by the differences in the characters
given above.
(a) Suborder-Homoptera. It is divided into 3 series which can be
identified as :
-
1.
Antennae short
Antennae long and filiform, rostrum
apparently arising from the sternum
between
inactive
the
forecoxae,
usually
2. Antennae 3 segmented, without a
terminal arista ; rostrum
4 segmented; tarsi 2 segmented Antennae variable with a terminal
arista; rostrum 3
segmented; tarsi
3- segmented
-
......................... 2
... Series Stemorrhyncha
.... Series Coleorrhyncha
-
.... Series Auchenorrhyncha
-
Series Auchenorrhyncha includes several economically important
families of which following are of common interest.
(i) Fam ily -- Jassidae, Cicadellidae (Jassids and leaf hoppers,
Fig. 20). Pronotum not produced backwards in the form of a hood over
the abdomen; very active and hop off at the least disturbance.
Numerous
species
are
of considerable
economic
importance
as
(Z-5 7)
Economically Important Insect Orders
50 J
26
Fig. 20 to 30. 20 . Leafhopper (Jassidae); 2 1 . Treehopper (Membracidae); 22. Perkinsiella
saccharicida (Delphacidae); 23. Pyrilla perpusilla (Lophopidae); 24. Psylla mali (Psyllidae) ;
25. Whitefly ( Aleyrodidae); 26. Aphid (Aphididae); 27. Pericerya purchasi (Margarodidae);
28. Female Kenia lacca ( Tachardiidae) , a, male; 29. Coccus hesperidum (Coccidae) ;
30. Quadraspidiotus pemicwsus (scale on a twig), a. Adult male (D1aspididae).
(Z-57)
Economically Important Insect Orders
[ 51
agricultural pest. Some transmit viral diseases. Examples, Idiocerus
atkinsoni (mango leaf-hopper), Nephotettix apicalis (green rice (=plant)
hopper).
(ii) Family-Membracidae (Tree hoppers, cowbugs, Fig. 2 1 ).
Pronotum enlarged bearing spine-like or variable process produced
backwardly over abdomen. Adults as well as nymphs excrete sugary fluid
(honeydew) from their anus that attracts ants. Examples, Gargara mixta,
Tricentrus assamensis, Leptocentrus taurus.
(iii) Family-Delphacidae (Fig. 22). Hind tibae with large spur;
ovipositor complex, male with large aedeagus; notorious pests. Example,
Nilapervata lugens (brown plant hopper, paddy pest); Perkinsiella
saccharicida (sugarcane leaf hopper, sugarcane pest in Queensland).
(iv) Family-Lophopidae (Fig. 23). Wings with reticulate system of
supernumerary veins and cross veins; proboscis like projection of head.
Example, Pyrilla perpusilla (sugarcane leaf-hopper).
Series
Sternorrhyncha
includes
several
economically
important
families of which some are beneficial.
(i) Family-Psyllidae (Jumping plant lice, Fig. 24). Small, mouthparts
functional in both sexes; antennae 1 0-segmented, femur thickened for
helping in taking leap. Example, Psylla mali (apple pest).
(ii) Family-Aleyrodidae (Whiteflies, Fig. 25). Mouthparts functional
in both sexes, antennae with 7-segments; legs long and slender; secrete
honeydew. Examples, Bemisia tabaci (cotton whitefly, on tobacco,
tomato, potato, cotton
- Northern India).
etc.), A leurolobus barodensis (sugarcane whitefly
(iii) Family-Aphididae (Aphids, plant lice, Fig. 26). Mouthparts in
both sexes; tarsi 2-segmented with paired claws; presence of abdominal
cornicles (siphunculi or honey tube) and 9 pairs of lateral spiracles are
characteristic features, cornicles produce waxy material which covers the
aphids. Honeydew is excreted through anus.
Aphids are polymorphic. They reproduce sexually (sexuparae) and
lay eggs or asexually (virginoparae) and larviposit (development by
parthenogenesis).
Winged
(alate) and wingless (apterous) forms
frequent. Notorious pests of vegetation, spread several viral diseases to
plants. Life-cycle complicated, sexual and asexual forms alternate
depending upon climatic conditions (temperature/photoperiod). Few host
specific but majorities are polyphagous. Examples, Myzus persicae (green
peach aphid, on potato, tomato), Aphis gossypii (cotton aphid, on
legumes, cotton, bhindi, brinjal, cucurbits etc.), Lipaphis erysimi (mustard
aphid, on crucifers), Rhopalosiphum maidis (corn aphid, on corn,
millets, sorghum), Eriosoma lanigerum (woolly aphid, on apple),
Brevicoryne brassicae (cabbage aphid).
52 1
Economically Important Insect Orders
(iv) Family-Margarodidae (Cushion scales, mealybugs, Fig. 27).
Female with distinctly segmented body, often covered with waxy
secretion. Examples, Pericerya (=Icerya) purchasi (cottony cushion scale,
on citrus; first biologically controlled by a coccinellid predator Rodolia
cardinalis in 1 888 in USA; the beetle was imported from Australia).
Example, Drosicha mangiferae (mango mealybug, pest in north India).
(v) Family-Tachardiidae (Lac insects, Fig. 28). Female highly
degenerate with vestigial antennae, no legs, variable body size and
shape, enclosed in resinous cells, male dipterous. Example, Kerria lacca.
The insect produce commercial lac.
(vi) Family-Coccidae (Scale insects, Fig. 29). Segmentation of
female obscure, integument may be naked or covered with wax,
antennae and legs considerably developed. Example, Coccus hesperidum.
(vii) Family-Diaspididae (Armoured scales, Fig. 30). Females
specialised by absence or vestigial antennae and loss of legs, covered
with hard waxy scales, red coloured. Examples, Aonidiella aurantii (citrus
red scale), Quadraspidiotus pemiciosus (San Jose scale, on apple),
Melanaspis glomerata (Sugarcane scale).
Series coleorrhyncha includes only one small family and is not
represented from India.
(b) Suborder - Heteroptera. This suborder is divided in to 2 series:
Gymnocerata (antennae long and conspicuous in front of the head,
families Reduviidae, Cirnicidae, Pyrrhocoridae, Coreidae, Scutellaridae,
Pentatomidae) and Cryptocerata (Antennae short and concealed beneath
the head, families Notonectidae, Nepidae).
(i) Family-Reduviidae (Assassin bugs, Fig. 3 1 ). Head longer than
broad; antennae filiform, 4-5 segmented; rostrum short, strong, curved
with 3 segments, the tip in striated furrow between forecoxae; forelegs
somewhat raptorial; abdomen dorsally often concave and broad at the
middle; predaceous on insects, few blood suckers. Examples, Triatoma
rubrofasciata (suspected to transmit Kala-azar in India), Acanthaspis siva
(predator on Apis indica).
(ii)
Family--Cimicidae
(Bedbugs,
Fig.
32).
Blood
sucking,
ectoparasite on man and animals; broadly oval and flattened insects
having very short hemielytra; rostrum short, triarticulate and lies in a
ventral groove; metapleural odoriferous glands present. Example, Cimex
lectularius (bedbug of temperate and subtropical countries).
(iii)" Family-Pyrrhocoridae (Red bugs, stainers, Fig. 33). Brightly
coloured with red/black markings and elongate-oval in shape; rostrum
4-segmented; antennae 4-segmented; tarsi 3-segmented; phytophagous.
Example : Dysdercus koenigii (red cotton bug, destructive to okra and
cotton) .
Economically Important Insect Orders
[ 53
32
31
35
34
37
Fig. 31 to 38. 3 1 . Assassin bug (Reduvi1dae); 32. Bedbug (Cimicidae); 33. Dysdercus sp.
(Pynhocoridae); 34. Leptocorysa varicornis (Coreidae); 35. Shield bug ( Scutellaridae) ;
36. Ne7AJU virdula ( Pentatomidae); 37. Backswimmer (Notonectidae) ; 38. Water scorpion
(Nepidae).
54 1
Economically lmponant Insect Orders
(iv) Family-Coreidae (Leaf footed bugs, stink bugs, Fig. 34). Body
more or less elongate; antennae with 4 segments, frequently dilated; eyes
and ocelli well developed; rostrum 4-segmented; scent gland openings on
metathorax;
in
some
species
leaf-like
legs;
tarsi
3-segmented;
phytophagous. Example, Leptocorisa varicomis ( = Leptocorixa varicornis)
(rice stink bug, a serious pest on paddy).
(v) Family-Scutelleridae (Shield backed bugs, Fig. 35). Scutellum
very large and extends up to apex of abdomen exposing the wings at its
edge; antennae short, stout with 5-segments; rostrum 4-segmented.
Example : Chrysocoris stollii (litchi bug).
(vi) Family-Pentatomidae (Stink bug, Fig. 36). Moderate to large
sized bugs; antennae 5-segmented, base of antennae are concealed by
the lateral margins of the head; scutellum large, triangular and never
covering the entire abdomen; terrestrial , phytophagous or predaceous on
caterpillars. Examples, Bagrada cruciferarum (painted bug, pest on
brassicas), Nezara virdula (feed on millet, paddy, cucurbits).
(vii) Family-Notonecridae (Backswimmers, Fig. 37). Light coloured
bugs, swim upside down, body more convex dorsally ; head inserted into
prothorax; antennae with 4 segments; tarsi 3-4 segmented; abdomen with
median ventral keel; predaceous. Examples, Notonecta glauca.
(viii) Family-Nepidae (Water scorpion, Fig. 38). Oval with terminal
respiratory siphon, spiracle 4-6 closed; antennae with 3 segments;
forelegs raptorial, hindlegs walking, tarsi uniarticulated wmgs well
developed; predaceous. Example, Nepa cinerea (Indian water scorpion).
11. Order-Coleoptera (Coleo- sheath, ptera-wing; beetles). Hard
bodied with forewings modified into horny or leathery elytra which
generally meet to form a straight median suture on the dorsum;
hindwings membranous, folded longitudinally and transversely beneath
the elytra during repose or often reduced or absent; mouthparts adapted
for biting, typically prognathus, ligula variably lobed; antennae variable,
never setaceous, usually
I I -jointed; prothorax large and mobile,
mesothorax much reduced; legs typically cursorial, some adapted for
swimming, jumping, or digging, number of tarsal segments variable and
taxonomically important; metamorphosis complete; larvae campodeifmm,
scarabeiform cruciform, seldom apodous with mandibulate mouthparts;
pupae adecticous and exarate, rarely obtect.
Coleoptera forms the largest order in the Animal Kingdom having
about 3,45,000 spp. and comprises approximately 40% of all species of
insects, however, they are not so conspicuous because of concealed
habit. They live very well in \Vater. air or on land. Mostly phytophagous,
some live wool, hides, furniture, stored food and oilseeds. Few are
predaceous and have been used in biological control, rarely parasitic.
The beetles contain a large number of very destructive pests of
Economically Important Insect Orders
[ 55
agricultural crops; stored grains, seeds, and grain products; other stored
products, such as tobacco, nuts, and chocolate; and shade trees and
shrubs. Several species serve as vectors of plant diseases. Beetles
measures 0.5 mm (or even less) to 1 5.0 cm. The order includes two
major suborders : Adephaga and Polyphaga which can be identified by
the following characters :
Hind coxae immovably fixed to the metasternum; ...... Adephaga
wings usually with 2 m-cu cross-veins forming an
oblongum; · notopleuraI sulcus
present
in
prothorax; testes tubular; ovarioles polytrophic;
malpighian tubules four and simple
H indcoxae movable; wings never with distinct
. . . .Polyphaga
oblongum; notopleural sulcus never distinct in
prothorax; testis folicular; ovariole acrotrophic;
malpighian tubules of various types
(a) Suborder - Adephaga. This suborder includes only one large
superfamily Caraboidea which includes Carabidae, Cicindellidae and
Dytiscidae as important families.
(i) Famlly--Carabidae (Ground beetles, Fig. 39). Mostly predaceous,
antennae filiform and project anteriorly, clypeus not extending laterally
infront of base of antennae; lacinia without a movable lobe; largely
ground living under stones, bark, rotten wood etc.; mostly carnivorous,
few seed feeders. Example, Calosoma sycophancta (introduced into
North America from Europe to control gypsy moth).
(ii) Family-Cicindellidae (Tiger beetles, Fig. 40). Predaceous; eyes
prominent; large and dentate mandibles, lacinia with a movable hook
apically; clypeus extending on each side in front of the antenna! sockets;
legs long; 6 visible abdominal segments present in females and 7 m
males. Example, Cicindella sexpunctata.
(iii) Family-Dytiscidae (True water beetle, diving beetles, Fig. 4 1 ).
Aquatic beetles, less convex above, broad head tightly fixed with thorax,
scutellum visible, hindcoxae larger and not produced into plates,
antennae filiform, hindlegs adapted for swimming, elytra cover the entire
abdomen and wings are large and functional. Example, Eretes sticticus
(edible in India).
(b) Suborder - Polyphaga. This suborder includes following
important families.
(i) Fam ily-Hydrophilidae (Water scavangers, Fig. 42). Aquatic,
subaquatic and terrestrial beetles, antennae fitted below the head, it
consists of a long basal joint, a club of 3-5 joints which are flattened
and pubescent, tarsi 5-segmented generally not exceeding 1 .25 cm,
herbivorous. Example, Hydrophilus kashmirensis.
56 1
Economically Important Insect Orders
50
Fig. 39 to 52. 39. Ground beetle {Carabidae); 40. Tiger beetle {Cicindellidae); 41. Water
beetle (D)tiscidae) ; 42. True water beetle (Hydrophilidae); 43. Stag beetle (Lucanidae);
44. Dung be-::tle (ScarabaeidaeJ; 45. Khapra beetle {Dermestidae); 46. Lad)'i>ird beetle
(Coccinellidae); 47. Red flour beetle \Tenebrionidae); 48. Blister b::etle {Meloidae) ;
49. Leaf beetle (Chr)'l>omelidae); 50. Long horn beetle (Cerambycidae) ; 5 1 . Pulse beetle
(Bruchidae);
52. Rice weevil (Curculionidae).
Economically Important Insect Orders
[ 57
(ii) Fam ily-Lucanidae (Staghorn beetles, stag beetles, Fig. 43).
Mandible of male enormously enlarged and antler-like, forwardly
projected in males, 5 visible sternite, abdomen concerned with unstriated
elytra, antennae geniculate, feed upon decaying vegetable matter,
sexually dimorphic. Example, Lucanus lunifer.
( iii) Fam ily-Scambaeidae (D ung beetles, chafers, Fig. 44) . Convex
beetles; 6 visible sternites; antennae 8- 10-segmented; roll dung into
convenient-sized balls, burry them in underground chambers and feed at
leisure; often nocturnal. Example, Oryctes rhinoceros (coconut beetle,
rhinoceros beetle). Orphnus picinus (cowdung beetle).
(iv) Family-Dermestidae (Skin beetles, Fig. 45). Metasternum of
normal length, without a transverse sulcus; often head with median
ocellus; household pests, often covered with fine hairs or scales; larvae
caraboid, densely covered by long and short tufts of hairs; feed on dead
organic matter especially skins, horn, hair, wool, meat etc. Example,
Trogoderma granarium (khapra beetle ) .
(v) Family-Coccinellidae (Lady bird beetles, Fig. 46). Body convex;
head partly concealed by pronotum; tarsi 4-4-4, third concealed in
deeply
bilobed
second
tarsus;
antennae
short;
carnivorous
or
phytophagous; economically important as notorious pests of crop and
beneficial as insect predator feeding upon aphids, scales and other soft
insects. Examples, Coccinella septempunctata, Rodolia cardinalis (used
against Pericerya purchasi) (all are predators on aphids, scale insects),
Epilachna vigintioctopunctata (vegetable pest).
(vi) Family-Tenebrionidae (Darkling beetle, Fig. 47). Tarsi S-5-4 in
both sexes; first 3 visible abdominal sternite connate; forecoxae not
projecting; claws · simple; antennae short; majority live in concealment;
mostly scavangers, few stored grain pests. Examples, Tribolium
castaneum (red flour beetle), T. confusum (confused flour beetle).
(vii) Family-Meloidae (Oil beetle, blister beetle, Fig. 48). Tarsi
5-5-4 in both sexes; head strongly deflexed, neck narrow; tarsal claws
bifid or dentate
(appendiculate);
adult soft bodied;
long legs;
phytophagous, some pest; some produce cantharidine oil. Examples,
Mylabris phalerata (blister beetle), Lytta vesicattoria (Spanish fly).
(viii) Family-Chrysomelidae (Leaf beetles, Fig. 49). Minute to small;
head hypognathous; antennae moderate length, not clubbed; eyes do not
embrace
their
points
of
insertion;
elytra
shining;
destructive
phytophagous. Examples, Leptinotaria 10-lineata (colorado beetle, a
pot�to pest in America), Hispa armigera (paddy hispa), Aulacophora indica
(red pumpkin beetle).
(ix) Family-Cerambycidae (Longicorn beetle, Fig. 50). Antennae
usually at least 2/3rd of the body and capable of being flexed
backwardly; all tibiae with 2 spurs; claws simple; some are serious pest
58 J
Economically Important Insect Orders
on fruit trees, coffee, destroy furniture. Example, Batocem rnfomaculata
(mango stem borer) .
(x) Family-Bruchidae (Pulse beetle, Fig. 5 1 ). Antennae short, often
pectmnate or serrate, not capable of being flexed backwards, at least
one tibia with one spur; ligula absent; serious pest on stored pulses.
Example, Callosobruchus chinensis.
(xi) Family-Curculionidae (Weevils, Fig. 52) . H ead more or less
produced into a rostrum; antennae geniculate and clubbed; lambrum
usually absent; trochanter very elongated; projecting seta or tuft of
bristles; notorious pests of standing crops and stored grains; very large
family, 60,000 spp. Examples, Anthonomus grandis (cotton boll weevil of
USA), Sitophilus orywe (rice weevil), Hypera postica (alfalfa beetle,
lucerne beetle on Medicago saliva).
12. Order-Diptera (Di-two, ptera-wing; flies, mosquitoes, midges,
gnats, fruit flies). Single pair of membranous wings with reduced
venation, hindwings reduced to halteres; mouthparts usually forming a
proboscis, sometimes adapted for piercing, mandible rarely present,
labium distally expanded into fleshy lobe; pro- and metathorax small and
fused with large mesothorax; tarsi 5-segmented; metamorphosis complete,
larvae eruciform. apodous, frequently with head reduced and retracted,
tracheal system variable, pupa either free or enclosed within puparia,
adecticous, primarily obtect, exarate in higher forms. About 85,000
species are known which are either nactar feeder, detritivorous,
frugivorous, omnivorous, carnivorous, or sanguivorous (only females);
some
transmit
human
disease
like
malaria,
sleeping
sickness,
elephantiasis, yellow fever. kala-azar etc.
The order Diptera is divided into 3 Suborders, Nematocera,
Brachycera and Cyclorrhapha and can be identified as follows :
1 . Antennae many segmented, usually longer than
. . . . Nematocera
head and thorax, segments alike, arista absent;
cu-cell when present widely open
Antennae generall y 3-4-segmented, shorter than
. . . . . . ..... . . . . . . . . . . . . 2
throax, last segments elongated with arista or
style; cu-cell contracted or closed
2. Antennae 3-segmented with terminal arista or
. . . . . .Brachycera
style ; labial palpi usually 1-2 segmented ; pupa
obtect ; larva with head. usually retractile and
with vertical biting mandible.
Antennae 3-4 segmented with dorsal arista ;
. .. Cyclorrhapha
labial palpi I-segmented ; pupa exarate ; larva
with vestigial head ; pupation in puparium
Economically Important Insect Orders
[ 59
(a) Suborder - Nematocera. It includes following important families.
(i) Family-Psychodidae (Moth flies, sand fly, Fig. 53). Minute,
moth-like, fragile, commonly met with in dark and moist places. Body
and wings clothed with coarse hairs and scales; ocelli absent; antennae
1 2- 1 6 segmented; wings with Rs 4-branched, no cross veins; larvae
usually aquatic and amphipneustic. Example, Phlebotomus argentipes
(spread kala-azar by transmitting Leishmania donovani during sucking
blood as meal).
(ii) Family-Culicidae (Mosquitoes, Fig. 54). Slender, generally with
elongate piercing proboscis; ocelli absent; palpi stiff not pendulous;
antennae 1 4- 1 5 segmented, pedicel large, plumose in males and pilose in
females; hindlegs raised while sitting; larvae metapneustic with enlarged
thoracic mass. Mosquitoes are of utmost public health importance
particularly in tropics and subtropics; spread malaria (Anopheles
masculipennis ), filaria ( Culex pipiens, C. fatigans), encephalitis (Aedes
vishnoi), yellow fever (A. aegyptii).
( iii) Family Cecidomyiidae (Gall midges, Fig. 55). Minute delicate
with long moniliform, 14-segmented antennae with conspicuous whorls of
hairs; wing with few longitudinal veins for the most part unbranched; no
cross-vein; coxae not elongate, tibiae devoid of apical spurs; larvae
peripneustic;
predaceous,
paras1t1c
(on
insects),
phytophagous,
detritivorus with sternal spatula "breast bone" on the ventral side of the
thorax. Example, Phytophaga (=Mayetiola)
destructor ( H e ssian fly wheat pest in USA).
(b) Suborder - Brachycera. It includes following important family :
Family - Tabanidae (H orse fly, Gad flies, Fig. 56). Medium sized,
bristleless flies, stoutly built with third antenna) segment annulated but
devoid of styles; eyes large, laterally extended; ocelli absent; proboscis
projecting, piercing in females, blood suckers on livestock; pulvilli and
arolium large and padlike. Example ,
Tabanus rnbidus (horse fly,
diurnal, female transmit surra disease in cattle, males live on honeydew
and plant sap ) .
(c) Suborder - Cyclorrhapha. This order i s divided into 3 sections
Aschiza, Schizophora and Pupipara as follows:
............... 2
1 . Wings well developed, head distinct
- Wings reduced or absent, head is closely united
...... Pupipara
with thorax
....... Aschiza
2. Frontal suture present; ptilinum present; cell Cu
elongated
- Frontal suture absent; ptilinum absent; cell Cu
.... Schizophora
short or vestigial
60 J
Economically Important Insect Orders
Fig. 53 to 64. 53. Sand fly (Psychodidae); 54. Mosquito (Cuhcidae); 55. Hessian fly
(Cecidomyidae); 56. Horse fly (Tabanidae); 57. Hover fly (Syrphidae); 58 Leaf miner
(Agromyzidae); 59. Drosophila (Drosophilidae); 60. Gasterophilus ( Gasterophilidae);
61. House fly (Muscidae) ; 62. Flesh fly ( Sarcophagidae) ; 63. Louse fly (Hippoboscidae) ;
64 . Rat flea (Pulicidae).
Economically Important Insect Orders
Section
[ 61
Aschiza includes following important family :
(i) Family-Syrphidae (Hover flies, drone flies, Fig. 57). Moderate to
large sized, brilliantly coloured markings; almost always bristleless, arista
dorsal; cell R5 closed. Found abundantly among flowering plants during
sunshine, feed on pollen and nectar. Larvae live generally in rotting
vegetables or animal matters (Saprophagous, Syritta); or predatory
feeding
on
aphids
(Aphidophagous,
Episyrphus balteatus);
or
phytopagous (Mesogramma); or parasitic and cause myiasis in human
intestine (Eristalis).
Section Schizophora includes following important families.
(i) Family-Agromyzidae (Leaf miners, Fig. 58). Small flies; antennae
short; anal cell is present; costa interrupted where Sc+R1 runs out;
vibrisae present; female with oviscapt. Example, Phytomyza atricomis
(pea leaf miner- on pea, crucifer crops, safflower, lentil etc.).
(ii) Family-Drosophilidae (Fruit flies, Fig. 59). Sm(lll, chubby flies
with large light eyes, life cycle within a week; vibrisae present; arista
dorsal and pectinate or . plumose, costa broken at the end of R 1 , Sc
present but reduced, Cu2 and IA present; third antenna! joint rounded.
Example, Dorsophila melanogaster, most common species, genetically well
known, largest genus, more than I 000 species.
(iii) Family-Gasterophilidae (Bot flies, Fig. 60). Hairy rather than
bristly; mouth-parts reduced; functionless; wings with cross vein r-m near
the base of wing, M I and M3 straight. Larval stage is passed in the
gut of mammals. Example : Gasterophilus intestinalis (lay eggs on hairs
of forelegs of horse and during licking eggs pass into intestine).
(iv) Family-Muscidae (House flies, Fig. 6 1 ). Small to large flies;
veins C u 1 and IA not reaching the apex of wing; lower calypter nearly
always longer than the upper one; prosternum sclerotised. Examples,
Musca domestica, M. nebulo, Glossina palpalis (tsetse fly- transmit
Trypanosoma gambiens causing sleeping sickness in South Africa) .
(v) Family-Calliphoridae (Blow flies, blue bottles, Fig. 62). Large
flies,
larvae
saprophagous,
flesh
feeders,
or
insect
parasitoids;
pteropleural and hypopleural
bristles present; post-scutellum well
developed; second abdominal sternite entirely overlapped by tergite.
Examples,
Calliphora erythrocephala (on meat or dead bodies),
Chrysomyia bezziana (cause myias1s in cattle, horse. elephants),
Cochliomyia hominivorax (screw-worm flies-cause cutaneous myiasis m
man in America), Sarcophaga ruficornis (flesh flies- lives in meat or
inside nostril of cattle).
(vi)
Fam ily- Tachinidae. M ostly
parasitic
upon
caterpillars,
pteropleural and hypopleural bristles present; post-scutellum little
developed, second abdominal sternite with its side visible. Examples,
62 J
Economically Imponant Insect Orders
Actia
monticola (parasitic on Spodoptera litura); Sturmiopsis inferens
(parasitic on Tryporyza nivella, T. incenulus, Chilo zanellus, C. panellus).
Section- Pupipara includes only one important family H ippoboscidae.
Family-Hippoboscidae (Louse flies, Fig. 63). Parasitic on cattle and
dogs; body flattened and leathery; head sunken into the anterior part of
thorax, retractile proboscis with a sheath formed of palpi ; legs short and
shout; wings present or absent. Examples, Hippobosca maculata
(on
cattle), H. capensis (on dog).
13. Order-Siphonaptera (Siphon -a tube, aptera
wingless; fleas) .
Parasitic, secondarily apterous, laterally compressed insects with highly
sclerotised integument; head set closely with thorax and bears comb
-
formed by a row of powerful spines on the latero-ventral border of
head; mouthparts adapted for piercing and sucking, blood suckers;
antennae clavate, short, concealed in grooves when at rest, in male
longer than in female and functioning as copulatory claspers, 1 1 - 1 2
segmented; eyes and ocelli absent; hindlegs strong, coxae enlarged,
adapted for gliding easily among hairs and for jumping or leaping; body
covered with backwardly inclined spines and bristles; thorax compact
and segments free. In some species pronotal comb present consisting of
a row of stout spines of the hind margin of the pronotum. Larvae blind,
eucephalous, apodous ; pupae free, exarate. The adults are exclusively
blood sucking ectoparasites of living birds and mammals. They can jump
a horizontal distance of nearly 33 cm. Fleas transmit several fatal
diseases, e.g. bubonic plague and murine endemic typhus. Plague is a
disease of wild rodents; and causative agent, plague bacillus Pasteurella
pestis, is transmitted from rodent to rodent by fleas. Under certain
conditions humans may be bitten by infected fleas and develop the
symptoms of plague. In regions of high population concentration and
poor sanitary conditions, the potential may exist for epidemics of this
disease, the black death. The order include only one important family
Pulicidae (Fig. 64). The distinguishing charatcers are: outer internal
ridge of midcoxae absent: hindtibia without an apical tooth on outside
and sensilium with 8 or 14 pits on each side. Examples, Pulex irritans
(notorious human flea; also attack dog, hog, rat, etc.), Xenopsylla
cheopis (Indian rat flea, chief vector of bubonic plague) .
14. Order - Hymenoptera (Hym en-membranous, ptera-wing; saw-flies,
fig insects, wasps, ants, bees, horntails). Two pairs of membranous
wings, venation often reduced, hindwing smaller and interlocked with
forewings by hooklets; mouthparts adapted for biting or lapping or
sucking; compound eyes usually well developed, commonly 3 dorsal
ocelli; ovipositor conspicuous and often modified for stinging, pricking
or sawing; parthenogenesis very common, males are usually haploid;
metamorphosis complete; larvae generally apodous, with more or less
Economically Important Insect Orders
[ 63
developed head; more rarely eruciform with locomotary appendages;
holopneustic, or peripneustic throughout life or at least in final instar; in
few hypermetamorphosis (larvae differ in shape in different instars);
pupa adecticous, exarate, rarely obtect and a cocoon generally present;
nervous system highly developed among non-chordates with mushroom
body in brain.
On the basis of complexity and diversity of behaviour the
Hymenoptera are generally recognised as the most advanced group of
insects. About 1 ,00,000 spp. are known; some show high degree of
specialisation; some are social (wasps, ants, bees); a large number of
females undergo structural changes to constitute a separate caste, i.e.,
worker whose power of reproduction is either checked or lay only male
producing eggs but do parental care; most solitary bees and wasps
exercise "mass provisioning" , i.e., they store their cells (in which they
reside) with sufficient food (for the proper development of the progeny)
and close them down before the egg hatch; some wasps exercise
' 'progressive provisioning' ' , i.e., they feed their larvae time to time;
colonies may be perennial having many fecundated Iemales (some
wasps) in higher groups colony has single fecundated female, the queen
(bees); few wasps are parasitic on insects, i.e., they are parasitoids and
several species have been used and are being in use in biological
control of certain insect pests of agricultural, horticultural. and medical
importance; parthenogenesis is more common, in some cases males
absent. In this order two suborders Syrnphyta (Chalastogastra) and
Apocrita (Clistogastra) may be identified as follows :
.....Symphyta
I . Abdomen broadly attached to the metathorax, no
marked constriction between first and second
abdominal segments; hindwings with 3 basal cells;
foretibiae with nearly two spurs, one smaller than
other; larvae cruciform
...... Apocrita
Abdomen deeply constricted between the first
(propodium) and second abdominal segments;
hindwings with 2 or 1 or no basal cells; foretibiae
with only one spur; larvae apodous
(a) Suborder - Symphyta. It includes only one important family.
Family-Tenthredinidae (Sawflies, Fig. 65). Antennae 9-segmented,
third segment short, subclavate; pronotum deeply emarginate behind;
post scutellum present; foretibiae with 2 apical spur; largest family
among Syrnphyta; ovipositor saw like . Example,
Athalia proxima
(mustard sawfly, a serious pest of mustard in India).
(b) Suborder - Apocrita. It includes following important families.
(i) Family-Ichneumonidae (Ichneumon wasp; death wasps, Fig. 66).
Hindfemur with divided trochanters; forewing with pterostigma at distal
(Z-57)
64 1
Economically Important Insect Orders
end of costa ; costal cell narrow, 2 m-cu present; hindwing with r-m
meeting Rs after it leaves Si::+R; ovipositor long; a majority of the
ichneumon flies parasitise larvae and pupae of Coleoptera, Lepidoptera,
Diptera; host specificity considerable; larvae peripneustic. Examples,
Isotima javensis (parasitic on sugarcane top-borer, Scirpophaga nivella),
Campoletis chlorideae (parasitic on cotton bollworm, Helicoverpa
armigera, rice-moth, Corcyra cephalonica). Most of the wasps are being
used in biological control of the insect pests.
(ii) Family-Braconidae (Fig. 67).
Hindfemur
with
divided
trochanters; forewing with pterostigma at distal end of costa; costal cell
narrow, 2 m-cu absent; hindwing with r-m meeting Sc+R before Rs
leaves it; first sector of M+Rs present, abdomen sessile, subsessile or
petiolate; larvae pripneustic; majority parasitise larvae of Lepidoptera,
Hymenoptera,
Coleoptera,
Diptera.
Examples,
Cotesia glomerata
(parasitic on grubs, caterpillars), Diaeretiella rapae (parasitic on brassica
aphids), Aphidius matricariae (widely used in biological control of green
peach aphid, Myzus persicae). Most of the wasps are being used in
biological control of the insect pests.
(iii) Family-Chalcidae (Fig. 68). Mandible stout with 3-4 teeth;
pronotum small, never extending back to tegulae; hindcoxae 5-6 times
longer than forecoxae; hindfemur is usually much swollen and dentate,
with a row of teeth; wings with single vein; ovipositer short; cocoon not
constructed, tiny insects; Example: Brachymeria. Mostly they are parasitic
upon larvae/egg of Lepidoptera.
(iv) Family-Agaonidae (Agaontidae) (Fig insects, Fig. 69). Females
black, winged; head oblong, deeply grooved above; pronotum small,
never extending back to tegulae; males always apterous, fore- and
hindlegs stout; tibia shorter than femora; ovipositor very long; help in
pollination of figs. Examples, Blastophaga psens (old world blastophaga),
caprifiers (live in fig receptacles).
(v) Family-Aphelinidae (Fig. 70). Parasitic on aphids or scale
insects; tarsi 5 segmented; axillae are advanced strongly in front of the
anterior margin of the scutellum and usually in front of tegulae;
forewing narrower with pubescence not in rows or lines. Examples,
Aphelinus mali (parasitic on woolly aphid, Eriosoma lanigerum).
(vi) Family-Eulophidae (Fig. 7 1 ). Forewing narrower, pubescence
not in rows, stigma! vein distinct; antennae with l -2 ring joints; axillae
triangularly produced forward into �he region of the scapulae; tarsi 4
segmented. Examples, Tetrastichus pyrillae (parasitic on eggs of Pyrilla
perpusilla).
(vii) Family-Tnchogrammatidae (Fig. 72). Minute species; forewing
broader, pubescence in rows; tarsi with 3 segments; antennae with 1
ring-joints; axillae extended back to tegulae; parasitic on eggs of
(Z-57)
Economically Important Insect Orders
[ 65
Fig. 65 to 77. 65. Sawfly (Tenthridinidae); 66. Ichneumon wasp (lchneumonidae) ;
67. Bracon wasp (Braconidae) ; 68. Chalcid wasp ( Chalcidae) ; 69. Fig wasp (Agoanidae);
70. Aphelinus mali (Aphelinidae); 71. Tetrastichus (Eulophidr.e); 72. Trichogramma
(Trichogrammatidae); 73. Ant (Fonnicidae); 74. Polistes (Vespidae); 75. Thread-waisted
wasp (Sphecidae); 76. Honey bees (Apidae); 77. Bumble bee (Bombidae)
(Z-57)
66 l
Economically Important Insect Orders
Lepidoptera, Hemiptera, Coleoptera, etc. Examples, Trichogramma spp.,
widely used in biological control of insect pests.
(viii) Family-Formicidae (Ants, Fig. 73). Best known insects, social;
labrum vestigial; submentum and mentum seperate; antennae geniculate,
male with one segment more than female; eyes and ocelli present in
male but vestigial in female; wings present in sexual forms, deciduous
with 1-2 cubital and 1 discal cell; first or first and second segments of
gaster scale like or nodiform and well seperated; polymorphic, 29
morphs may be present in one colony; colony consists of (a) workers or
ergates - sterile, wingless females, (b) soldiers or dinergates - modified
workers
with enormous head and mandibles, (c) gyne or fertile
females or queens (d) Aner or fertile male; the queen once mated,
dealates herself and establishes the first nest and rears her first brood.
She draws her nourishment from the now useless flight muscles and
stored up fat. When the first larvae appear, they are fed with a special
nutritive secretion of the salivary gland and as soon as the first workers
appear, they go forth into the world foraging. They take overall duties
of the rearing of the brood, foraging, nest building, cleaning, nursing,
fighting, etc. The queen survived upto 1 5 years. The population of a
single colony varies considerably from a few thousand to over 5,00,000
individuals. The ant nests or formacaria is established in a bewildering
variety of situations such as underground, inside hollow stems, fruits,
thorns, galls, among leaves, under stones etc. The tropical Oecophylla
smaragdina webs leaves of various trees with silken threads into a nest.
The larvae secrete the silk and are used by workers as a kind of living
thread ball. Most of these ants tend aphids, membracids and others for
honeydew. Examples, Formica spp., Monomorium destructor (inside
house), Mermis sp.
(ix) Family-Vespidae (True wasps, Fig. 74). Eye usually emarginate
on inner sides; antennae 1 1- 1 3 segmented; pronotum produced back to
tegulae; forewings folded longitudinally when in repose, first discoidal
cell very long; hindwing with anal cell, median cell narrow; mandible
short, broad, obliquely truncate or toothed at tip; foretibia with a comb
for cleaning the antennae; often red or yellow; sting present; generally
predaceous, make nests (papery). Social wasps, colony trimorphic.
Examples, Polistes stigmata, Vespa orientalis (hornets).
(x) Family-Sphecidae (Solitary wasps, Fig. 75). Antennae 13
segments in males and 12 segments in females; hindwings with anal
lobe; abdomen with long petiole; pronotum not extending back to
tegulae; tracbanters undivided; bindtarsi Slender; graceful and attractive
wasps, very intelligent, they run and sting the prey, poison is antiseptic;
prey upon spiders, caterpillars, crickets, grasshoppers; construct mud
(Z-57)
Economically Important Insect Orders
{ 67
nest (cell), provision food (stung prey) for larvae
Examples, Sphex lobatus.
inside mud cell.
(xi) Family-Apidae (Honey bees, Fig 76). Body highly pubescence,
hindtarsi more or less broadened, hindtibiae without apical spur and
with a pollen basket (corbicle); mandible cutting and forms tongue
alongwith glossae for lapping and imbibing liquid; abdomen with short
petiole; colony trimorphic: queens, workers and drones; pronotum not
extending back to tegulae; trochanter undivided; hindlegs adapted for
pollen collection. Mostly social, pollinate flowers of plants of economic
importance, produce honey of commercial importance. Examples, Apis
dorsata (giant bee), A. cerana indica (Indian bee), A. florea (rock bee),
A. mellifera (European bee).
(xii) Family-Bombidae (Bumble bees, Fig. 77). Densely hairy; black,
red or yellow; antennae geniculate; glossa long; eyes not reaching base
of antennae, gena longer than antenna! pedicel ; good pollinator in
Himalayan region. Example, Bombus tunicatus.
15. Order-Lepidoptera (Lepido - scale; ptero.
wing; moths and
butterflies). Body covered with broad scales which are modified
macrotrichia, beautifully coloured ; mandibles always vestigial or absent,
principle mouthparts usually long, sucking proboscis formed by the
maxillae (galea); well developed compound eyes; two pairs membranous
wings, cross-veins few, tracheation complete; forewings always larger
than hindwings, both wings are coupled together in 4 different ways,
jugate, frenate, jugofrenate, and amplexiform (see wing venation and
coupling mechanism); tarsi 5-segmented; larvae eruciform, peripneustic,
frequently with 8 pairs of limbs; pupae usually adecticous, more or less
obtect and generally enclosed in a cocoon or an earthen cell. Members
in several families have organs for hearing.
Most Lepidoptera feed on flowering plants and are particularly
voracious in the larval stages, most adults being capable of obtaining
only fluid meals from flowers and other sources and being unable to
masticate plant material. Their plant feeding habits make several species
among the most important agricultural pests. Many attack stored grain
and some attack fibre in clothing. Most pest species are moths.
However, silkworm moth, Bombyx mori is highly valuable moth. About
1 ,00,000 species are described. The order Lepidoptera is divided into 3
suborders, Zeugloptera, Monotrysia and Ditrysia that can be identified
as follows :
-
1.
Adults with functional mandibles, maxilla
with lacinia developed, galea
not
haustellate
. ............... 2
Economically Important Insect Orders
68 J
..Ditrysia (Frenatae)
Adult with vestigial mandibles, maxilla
without lacinia, galea haustallate; wing
interlocked by frenulum; larvae with less
than 5 pairs of abdominal legs
2. Larvae with not more than 7 pairs of . .Monotrysia (Jugateae)
abdominal legs
... 2eugloptera
- Larvae with 8 pairs of abdominal legs
(a) Suborder - Zeugloptera. Very primitive Lepidoptera. No Indian
species is recorded.
(b) Suborder - Monotrysia. The majority of the members belong to
the Family Hepialidae (swift moth). The distinguishing characters are:
antennae very short, mouthparts vestigial, venation of fore- and
hindwings similar and wing-coupling apparatus jugate type, tibial spurs
absent. The larvae apparently feed on mosses or liverworts. Example,
-
Hepialus.
(c) Suborder - Ditrysia.
It includes majority (97% ) of the
Lepidoptera. Following are some important families.
(i) Family-Plutellidae (Fig. 78). Maxillary palpi well developed and
projecting,
larvae remain
in
silken
cocoon. Examples,
Plutella
maculipennis (diamond back moth) - it is a brassica pest.
(ii) Family-Gelechiidae (Fig. 79). Antennae rarely with basal
pecten; forewings trapezoidal; hindwings with Rs and M1 stalked at the
base,
includes
notorious
pests.
Examples,
Phthorimaea
(potato tuber moth), Pectinophora
(=Gnorimoschema) operculella
gossypiella {pink bollworm), Sitotroga cerealella (Angoumois grain moth),
Holcocera pulverea (predator on lac insects).
(iii) Family-Pyralidae (Fig. 80). Maxillary palpi usually present; legs
always long; tympanal organ present at base of abdomen ; Sc+R1
approximated to or fused with Rs distally ; haustellum scaly. Examples,
Gal/aria me/lone/la (wax moth on bee hives- larvae feed on wax),
Corcyra cephalonica (rice moth), Diatraea saccharalis
(American
sugarcane
borer),
Chilo zanellus (com borer), Tryporyza nivella
(sugarcane top borer), Ephestia kuhniella (Mediterranean flour moth),
Plodia interpunctatta (Indian meal moth).
(iv) Family-Nymphalidae (Fig. 8 1 ). Antennae slender, clubbed,
clavate; labial palps moderately long, terminal rather pointed; forewing
with Cu2 absent, hindwing without frenulum, Cu 2 absent, humeral lobe
well developed; forelegs being reduced in size in both sexes and useless
for walking, tibiae short and clothed with long hairs (brush footed
butter flies); cell of forewing rarely open, tarsus in female with small
knob, vein 2A single.
Brightly coloured. E xample, Kallima sp . (leaf
butterfly).
Economically Important Insect Orders
[ 69
84
Fig. 78 to 88 78. Piute/la (Plutellidae); 79. Potato tuber moth (Gelechiidae); 80. Chilo
(Pyralidae); 8 1 . Leaf butterfly (Nymphahdae); 82. Whites (Pieridae); 83. Swallow-tails
(Papilionidae); 84. Bombyx mori (Bombycidae) ; 85. Emperor moth ( Saturniidae);
86. H awk-moth ( Sphingidae); 87. Cutworm (Noctuidae); 88. Tiger moth ( Arctiidae).
(v) Family-Pieridae (Whites, Fig. 82). Antennae slender, clubbed,
clavate; labial palps moderately long, tenninal rather pointed; forewing
with Cu2 absent, hindwing without frenulum, Cu2 absent, humeral lobe
well developed; legs normal, hindwing with two anal veins; yellow orange
or white; pests of crucifer and leguminous crops. Example, Pieris
brassicae (cabbage butterfly).
(vi) Family-Papilionidae (Swallow-tails, Fig. 83). Antennae slender,
clubbed, clavate; labial palps moderately long, terminal rather pointed;
forewing with Cu2 absent, hindwing without frenulum, Cu2 absent,
humeral lobe well developed; legs nonnal, hindwing with one anal vein.
The wings of these insects are extra ordinarily variable in shape, the
70 J
Economically Important Insect Orders
hindwing is provided with conspicuous tail-like prolongation which are
marginal extensions in the region of vein M3 . Example, Papilio demolius
(citrus pest).
(vii) Family-Bombycidae (Silkworm, Fig. 84): Maxillary palps and
tympanal organs absent, frenulum always atrophied; proboscis rarely
developed; antennae pectinated; especially in male; Cu2 absent from
both wings; hindwing with Sc+R 1 connected with cell by a crossvein;
forewing with M 1 free or shortly stalked on Rs; proboscis absent.
Example, Bombyx mori (silkworm moth).
(viii) Family-Saturniidae (Fig. 85). Maxillary palps and tympanal
organs absent, frenulum always atrophied; proboscis rarely developed;
antennae pectinated; especially in male; Cu2 absent from both wings;
brightly coloured large moths, densely covered with scales; hindwing
with Sc+R1 diverging from cell base; M2 arising at or infront of middle
of cell, nearer M 1 than Cu1a, Cu2 and frenululm absent, tibiae with
spine; antennae bipectinated in both sexes. Example: Saturnia pyri
(Emperor moth-largest European moth), Attacus atlas (among largest
moth, wing span 27 cm); Samia cynthia ricini (=Philosamia ricim) (Eri
silkworm).
(ix) Family-Sphingidae (Hawk moth, Fig. 86): Antennae clubbed
with apex pointed; proboscis and frenulum develped; Cu2 absent from
both wings; forewing with M 1 arising from stem of R 3_ 5 or basally
approximated to it. Hindwing with Sc+R 1 connected with cell by a
cross-vein and approximated to Rs beyond the cell; tympanal organ
absent. Large robust moth, predaceous. Antennae often hooked apically.
Examples, Acherontia (deaths head moth).
(x) Family-Noctuidae (= Agrotidae, Fig. 87). Maxillary palpi minute,
tympanal organ present on metathorax; Cu2 absent from both wings;
forewing usually with M2 basally approximated to M 3 and with I A+2A
not forming a definite basal fork; hindwing with Sc+R 1 separate from
Rs, connected with the cell by a bar; antennae with shaft not dilated;
forewing with areole. Examples, Eublemma amabilis (enemy of lac
insects), Plusia ni (crucifer pest), Agrotis segetum (cut worm), Leucania
insularis
(armyworm),
Helicoverpa
armigera
(gram
pod
borer-polyphagous-cotton, gram, tomato, corn etc. - also known as
cotton bollworm), Earias fabia (okra pest), E. insulana (cotton
bollworm), Spodoptera litura.
(xi) Family-Arctiidae (Tiger moth, Fig. 88). Maxillary palpi minute,
tympanal organ present on metathorax; Cu2 absent from both wings;
forewing with M2 basally approximated to M 3 , Sc+R1 separate from
Rs; hindwing with Sc+R 1 anastomosing with cell to or to beyond
middle, stout bodied moth. Examples, Diacrisia obliqua (Bihar hairy
catterpillar), A msacta m oorei.
Economically Important Insect Orders
[ 71
Important Questions
1.
2.
3.
4.
Give an outline classification of class lnsecta giving examples from each order.
Describe the characteristic features of the important families of order Orthoptora or
Diptera or Coleoptera or Hymenoptera or Lepidoptera or Hemiptera.
Differentiate the following families : (i) Acridtdae and Gryllidae (ii) Jassidae and
Membracidae (iii) Pyrrhocoridae and Coreidae (iv) Muscidae and Drosophilidae
(v) Culicidae and Simulidae (vi) Coccinellidae and Cicindellidae (vii) D)tiscidae and
Hydrophillidae (viii) Nepidae and Belostomatidae (ix) Ici)Ileumonidae and Braconidae
(x) Noctuidae and Arctiidae (xi) Satumiidae and Bombycidae (xii) Blattidae and
Mantidae.
Write short notes on : (i) Formicidae; (ii) Isoptera; (iii) Bombycidae; (iv) Tachardiidae;
(v) Silverfish and (vi) Ladybird beetles.
Insect Integuinent
The general body covering or integument is a complex vulnerable organ
system of diverse structure and functions. It is the medium through which
and by which all of an insect's activities are moderated. The integument is
not only the characteristic feature of all arthropods but is responsible for
the great success of insects as terrestrial animals. The integument is also a
central subject intimately related to a variety of applied problems such as
the mode of action of insecticides, water metabolism and ecology and
intricate relationship of endocrines and the cuticle.
Histology of the Integument
The insect
epidermis
membrane
epidermis)
integument consists of 3 basic layers: one cellular layer, the
(hypodermis) and two non-cellular layers, the basement
(underneath the epidermis) and the cuticle (above the
(Fig. I ).
[ I] Epidermis
The epidermis is typically one cell thick and derived from embryonic
ectoderm. Though the plasma membranes of the adjacent cells are joined
by septate desmosomes, it forms a functional syncytial layer. The
diffusional molecular transfer through cytoplasmic channels in the plasma
membrane functionally integrates all the cells with each other.
The ultra structure of the cells varies with their cycles of secretary
activities. The cells contain numerous mitochondria, Golgi vesicles and
cistemae of smoothed surfaced endoplasmic reticulum as well as
cytoskeletal structures in the form of oriented microfibres and
Insect Integument
{ 73
microtubules. The muscle attachments penetrate the epidennis and the
oenocytes (excretory organ) which are originated from epidermal cells
some times remain closely associated with this layer. Interspersed among
the epidermal cells are dermal glands, some of which play a part in
secreting a portion of the cuticle and hence shows greatest activities
during moulting. It may secrete the basement membrane also. Other
types of dermal glands, e.g., exocrine glands carry out a variety of
functions such as secretion of defensive substances, silk, pheromones,
kairomones etc. During moulting, it secretes moulting fluid which
dissolves the old endocuticle before the immature insect moults.
[ II] Basement membrane
The basement membrane, generally 0.5 µm or less in thickness, is a
continuous amorphous granular layer. It contains neutral mucopoly­
saccharides secreted by haemocytes.
[ III] Cuticle
The cuticle is a complex, non-cellular, outermost layer secreted by
epidermal cells and is the seat of several metabolic activities. It also lines
stomodeum, proctodeum, tracheae, some glands and the parts of the
reproductive tract.
1. Histology of the cuticle. The cuticle principally consists of two
layers: epicuticle (outermost non-chitinous layer of the cuticle) and the
procuticle (innermost layer of the cuticle (Fig. 1).
(a) Epicuticle. The epicuticle, despite its comparative thinness
(0.03-4.0 µm), is exceedingly complex and extremely important layer of
cuticle. It is at least 4 layered structure and is penetrated by wax canal
that contains wax filaments. The wax canals are involved in transporting
wax molecules to the epicuticle via pore canals from their secretion
(Fig. I C).
(i) Outer cement layer or tectocuticle or roof cuticle. The outer
cement layer of unknown composition (absent in insects having scales) is
secreted by dermal glands. It is similar to shellac in chemical
composition and is less than 0. 1 µm thick. It determines the surface
properties of the cuticle, i.e., whether the cuticle will be water repellent
(hydrophobic) or water attractant (hydrophilic). It serves as protective
barrier for the more vulnerable layers beneath. A waxy bloom appears
on the surface of cement layer in some bugs.
(ii) Waxy layer. The waxy layer consists of an ordered monolayer of
lipid directly associated with the cuticulin layer and is responsible for
many of the permeability characteristics of the cuticle.
( iii) Cuticulin layer. Cuticulin is the first layer of about 100-200 A
thick secreted by epidermis during the formation of cuticle as small
Insect Integument
74 J
epicuticle
cuticle
exocuticle
endocuticle
epidermis
{{
{
A
epicuticle
�------o
.
.,._...._,,;.,;_..;....,;...;i
_
_
.
basement
membrane
] night
(lamellated)
""""�--����""""4-- day
...._
8
exocuticle
-���.._ pore canal
c
J
par canal
D
E
Fig. I . Structure of the integument (diagrarrunatic) . (A) section of generalised integument,
(B) daily growth layers and lamellar pattern, (C) generalised epicuticle, (D) hehcoidal and
preferred structure of layers of endocuticle, (E) transverse section of endocuticle showing
parabolic effect
Insect Integument
[ 75
plaques. It is quite impermeable to water and is very resistant to acids
and
organic
solvents
and
it
covers
the
entire
integumental
surface
including the tracheoles and ducts. Its chemistry is not well understood
but is said to be a poythene-like polymer with quinone-tanned proteins
or lipoprotein.
(iv) Inner epicuticle.
layer
and
chitin
The inner or protein epicuticle is the innermost
1 µm thick and appears as homogenous, dense, refractile
layer, about
is
composed
by
quinone-tanned,
It
the
amount
fibres.
limits
of
amorphous
expansion
of
protein
without
cuticle
possible
between moults.
Beneath
(b) Procuticle.
the
epicuticle,
the
part
hardened
of
the
cuticle is the procuticle and is about 200 µm thick. It is divided into
two layers, exocuticle and endocuticle.
(i) Exocuticle or outer procuticle.
The exocuticle, thin in soft bodied
and thick in hard bodied insects, i s highly stablised pigmented layer of
the procuticle. It is
hardened
through
composed of a homogeneous
sclerotisation
and
is
resistant
electron-dense matrix
to
the
exuvial
fluid
during ecdysis.
(ii) Endocuticle or inner procuticle.
The endocuticle is composed of
successive light (deposited during the night) and dark layers (deposited
during the day) which correspond to daily gr�wth layers (Fig.
light
layers
are
again
subdivided
into
light
lamellar patterns are considered to be the
layers
of microfibrils
protein
matrix.
of
These
Within each sheet,
chitin
and
microfibrils
result
probably
are
the microfibrils
and
laid
dark
of the
protein
down
are parallel
l B ) . The
lamellae.
embedded
in
The
orientation
layered
of
in
a
sheets.
to one another, but in
successive sheets they are aligned at regularly changing angles (Fig. I D).
This
helicoidal
arrangement
is
responsible
for
the
parabolic
patterning
of endocuticle (Fig. I E). In the dark layers, the microfibrils are oriented
in successive sheets
lamellar
The
vertically
surface
pore
canals
are
tiny
and
extend
from
the
of
apparently
layers.
in a preferred direction and hence do not have a
appearance.
the
They
endocuticle
epicuticle.
serve
run
and
miJlion per mm
2
as
These
connecting
spirally
may
tubes
are
tubes
through
number
(usually
epidermal
from
layer
ribbon-like
between
the
1
in
diam.)
to
and
arranged
thousand
twisted
the external
appearance
epidermis
helicoidally
several
µm
nearly
to
and
cuticular
layers
well
over
of
a
of the integument.
2. Chemical composition of the cuticle. The polysaccharide chitin and
various structural proteins are the major cuticular constituents. Chitin is a
high molecular weight polymer of anhydro N-acetyl-D-glucosamine and
D-glucosamine linked through 1 , 4 �-glucosidic bonds mostly in proportion
of 9: 1 (Fig. 2). It may make up to 25 to 60% of the dry weight of exo- and
76 1
Insect Integument
0
o
o�
�
� o
=
C - CH3
I
NH
CH20H
=
NH2
6 - CH3
Acetyl gltlcosamine Acetylglucosamine glucos31Tline
Fig. 2 . Part of the chitin chain.
endocuticle. Chitin is absent in epicuticle. It does not exist in a pure state
naturally, but is combined with a protein as a glycoprotein. Apparently
chitin chains are attached to one another by hydrogen bonds forming
elongate microfibrils and probably also link the 02 atoms of adjacent
acetylglucosamine residues. It seems likely that chitin microfibrils and
protein chains are in intimate combination with one another and that this
complex is impregnated with loosely bound protein.
Cuticular proteins usually make up more than 50% of the dry
weight of the cuticle. Included in this category are (i) arthropodins- a
group of soluble proteins; (ii) resilin- · a protein that forms a rubber like
framework and is found some times in pure form in skeletal articulation,
and (iii) sclerotins, a stabilised protein that is responsible for the hard
horny characters of the cuticle. In addition to above, polyhydric phenols
and quinones which play a role in sclerotisation (hardening) and
melanisation (darkening) processes, lipids of various sorts associated
with the epicuticle, enzymes which catalyse the many complex
biochemical reactions involved in the moulting and subsequent processes
and very small amount of inorganic compounds are also present in the
cuticle.
3. Sclerotisation of the cuticle. Chitin is not the agent responsible
for the hardness of the cuticle, although it undoubtedly lends strength to
it. In fact, highly sclerotised skeletal regions may contain less chitin than
softer membranous areas.
Immediately after ecdysis (shedding of the cuticle) the cuticle is
often soft and pliable, pale in colour and the cuticular proteins can
easily be extracted without degradation. After few hours later the cuticle
become hard and solid, darker in colour ranging from light amber to
dark brown or black, and only small amount of proteins can now be
extracted. The changes have been explained as due to chemical
modifications of the proteins whereby intermolecular cross-links are
introduced, and changes appear to be catalysed by enzymes present in
the cuticle.
Insect Integument
It
is
[ 77
difficult
to
draw
any
firm
conclusion
from
the
available
evidence and the processes are only hypothetical. From the evidences it
seems reasonable to conclude that insects have several ways of stablising
their cuticle, and different chemical processes occur in different insects
and in all parts of the cuticle of a single insect.
cross-
linking
cuticular
2. �-
Quinone-tanning,
formation.
In
the
proteins
sclerotisation
first
two
acetyldoparnine is utilised.
3.
and,
processes
reported
di-
the
3 kinds of
So far,
been
and
same
l.
trityrosine
substrate,
N­
Q uinone is derived from tyrosine as: tyrosine
--+ dihydroxyphenylalanine
N-acetyldopamine
have
--+ dopamine
(dopa)
quinone.
The
--+ N-acetyldopamine
N-acetyldopamine
(a
diphenol)
--+
passes
out through the pore canals of newly formed cuticle and concentrated
in
the
epicuticle
quinone tans
where
the
phenol
is
oxidised
the protein of the cuticulin and
to
a
diffuses
quinone.
The
inwards tanning
the proteins of the outer epicuticle forming exocuticle.
In tanning, the quinone forms links with the terminal amino groups
and the amino groups of dibasic amino acids in the protein molecule.
The quinone reacts with the N-terminal amino groups of the protein to
produce a catechol type protein.
It is
then
oxidised in the excess
of
quinone to a quinonoid protein which then links on to another protein
molecule. In this way end-to-end linkage between protein molecules are
produced. As a result of tanning the cuticle becomes hard and brittle.
The water soluble arthropodin is converted to the insoluble sclerotin. As
the cuticle hardens it usually also darkens due to sclerotin formation as
well as by polymerisation of excess quinorres to form melanin.
4. Physical properties of the cuticle. Insect cuticle is rigid, elastic,
permeable,
impermeable,
structure and
flexible
and so
on
as appropriate to a given
function. The physical characteristics
of the cuticle in
Rhodnius
given region may change perodically, e.g., when
a
(assassin bug)
takes a blood meal, a plasticising factor is secreted that changes the pH
of cuticle, thus the cuticle becomes more flexible that allows expansion
of the abdomen.
Insect colour may be due to the various pigments
5. Colouration.
present in the cuticle, scales on the cuticle, epidermal cells or fat body;
to
physical
characteristics
of
the
cuticle
and
the
scales,
or
their
combination. Colouration is commonly produced by complex mixture of
different pigments
ways.
Very
and
common
the
same
pigments
colour may
are:
be achieved
melanins
(yellow,
carotenoids (red and yellow) developed from plant tissues,
and
yellow,
pigments,
white),
ommochromes
anthroquinones
(red,
derivatives
(green),
chlorophyll
anthoxanthins
(whitish,
yellow),
(re�,
orange,
yellow,
yellow)
haemoglobin
brown)
different
black),
pterins (red
found
confined
derivatives
flavins (greenish yellow).
in
brown,
to
as
eye
aphids,
(reddish),
Some insects
78 I
Insect Integument
are able to change the colour reversibly which is accomplished with the
epidermal cells or in the cuticle.
6. Permeabilty. Insects being essentially terrestrial animals, are
continuously faced with the problem of losing water, especially in
extremely arid habitats. The small size of the insects makes this problem
more acute, since transpiration rate varies inversely with the ratio of
surface area to volume and hence the greater the tendency to lose
water. Epicuticle plays a vital role in integumental permeability. If a
portion of epicuticle is dissolved by means of organic solvents,
insecticides etc. increases the rate of transpiration resulting in death.
Moulting
The periodic shedding of cuticle followed by formation of new cuticle is a
mechanism facilitating growth despite a more or less inflexible integument.
At the onset of moulting the epidermal cells show much activjties in
increasing in size and number. During moulting epidermal cells separate
from the old cuticle (apolysis) and begin to secrete the new. Then the
epidermal cells secrete moulting fluid that contains chitinase and protease
digesting about 80-90% of the endocuticle. The digested material is
then absorbed by them. At apolysis a thin homogenous, transparent
excuvial membrane appears between epidermis and old cuticle. It is
resistant to moulting fluid. Figure 3A-F illustrate the changes occurring
in the integument during the moulting cycle .
Exo- and epicuticles are also resistant to the action of the moulting
fluid and make up the portion of the integument that is shed at ecdysis.
As the old endocuticle is digested forming the exuvial space, new cuticle
is deposited, cuticulin layer first, then the protein epicuticle and finally
exo- and endocuticle. The new cuticle is typically wrinkled beneath the
old indicative of the greater surface area to be occupied in the
expanded insect after the remaining old cuticle is shed that determines
the maximum to which cuticle can be expanded.
The wax layers are laid down shortly before ecdysis, assuring the
water proofing of the newly emerged insect. The cement layer is last
secreted by the dermal glands, the canal of which perforated the wax
layer and hence allow the cement layer to be formed over the wax
layer. Pore canals apparently serve as routes for secretion gf the wax
layer.
When the secretion of the new cuticle is complete, the insect
emerges, the act of ecdysis, leaving behind what remains of the old
cuticle and the tracheal and gland duct linings, e.g. exuvia. This process
is facilitated by ecdysial lines beneath which only epicuticle and
endocuticle are present. Since the endocuticle is digested during the
moulting process, a line of weakness develops. When ready to emerge
Insect Integument
.
J;<··: ?::i?.?: · ;.}�;;i\{\·;:\�;
·
:. >- : .:
.
...
.
. . .· . · ·
.
.
.
.
.
[ 79
epicuticle
exocuticle
. :__endocuticle
__
APOLYSIS
B
NEW CUTICULIN
LAYER PRODUCED
ENDOCUTICLE
DIGESTED
c
D
. . . . . . . .·. .·.·. :.·.·.·.·. . .·.· ·.·:.·
MOULTING FLUID
RESORBED
E
.
.._
ecdysial
membrane
REMAINS OLD
CUTICLE
CAST-OFF
F
Fig. 3. Diagrammatic representation of the changes occurring in the integument during the
moulting cycle.
the insect may gulp air, or water or increase the hydrostatic pressure of
the blood by contracting body muscles. These actions exert an internal
force of the ecdysial lines and subsequently the old cuticle splits
(Z-57)
Insect Integument
80 1
'" �
soc ket
setal membrane
epicuticle
. '!i 11.1�--'
__.,,.-J:
�:---'�.._;_ tormogen
ceH
trichogen cell
Fig. 4. A seta and its socket showing
trichogen and tormogen cells.
wherever they are located. These lines of weakness are usually located
on the dorsum of the head and thorax with an anterior and posterior
orientation. Following ecdysis an insect may consume the exuviae and
hence reclaim nearly all nutrients that may have been lost during
moulting. Sclerotisation and melanisation follow subsequent to ecdysis.
External Integumentary Processes
The integument of various insects bears a great number of different
external processes and these can be classed as non-cellular and cellular.
Non-cellular processes are composed entirely of cuticle and may take
any of several forms, such as spines, ridges, nodules, minute fixed hairs
(microtrichia) that lack the basal articulation. The cellular processes may
be unicellular or multicellular. Multicellular processes are hollow
outgrowth of the integument and are lined with epidermal cells taking
the form of spines (e.g., spines of hindtibiae of locusts). Unicellular
processes are all referred as setae (macrotrichia) with diversity of forms.
They are commonly hair like, but may be flattened into scales, may bear
branches and appear plumose etc. The setal shaft is formed by a
protoplasmic outgrowth of a specialised hair forming or trichogen cell.
This projection is surrounded by a setal membrane and lies within a
socket. The membrane and socket are formed by a second cell, the
tormogen cell (Fig. 4).
Important Questions
I.
2.
Give an account of histology of insect integument.
Write short notes on : (i) Sclerotisation of cuticle, (ii) Pore canal, ( iii) Moult ing,
( iv) Phy.;ical properties of cuticle.
(Z-57)
5
Segmentation and Body Regions
The insect body is heteronomously segmented: the segments in different
regions of the body differ in size, shape and other details. The body
segments are thus segregated into groups giving rise to tagmata or sections
of the body, viz., the head, thorax and abdomen.
Head
The insect head is a composite structure developed from the fusion of the
prostomium with six post-oral segments and is composed of a hardened
capsule, the cranium that bears the antennae, eyes and mouthparts. The
maxillary and labial segments are separated by the post-occipital suture of .
the cranium. The head is attached to the thorax by means of a flexible
membranous neck (cervix) that allows its movement.
[ I] Cranial structure
TQe cranium is divided into various regions by a series of sutures. The
epicranial suture is an inverted Y-shaped, the stem (the coronal suture)
forms the dorsal midline of the cranium and the arms (the frontal suture)
diverge ventrally across the anterior portion of the head. The region
between the frontal sutures is called the frons, and the dorsal portion of
the cranium is the vertex. These three sutures are lines along which the
shed cuticle of the cranium splits during ecdysis. The frons bears the
median ocellus, if present, and internally bears the origins of the muscles
of the anterior mouthpart, the labrum. The occipital suture forms a line
from the posterior termination of the coronal suture to just above the
mandibles on either side of the cranium. The postoccipital suture lies
(Z-57)
82 J
Segmentation and Body Regions
posterior to, and in the same plane as, the occipital suture. This suture
encircles the posterior opening of the head capsule, the foramen magnum,
through which the internal organs communicate between the head and
thorax. The postoccipital suture internally provides attachment to the
muscles that move the head. On either side of the cranium immediately
above the bases of the mandibles and maxillae are the subgenal sutures.
The area beneath the eyes and posterior to the frons is called as cheek or
gena. A postgena lies adjacent to the gena, but posterior to the occipital
suture. Dorsally, the region between the occipital and postoccipital sutures
is called the occiput. The plate posterior to the post-occipital suture, which
surrounds the better part of the foramen magnum, is the post-occiput. The
subgenal sutures may be connected across the front of the cranium, just
beneath the frontal suture, by the �pistomal suture, or fronto-clypeal
suture. The clypeus lies beneath the epistomal suture and is hinged
with the labrum. The ocular sutures commonly surround the compound
eyes. Similarly, an antenna! suture surrounds the base of each antenna
(Fig. 1 . A-C).
[ II] Tentoriuw
The head is strengthened internally by a set of sclerotised apodemes or
invagination of the body wall that have evolved primarily as more rigid
support for the attachment of muscles connected with the mouthparts. The
occiput
postoccipital
suture
maxilla
epistoma
sulcus
labium
labial palp
post occiput
postgena
maxilla
epistomal sulcus
Fig.
l . Generalised insect head. (A) Anterior view, (B) Lateral view, (C) Posterior view
and (D) Tentoriurn
(Z-57)
Segmentation and Body Regions
[ 83
tentorium provides many points for muscle attachment, makes the head
capsule rigid, and provides support for the brain. Typically the tentorium
is composed of a pair of anterior arms invaginated from the anterior
tentorial pits,
which fuse with the posterior arms invaginated from the
posterior tentorial pits. At the point of fusion (central mass), a tentorial
bridge, or corporotentorium is formed. In addition, a third pair of arms
may arise dorsally from the anterior arms (Fig. l D).
[ III] Compound eyes and ocelli
The compound eyes are located on each side of the head. It is composed
of many individual units, the ommatidium. The surface of each eye is
divided into a large number of usually hexagonal facets serving as corneal
lenses. The dorsal ocelli, or simple eyes, are commonly three in number
and are located on the anterior portion of the cranium, one on either side
of the coronal suture and the third between the frontal sutures. Details of
the compound eyes are given in Chapter 1 4.
[ IV] Antennae
Almost all adult insects bear a pair of movable, segmented and sensory
appendages called as antennae on the head between the compound eyes.
Typically, they are composed of three basic parts, the scape, the pedicel,
and the multi-segmented flagellum. The scape articulates the head capsule
with an antenna! socket (Fig. 2. A).
Antennae are extremely
flagellum
and
they
can
varied
in
usually be
(Fig. 2. B-N) : tilifonn (threadlike,
shape
and size mostly
described
e.g.,
in
using following
cockroaches),
the
terms
setaceous
(bristlelike, tapering, e.g., dragonfly, crickets), monilifonn (beadlike, e.g.,
termites),
serrate (sawlike, e.g., click beetle), pectinate (comblike, e.g.,
bipectinate (double comblike, e.g., silkworm), clavate (the
distal segments gradually increased in diameter, e.g., butterflies), capitate
sawflies),
(the distal segments suddenly increased in diameter, e.g., khapra beetle),
lamellate (the distal segments expanded laterally, e.g., dung beetle),
tlabellate (the distal segments have long parallel-sided, sheet-like lobes
extending laterally, e.g., sandalid beetle), geniculate (elbowed, e.g., ants),
plumose (flagellar segments with whorl of long bushy hairs, e.g., male
mosquitoes), pilose (flagellar segments with whorl of long sparse hairs,
e.g., female mosquitoes), aristate (last segment enlarged bearing a
dorsal bristle, the arista, e.g., house flies), stylate (last segment bearing
an elongated terminal stylelike process, e.g., robber flies).
In
certain insects,
mosquitoes.
insects
Antennal
antennae
structure
are
useful
is closely
antennae serve exclusively
in
sex
related to
identification,
function.
e.g.,
In most
as sensory structures but in certain
Segmentation and Body Regions
84 J
antenna! socket
E
F
K
Fig. :l. Different types of insect antennae. (A) Typical, ( B ) Filliform, (C) Setaceous,
(D) Moniliform, (E) Serrate, (F) Clavate, (G) Plumose, ( H ) Aristate, (I) Stylate,
(J) Pectinate, ( K) Capitate, (L) Lamellate, (M) Flabellate, and (N) Geniculate
insects it is used for prey capture. It is also used by male insect of
some species as claspers to hold females during copulation.
[ V] Mouthparts (typical mandibulate mouthparts)
The mandibulate mouthparts are considered to
be
the primitive form. They
typically consist of an anterior upper lip or labrum, the hypopharynx, a pair
of mandibles, a pair o{ maxillae and a posterior lower lip or labium. The
mandibles, maxillae and labium represent modification of typical paired
appendages of three primitive segments (Fig. 3).
1. Labrum. Typically this is a movable flap hanging down from the
edge of the clypeus and covering the mouth. Its inner side forms the
front of the pre-oral cavity and is called the epipharynx.
2. Hypopharynx. The hypopharynx is an unsegmented outgrowth of
the body wall and lies in the preoral cavity like a tongue. The portion
of
the
preoral
cavity
between
the
hypopharynx
and
labrum
is
the
Segmentation and Body Regions
Q
'
.
mandible
[ 85
n
t;7:
labrum
kl
m
·
molar cusps
cisor cusps
cardo
hypopharynx
maxilla
labium
Fig. 3. Typical mandibulate mouthparts of
as
exemplified by cockroach.
cibarium. The portion of the preoral cavity between the hypopharynx
and labium forms the salivarium.
3. Mandibles. The mandibles are the paired appendages of fourth
head segment behind the mouth and are highly sclerotised unsegmented
jaws. Each mandible forms two articulations with the cranium. Each
mandible has a proximal molar or grinding and a distal incisor or
cutting regions. The palp is absent.
4. Maxillae. The maxillae lie directly behind the mandibles and are
the paired appendages of fifth head segment and serve as accessory
jaws, helping in holding and chewing food. Each maxilla is composed of
the following parts.
(a) Cardo. It is the triangular basal sclerite that is attached to the
head capsule, and that serves as a hinge for the movement of the rest
of the maxilla.
(b) Stipes. Stipes lies above the cardo and is the central part of the
maxilla and somewhat rectangular in shape. It 1s the basis for the
remaining parts of the maxilla.
(c) Galea. The galea is the outer (lateral) lobe attached at the end
of stipes and functions as sensory pad.
(d) Lacinia. The lacinia is the inner lobe attached at the end of stipes and is mandible-like in general form with a series of spines or
teeth along its inner edge.
86 1
Segmentation and Body Regions
(e) Palpus. The palpus is an antenna-like segmented (usually 5
segments) sensory appendage attached laterally to the stipes through a
sclerite, the palpifer.
S. Labium. The labium is a composite structure formed from the
fusion of two primitive segmental appendages of sixth head segment. It
appears to be a single unit but really it consists of a second P.air of
maxillae that have fused on the inner side to form a single functional
structure. It consists of a basal postmentum attached to the cervix
ventral to the foramen magnum and is commonly divided transversely
into two portions, a proximal submentum and a distal mentum. The
apical portion of the labium is the prementum, which is hinged to the
postmentum by the labial suture. The prementum bears laterally a pair
of segmented labial palpi and distally four lobes, two inner lobes, the
glossae, and two outer lobes, the paraglossae. The labial palpi are
attached to lateral scierites on the prelabium, the palpigers.
The muscles responsible for the movement of the mouthparts are
attached at various points on the head capsule and tentorium.
Types of mouthparts
Insect mouthparts have become modified in various groups to perform the
ingestion of different types of food and by different methods. Indeed the
modifications in the mouthparts to ingest almost all kinds of the food
material, are one of the factors for the success of the group. Following are
the most interesting types.
1. Chewing type. The primitive type of mouthparts, as descirbed
above, is the generalised one from which the other types developed. The
mandibles cut off and grind solid food, and the maxillae and labium
push it into the oesophagous. It is found not only in primitive orders of
the Insecta like Thysanura, Orthoptera, Dictyoptera, Isoptera but also in
developed orders like Coleoptera and most of the Hymenoptera. Even
the larvae of Lepidoptera have chewing type of mouthparts.
2. Sponging type. A large number of the non-biting flies, e.g., house
flies, have this type, adapted for ingesting only foods that are either
liquid or readily soluble in saliva. The mouthparts comprise a fleshy and
retractile proboscis which lies under the head and is formed by three
components, the basal rostrum (basiproboscis),
middle haustellum
(mediproboscis) and the distal pair of labella (distiproboscis). The
mandibles are absent and maxillae are represented by its palpi that arise
at the distal end of the rostrum. The labrum and hypopharynx are
slender and lie in an anterior groove of the labium, which forms the
bulk of haustellum. The salivary channel is in the hypopharynx, and the
food channel lies between the labrum and the hypopharynx. At the apex
of the labium are labella, a pair of large, soft, oval lobes. The lower
Segmentation and Body Regions
Fig.
4.
[ 87
(A) Sponging type of mouthparts (house fly). (B) Piercing and sucking type of
mouthparts
(female
mosquito)
surface of these lobes bear numerous transverse grooves which serve as
food channel. The proboscis can usually be folded up against the lower
side of the head. The flies sip the
liquid food; this food may be
already in liquid form, or it may first be liquefied by salivary secretions
of the fly (Fig. 4. A).
3. Piercing and sucking type. This type of mouthparts are found in
female mosquitoes in which paired mandibles, paired maxillae,
labrum-epipharynx and hypopharynx, all
six parts are modified to
needle-like stylets ensheathed by the broad tubular labium, the
proboscis. The labrum-epipharynx, formed by the fusion of labrum and
epipharynx, is the dorsal most stylet covering the opening of the groove
of proboscis. The inner depression due to which it appears C-shaped in
transverse section, function as food channel. Hypopharynx is somewhat
flattened and double edged, sword-like stylet cover over the food
channel and has a salivary duct inside. Needle-like mandibles lie on
each side of the labium-epipharynx, the distal end being serrated for
pricking the host skin. The maxillae are also needle-like, distally serrated
and located laterally beneath the hypopharynx in the groove of the
proboscis (Fig. 4. B).
4. Siphoning type. This type of mouthparts are found in butterflies
and moth. The long coiled proboscis is formed by the two galeae of the
maxillae; the food channel is between tl1e galeae. The labrum is reduced
to a narrow transverse band across the lower margin of the face, and
Segmentation and Body Regions
88 J
A
Fig. 5. (A) Siphoning type of mouthparts (bunerfly). (B) Chewing and lapping type of
mouthparts (honey bees).
the mandibles and hypopharynx are lacking. The maxillary palpi are
usually reduced or absent, but the labial palpi are usually well
developed. There is no special salivary channel. The liquid food is
sucked or siphoned up through the proboscis. When used, the proboscis
is uncoiled by blood pressure; it recoils by its own elasticity (Fig. 5. A).
5. Chewing-lapping type. Such type of mouthparts are also adapted
for taking liquid food and found in honeybees and wasps. The
mandibles and labrum are of the chewing type and are used for
grasping prey (as in wasps) or moulding wax (as in worker bees). The
maxillae and labium form flattened elongate structures, of which the
glossa forms an extensile channelled organ with a small labellum at the
tip. This latter is used to probe deep into nectaries of flowers. The
other flaps of the maxillae and labium fit up against the glossa and
forms salivary channel (Fig. 5. B).
Position of the mouthparts
There are basically three positions of the mouthparts relative to the head
capsule. The mouthparts either hang ventrally from the head capsule, e.g.,
cockroaches, grasshoppers (hypognathous condition), considered to be the
most primitive condition as the mouthparts are apparently modified
locomotor appendages and have retained a similar position relative to the
insect body; or projected anteriorly, e.g. termites (prognathous condition);
Segmentation and Body Regions
or directed ventroposteriorly relative to the head capsule,
( opisthognathous condition).
{ 89
e.g.,
bugs
Thorax
The insect thorax is composed of three segments: an anterior prothorax, a
middle mesothorax, and a posterior metathorax. Each segment bears a pair
of legs. The last two segments often called as pterothorax may bear wings.
In most winged insects the prothorax is usually separate froni, and
somewhat less developed than, the remaining segments. In many insects at
least part of the first abdominal segment has become intimately associated
with the thorax, and in many of the Hymenoptera it has literally become a
part of the thorax, being separated from the rest of the abdomen by a
constriction.
Each thoracic segment typically can be divided into four distinct
regions: a dorsal tergum, or notum; a pair of bilateral pleura (sing.,
pleuron); and a ventral sternum. Each of these regions is commonly
sub-divided into two or more sclerites. The legs arise on the pleura; the
wings articulate between the notal and pleural regions. Spiracles are
usually found one in each of the pleural regions between the prothorax
and mesothorax, and between mesothorax and metathorax.
[ I] Legs
The typical thoracic leg consists of six parts, basal coxa that articulates with
the thorax in the pleural region, small trochanter, femur, tibia, segmented
tarsus, and pretarsus.
The coxa is often divided into two parts, the posterior and the
anterior (usually the larger part) being called the meron. The trochanter
articulates with the coxa, but usually forms an immovable attachment
with the femur. The femur and tibia are typically the longest leg
segments. The tarsus, which is derived from a single segment, - is usually
sub-divided into individual tarsomeres. The pretarsus may consist of a
single claw, but it is usually composed of a pair of moveable claws and
one or more pads or bristles. Legs are usually looked upon as the
principal organs of terrestrial locomotion. They have undergone many
modificati.ons and have been adapted to a wide variety of functions
including swimming, prey capture, pollen collection and digging.
1. Cursorial legs. The simple unmodified legs are walking type or
running type as found in cockroaches (Fig. 6A).
2. Fossorial legs. The legs modified for digging are best known in
mole cricket (Gryllotalpa) and dung beetles. In Gryllotalpa the foreleg is
very short and broad, the tibia and tarsomeres bearing stout lobes which
are used in digging (Fig. 6B).
Segmentation a11d Body Regions
90 J
A
D
�
��
F
Fig. 6. Types of legs. (A) Cursorial,
(B)
Fossorial, (C)
Raptorial, (D)
Saltatorial,
(E) Pollen carrying, (F) Clinging, and (G) Natatorial.
3. Raptorial legs. This type of leg modification is found in
predatory insects, e.g., praying mantids, water scorpions (nepid bugs). In
mantids the fore legs are modified to capture prey. The coxae are
elongate and mobile while the femora are thickly spinose and grooved
along their lower side. The tibiae, which are also spinose, can fit into
the grooves. While waiting for a prey, its forelegs are held folded
against the prothorax. When the prey is in capturing range, spined
forelegs suddenly shoot forward to seize the prey. Held in a pincer-like
grip, the prey is brought the jaws and devoured (Fig. 6C).
4. Saltatorial legs. The femora of the hindlegs of grasshoppers and
katydids are enlarged, accommodating the muscles used in jumping
(Fig. 6. D).
5. Pollen-carrying legs. The hindlegs of the honeybees are adapted
for carrying pollen. The hindtibia is more or less dilated and either
bears a large pollen brush or scopa or is margined with long hairs,
being thus modified to form a corbicula or pollen basket. The basitarsus
is flattened on its inner aspect, and provided with several rows of short
stiff spines which form a brush; by means of latter the bees collects the
pollen adhering to the body hairs. When sufficient quantity has
accumulated on the brushes, it is scraped off over the edge of the
hindtibia of opposite side and stored in the pollen basket (Fig. 6. E).
6. Clinging legs. Clinging legs are found in lice and is adapted to
grip the hairs of the host. The tarsi are single-segmented, and each
ends in a powerful claw which works against a tibial process (Fig. 6. F).
7. Natatorial legs. The legs of several aquatic insects are modified
in such a way that they facilitate swimming, e.g., water beetles which
Segmentation and Body Regions
{ 91
bear two rows of ' 'swimming hairs' ' on the edges of the flattened tibiae
and tarsi of the middle and hindlegs. During swimming, they greatly
expand the surface area being applied against the water in the paddling
action (Fig. 6. G).
[ II] Wings
The insects are the only invertebrates which are magnificently characterised
by having the wings. Indeed the success of the insects as terrestrial animals
is at least partly due to their ability to fly. Except the apterygotes and some
secondarily wingless pterygotes all modem insects bear laterally a pair of
wings on each meso- and metathorax and are called fore- and hindwings,
respectively.
The wings are outgrowths of the body wall located dorsolaterally
between the tergal and pleural sclerites. They are thus composed of two
layers of the integument. At the beginning, the wings arise as saclike
outgrowths, but in adults they are solid structures, with the only cavities
being those of veins which are formed by the tracheation. The cuticle is
often thicker in the region of these veins, lending further rigidity. The
body cavity, or haemocoel, of the insect is evident only around the
veins, because in the other parts of the wing, the two layers of body
wall become closely adhered to one another. The wings are, of course,
the organs of aerial locomotion in most cases, but, like the legs, they
have undergone extensive adaptive modification.
1. Morphological variations. Hindwings in Diptera are modified into
the dumb-bell-shaped structures, the halteres. In some groups of insects,
for example, Psocoptera, Mallophaga, Anoplura, Siphonaptera and some
Hemiptera, both pairs of wings are lost completely providing apterous
condition suitable for their parasitic mode of life. Both pairs of wings of
termites resemble one another in their shape and size. In termites as
well as in black ants, the wings are present only in virgin reproductive
individuals which are shed after the nuptial flight.
The forewings appear to be extensively sclerotised in Orthoptera,
Dictyoptera and Coleoptera and are commonly called as the elytra or
tegmina while in Heteroptera only the basal part of the wings is
sclerotised forming the hemelytra. The elytra are leathery or hard
structures and provide protection to the hindwings.
2. Structure. The wings appear as the thin or thick, transparent or
leathery, partially pigmented or darkly sclerotised fan-like, flattened
membranous structures. The wings bear group of sclerites at their base,
a complex of longitudinal and cross veins throughout the wing-body, and
various types of sense organs and pigments. The basal sclerites help in
articulation of wings with the thorax as they rotate with one another.
The wings bear a specific pattern of venation, which is derived from
92 J
Segmentation and Body Regions
pterostigma
costal margin
vannal fold
_}
7----�-..,_
--
anal margin
apical angle
,,.,,., m'"''"
anal angle
A
r--:--'"-----I R4
Rs
axillary
sclentes
__,..____,--c
Cu 4fp<
·
Cu.,,, 06 '<1
c
MP1
M4 1
M-4.?
~
�
Hamuli
�
E
�
frenulum
hlod-Mng
F
Fig. 7. (A) General features of the wing, (B) Wing bases, (C) Hypothetical wing venation,
(D) Coupling mechanism in honeybee, (E) Jugate wing coupling in moth, and (F) Frenate
wing coupling in moth.
Segmentation and Body Regions
[ 93
unique arrangement of veins. The veins provide mechanical support and
folding to the wings. The sense organs are mostly concentrated at the
base of the wing at proximal parts of the veins. They play important
role in determination of direction of the wind, wing movements and
navigation during flight. The wings have often smooth dorsal surface but
in the Lepidoptera they are covered with the scales while in the
Trichoptera and Diptera with the hairs which provide the brilliant
pigmentation and hairy covering respectively.
(a) The wing areas or regions. The edges, or margins, of wings are
named as the anterior margin, or costal margin, the posterior margin, or
anal margin; and the outer margin, or apical margin. The entire body of
wing is differentiated broadly into the basal articular or axillary region
and rest as the alar region. In Diptera, pair of membranous lobes at
posterior margin of wing base are predominantly evident and are known
as the outer and inner squamma. They constitute the area known as the
alula. It is well developed in the housefly. The angle between the costal
and apical margin is the apical angle, that between the outer and anal
margin is the anal angle, while the angle at the base of the wing is
called the humeral angle (Fig. 7. A).
(b) The wing bases. In most of the insects several sclerites are
uniformly grouped at the wing base of each wing. They form a
composite hinge by which the wing articulates with the thorax. They
include the tegula, the axillary cord, axillary sclerites, and variable forms
of skeletal plates. The tegula are mostly confined to the forewings,
particularly in the Lepidoptera, Hymenoptera and Diptera, each lies at
the base of the costal vein as a small hairy chitinous pad or a scale-like
sclerite. The axillary cord is lobe-like structure lying closely with the
posterior region of the wing. There are three axillaries at the base of
wings. The first axillary lies at the base of the subcostal vein. It
articulates with the anterior notal wing process and sometimes divided
into two parts. The second axillary lies at the base of the radius vein
and articulates with the first axillary proximally and with the wing
process of the pleuron distally. The third axillary lies between the base
of the anal veins and the fourth axillary. It articulates directly with the
posterior notal wing process. The additional fourth axillary is present
only in the Orthoptera and Hymenoptera (Fig. 7. B).
The median plates occur in pair. One of the median plates is
associated with the bases of the media, cubitus and the first anal veins,
while the other is often fused or closely opposed with the third axillary.
In dragonflies, besides the anterior well-developed humeral plate, there
is a posterior plate, the axillary plate, at the bases of the four
post-costal veins. It articulates with the posterior half of the lateral
94 1
Segmentation and Body Regions
membranous margin and with one of the arms of the pleural wing
process.
(c) The wing venation. The veins are hollow, tubular, s�lerotised
structures and each is typically provided with a nerve, trachea and the
circulating blood. The precursors of the veins in the nymphal wing pads
are the lacunae, free spaces that are surrounded by the spongy
columnar epidermal cells. The wing-venation represents the complex
organisation of veins within the wing. The veins may be unbranched, or
branched, isolated or jointed with one another by cross veins and
convex or concave. Comstock ( 1 9 1 8) and Hamilton ( 1972) have dealt in
detail with wing venation. On the basis of morphological characteristic
features, the veins of the generalised insects representing simple system
of venation, the archaetype venation, can be described as follows.
Comstock-Needham system of nomenclature was follgwed (Fig. 7. C) :
(i) The costa (C). It is the first anterior marginal vein due to which
the anterior margin of the wing is commonly designated as the costal
margin. It starts from the humeral plate. It is the convex vein and may
bear sometimes, the stigma as can be seen on the fore- and hindwings
of dragonflies, and only on the forewings of the Hymenoptera.
(ii) The subcosta (Sc). It is the second vein and distally divided into
two branches. The outer branch and the inner branch are usually
designated as the Sc1 and Sc2, respectively. It is the concave vein.
(iii) The radius (R). It starts from the anterior end of the second
axillary sclerite at the base of the wing. After running for a short
distance, it divides into outer branch R1 (convex vein) reaching the
outer margin. The second branch, the radial sector (Rs) (concave vein)
divides into four branches, R2, R3, R4 and R5.
(iv) The media (M). It is the fourth major vein, starting from the
distal medial plate and mostly fused with the radius. At the base, it is
largely sclerotised and convex and after running for a short distance it
gives two branches, the anterior (MA) and posteriors media (MP),
respectively. MA further divides into two convex branches MA1 and
MA2 while MP on the contrary is the concave and divides into four
branches MP1, MP2, MP3 and MP 4.
(v) The cubitus (Cu). It is the fifth vein and starts from the distal
medial plate at the wing base. At the beginning it is a convex vein but
later on it divides into the first cubitus (Cui- convex) and the second
cubitus (Cu2- concave) veins.
(vi) The anal or vennal veins (A). These veins start from the base of
the third axillary sclerite and constitute the l A to 3A convex and
unbranched veins in the anal region.
(vii) The jugal veins (J). Commonly, there are two unbranched jugal
veins J 1 and J2 in the jugal lobe of the wing of some insects.
Segmentation and Body Regions
[ 95
(viii) The cross veins. The various types of longitudinal veins
described above are linked with one another by the cross-veins. The
cross-veins run in vertical plane and are confined to the adjacent
longitudinal veins only. From their location, these veins are termed as :
(a} humeral cross-vein (h) lying between the costa and subcosta,
(b) radial cross-vein (r) lying between the first and second branch of
radius, (c) sectorial cross-vein (s) lying between the second or radius
sector, (d) radio-medial cross-veins (r-m) lying between the radius and
media veins, (e) media cross-veins (m) lying between the branches of
the posterior media, and (f) media-cubital cross-vein (m-cu) lying
between the media and cubitus.
Due to unique distribution of longitudinal and cross veins, the wing
becomes divided into a large number of small spaces or cells. They can
be divided into two types; basal cells lying towards the base of wings
and the distal cells lying in between the branches of principal veins. The
venation of wings is greatly modified in different groups of insects.
(d) Wing coupling apparatus. In several insects the fore- and
hindwings of either side are coupled with each other so that they move
together as a single unit. The wing coupling is done mostly with the
help of lobes or spines lying at the wing and a humeral lobe at the
base of costal margin of hindwing. Both lobes contain setae. The
humeral lobe specially bears the frenular bristles. From this primitive
type of wing coupling mechanism, complex types have been evolved.
Some Trictloptera have a strong jugal lobe which lies beneath the
costal margin of the hindwing so that this is held between the jugum and
the rest of the forewing. This is called jugate wing coupling (Fig. 7. E). In
Micropterygidae the jugum is folded under the forewings and holds the
frenular bristles. This is jugo-frenate coupling.
Many other Lepidoptera have the frenulum well developed and
engaging with a catch or retinaculum on the underside of the forewing so
that the wings are firmly coupled. This is frenate coupling (Fig. 7. F).
Female noctuids, for instance, have from 2 to 20 frenular bristles and a
retinaculum of forwardly directed hairs on the underside of the cubital
vein, in the male the frenular bristles. are fused together to form a single
stout spine and the retinaculum is a cuticular clasp projecting down from
the radial or subcostal.
Other insects have the wings coupled by more distal modifications
which hold the costal margin of the hindwing to the anal margin of the
forewing. Thus, Hymenoptera have a row of hooks, the hamuli, along the
costal margin of the hind wing which catch into a fold of the forewing
(Fig. 7. D).
(Z-57)
96 1
Segmentation and Body Regions
Abdomen
The primitive number of abdominal segments is considered to have been
1 1 true metameres plus a terminal segment, the periproct or telson, that
contained the anus. The tendency in insectan evolution has been toward a
reduction in the number of segments, and in the generalised insect
abdomen there are 1 1 segments, the eleventh being reduced and divided
into lobes that surround the anus. This terminal segment may bear a pair
of appendages, the cerci. These are considered to be serially homologous
with the legs and mouthparts. The plates of the eleventh segment are
generally three in number, the epiproct dorsal to the anus, and the two
paraprocts . on either side of the anus. Spiracles, the external openings of
the respiratory system, are typically found one on either side of the first
eight abdominal segments.
In the generalised female pterygote insect, modified appendages of the
eighth and ninth abdominal segments form the ovipositor, or egg-laying
apparatus, which is composed of two pairs of basal valvifers. The valvifers
in turn bear the valvulae, one pair on the eighth segmental appendage and
two pairs on the ninth segmental appendage. The female gonopore is
usually on or posterior to the eighth or ninth segment. The male external
copulatory apparatus, penis, or aedeagus, is usually borne on the ninth
abdominal segment.
[I] Non-genital abdominal appendages
Unlike the pregenital and genital segments in adult pterygotes, the
pregenital and genital segments of many larval pterygotes and many
apterygote insects bear appendages of various sorts. For example, certain
abdominal segments of adult thysanurans and proturans bear simple styli.
Collembolans bear three unique abdominal structures, an anterior
collophore (supposed to be the organ of adhesion) is located on the venter
of the first abdominal segment, and two posterior structures, the tenaculum
and the furcula on the venter of the third and fifth segments, respectively.
These two structures help the insect in springing through the air (hence
they are known as spring-tails).
The structure of the terminal abdominal segments (e.g., the cerci
and the epiprocts and paraprocts) is variously modified. For example,
the cerci may be forcep-like or clasperlike (earwigs), feelerlike
(crickets), reduced (cockroaches), or absent (in bugs, endopterygotes).
[ II] Genital abdominal appendages (external genitalia)
External genitalia are copulatory apparatus in male and ovipositor in
females. These structures are formed by the modification of certain
abdominal segments. In the generalised female pterygote insect, ovipositor
(Z-57)
Segmentation and Body Regions
[ 97
is formed by the modification of the eighth and ninth abdominal segments
and is composed of two pairs of basal val vifers, that bear the valvulae, one
pair on eighth and two pairs on ninth segmental appendages. The female
gonopore is usually on or posterior to the eighth or ninth segment. The
male copulatory apparatus is penis or aedeagus and is usually borne on
ninth abdominal segment.
Female external genitalia. It consists of three pairs of valve s
1.
which
form
the
co-adaptation
deposited.
of
ovipositor.
the
valves
The
varies
degree
of
according
Female insects deposit their eggs
development
to
in
the
one
ways
and
eggs
of the three
are
ways:
directly from the external opening of the reproductive system, by use of
tubular
reversible
abdomen ,
considered to
be prototypic
derived,
Collembola
e.g.,
and
by
method
lay
use
in
eggs
of
an
insects.
directly
ovipositor,
which
is
Remaining methods are
from
the
genital
opening
because at no point in their phylogeny has an ovipositor been developed
whereas moths
lay
eggs
in
the
same
way
because
the
ovipositor has
been lost.
Thysanura
present
a
generalised
structure
and
possess
a
very
primitive ovipositor composed of paired appendages borne on the venter
of eighth and ninth abdominal segment (Fig. 8A).
Each appendage or
gonopod or gonapophysis or valvulae is borne on a basal coxopodite or
gon�coxae which may or may not bear stylus. The second gonapophysis
of the
two
pass
At
�.
sides
the
gonangulum
segment
are
united
base
derived
attached
forming
of the
from
with
ovipositor
the
the
a
tube
there
through
is
a
which
small
anterior part of exopodite
first
gonapophysis
and
the
egg
sclerite,
of the
articulate
the
ninth
with
the
second gonocoxae and tergum of ninth segment.
The ovipositor of Orthoptera represents a basic pattern found in the
more advanced pterygote orders (Fig. 8B). It is made of the coxopodite
or gonocoxae
or coxae
or valvifers
of the eighth
and
ninth
segments
having three elongated processes, i.e., first, second and third valvulae.
In
certain orthopterans with only two pairs of valvulae it is the second pair
that is missing. In locusts, the third pair is blade like not ensheathing
the distal part of the ovipositor shaft formed by the first and second
pairs
of
valvulae,
as
they
do
in
other
higher
insects
having
well
developed ovi�ositor.
The Hymenoptera have the full set of six valvulae but a great deal
of variation is found. In the sawfly the basic structure of the ovipositor
is similar to that of other Hymenoptera but the ovipositor has a form
different
from
that
of
a
typical . hymenopterous
ovipositor
which
is
usually long and slender. The -shaft, which is composed of the first and
second
pairs
strong
lateral
of valvulae,
ridges.
It
is
pair of valvulae (Fig. 8C).
is
short and broad with an
usually
acute
apex and
ensheathed between the broad third
(Z-57)
98: J
Segmentation and Body Region:
anus
cercus
A
valvifer 1
poison gland
rami of
gonapophysis
gonapophysis 1
oancet)
D
Fig.
8.
Modification
of
female
genitalia.
(A)
Firebat
(Thysanura),
(B)
Katydid
(01thorptera), (C) Sawfly (Hymenoptera), (D) Honey bee and (E) House fly.
In bees and other stinging Hymenoptera the ovipositor is specialised
as a stinging apparatus instead of being an egg-laying organ. The
stinging apparatus consists of the basal part of the ovipositor and the
adjoining sclerites of the ninth tergum as well as the spiracular plates of
the eighth tergum. Th� anterior pair of valvulae forms the lancet which
is modified for introducing the liquid venom through the shaft. The
posterior valvulae are united to form a trough-like organ, the proximal
part of which, is swollen to form bulb-like base of the shaft while the
distal part, the stylet, is slender. The venom is discharged into the base
of the bulb. At rest the sting lies retracted into the seventh tergum
ensheathed between the lateral valvulae (Fig. 8D). During stinging, the
sting is pushed out of the chamber and deflected at right angle. The
eggs are pushed out not through the lumen of the shaft as in
(Z-57)
Segmentation and Body Regions
[ 99
non-stinging Hymenoptera but are ejected through the gonopore at the
base of the ovipositor.
In house fly, the ovipositor is absent and for egg laying the posterior
abdominal segments being telescopic, projected and the eggs are
discharged from the gonopore (Fig. 8E).
2. Male external genitalia. The male external genitalia are used in
holding the female genitalia for sperm transfer. The principal male
genitalia in advanced pterygotes consists of a pair of moveable
appendages or plates at the ventro-posterior surface of ninth segment
and is generally called claspers or harpogones or subgenital plates or
phallomeres or phallic lobes or phallus or gonapophysis. The phallic
lobes divide to form an inner mesomeres and outer parameres. The
mesomeres unite to form aedeagus, an intromittent organ. The inner
wall of the distal part of the aedeagus is in continuation of the
ejaculatory duct and is called endophallus (Fig. 9A). The opening of the
duct at the tip of the aedeagus is called as phallotreme. The true
gonopore is at the junction of the ejaculatory duct and endophallus and
hence is internal, but in many insects the duct opens externally during
copulation. The parameres develop into claspers which are very variable
in form and shape. The claspers are coxites and styles of the ninth
segment or only styles in some insects. The coxites are fused with the
sternum of the ninth segment.
The parameres and mesomeres may be mounted on a common
basal plate, the phallobase. The term phallus invariably used with male
genitalia mean either the parameres together with aedeagus, but it is
either restricts only of aedeagus alone. The endophallus together with
phallotereme is termed as penis. Although penis is some times used
instead of phallus.
In Thysanura, the genitalia are very simple in structure and the
phallus is differentiated in to proximal phallobase and the distal
aedeagus. The coxites of the ninth sternum are prolonged into a pair of
appendages with slender, fingerlink style which form the clasper
(Fig. 9B).
The Orthoptera and Dictyoptera deviate from the basic pattern in
that the phallomeres do not form a typical aedeagus and parameres.
Phallic lobes are 3 in number and probably median one represents
aedeagus. The remaining two parameres of which the right one is
complex in Dictyoptera having basal paired plates opposing each other
along with distal plate. The proximal part of distal plate is serrated and
distal part has a hook. The left paramere is also composed by complex
structures, having four processes: a long hooked titilator, a slender
pseudopenis, a spiny asperate lobe, and a hooked accutolobus (Fig. 9C).
Segmentation and Body Regions
J OO ]
vas deferens
endophallus
aedeagus
primary phallic lobe
Origin and development of male genitalia
stemite 9
steri. ,ite 8
aedeagus
gonopophysis
stylus
A
left
phallomere
ventral
phallomere
right
phallomere
tergum 9
alimentary
canal
anal style
c
D
Fig. 9. Origin, development and modification of male genitalia. (A) First gonopods showing
gonapophyses of eighth segment and (B) Second gonopods and median aedeagus of
bristle-tails (Thysanura), (C) Cockroach, and (D) Mosquito.
Segmentation and Body Regions
[ 101
In Diptera, the genitalia are very complicated. In most of them the
terminal segments rotated at 1 80° or 360° so that the related positions
of the genitalia are changed. In mosquitoes, the terminal segments are
1 800 soon after emergence. Thus the aedeagus comes to above the anus
instead of below it and the hindgut is twisted over the reproductive
ducts. In house fly, the terminal segments have rotated through 360° so
that the genitalia are in their normal position ·but the movement is
indicated by some asymmetry of the sclerites and by ejaculatory duct
looping right round the gut (Fig. 9D).
Important Questions
1.
2.
3.
4.
Describe different types of mouthparts of insects.
Give an account of structure and venation of insect wings.
Describe the external genitalia of either male or female msects.
Wnte short notes on : (i) Structural modification of insect antennae; (ii) Adaptive
modification of legs of insects; (iii) Sutures of head capsule; (iv) Tentorium and
(v) Wing coupling.
6
Digestive System
Digestive system is composed of alimentary canal (digestive tract) and
various glands related with it either directly (salivary glands, gastric caeca),
or indirectly (Malpighian tubules).
Description of anatomy, histology etc. of a generalised insect is
based on the orthopteroid plan (e.g., grasshoppers, crickets, and locusts)
as it exemplify the basic, primitive design from which other insect
groups evolved modifications and specialisations.
One of the major
reasons for the biological success of insects is their ability to eat, digest,
and utilise an enormous diversity of foods. This ability allows the
extreme diversity observed in the modifications and specialisations of the
alimentary
system
of
insects.
The
structural
and
biochemical
modifications of the alimentary system of a particular species depend
upon the type of food eaten. There are structural and functional
differences in the way foods are obtained, stored, processed, and
absorbed between the different life stages and between the sexes, e.g.,
caterpillars chew up plant material, whereas adults suck up only floral
nectar and female mosquitoes suck up a vertebrate blood, whereas
males suck up plant sap.
Alimentary Canal
The alimentary canal of the insects is a tube, which extends from the
anterior oral opening, the mouth, to the posterior anus. The gut is formed
by a one-cell-thick layer of epithelial cells lying on a non-cellular basement
membrane (basal lamina). Ingestion, trituration (chewing), digestion,
absorption into the haemolymph, and egestion are associated with it.
Pigestive System
[ 1 03
foregut
midgut
hindgut
to right salivary
gland and reservoir
foregut intima
salivary duct
oesophagus
hypopharynx
pharynx
pyloric
valve
proventriculus
peritrophic
membrane
Fig. 1. Alimentary canal of a generalised msect
Alimentary canal (Fig. 1) is divided into three distinct regions, the
anterior foregut (stomodaeum), the midgut (mesenteron) and the
posterior hindgut (proctodaeum). The foregut and hindgut arise as
invaginations of the ectoderm, are lined with a chitinous intima, which is
continuous with the cuticle of the integument and therefore, at the
moult, both foregut and hindgut and their contents are shed. The
midgut is derived from endoderm. Longitudinal and circular (intrinsic)
muscles are associated with the alimentary canal. The anterior
alimentary canal muscles are innervated by the stomatogastric system,
and the posterior muscles are innervated by nerves from the posterior
ganglion of the ventral chain. In addition to muscles, tracheae and
tracheoles provide support for the gut. The alimentary canal tends to be
shorter in species that exist on high-protein diets (carnivorous) and
longer in those with high-carbohydrate diets (phytophagous).
[ I] Foregut or stomodeum
Foregut with its various morphological divisions serves mainly as a
conducting tube, carrying food from the pre�oral cavity to Hie midgut. The
mouth lies at the base of hypopharynx within the preoral cavity (cibarium)
formed by the mouthparts. It communicates directly with the pharynx that
varies greatly among different insects. In sucking insects (e.g., mosquitoes,
bugs) the cibarium and pharynx both form suctorial pump with
well-developed dialator muscles. The pharynx leads to the oesophagus,
which is commonly enlarged posteriorly to form the crop to store the food
(Fig. 1 ). Immediately posterior to the crop is the proventriculus or gizzard
Digestive System
104 J
midgut
mouth of proventriculus
oesophagus
Fig.
2 . Longitudinal section of the proventriculus of honey bee
which is variously modified in different insects. It is absent in fluid feeder
but is well developed in orthopteroid insects (e.g., cockroach). In
cockroach and crickets, the intima in the proventriculus is developed into
six strong teeth for grinding the food. Spines in the proventricular region
may act together as a food sieve or filter. The proventriculus typically
communicates with the midgut by stomodaeal valve. In honey bees, for
example, proventricular spines allow the movement of pollen into the
midgut without admitting ingested flower nectar. The stomodaeal valve is
developed to varying degrees in different insects (Fig. 2) .
Although the foregut is not the major digestive region of the
alimentary canal, some digestion may occur in the crop by the action of
salivary enzymes and enzymes regurgitated from the midgut (e.g., in the
cockroaches). Due to presence of impermeable mtima, the foregut
probably plays no major role in the absorption of digested food.
[ II] Midgut or mesenteron or ventriculus
The midgut (Fig. I ) does not have a cuticular mt1ma but, in the ma1onty
of insects, it is lined by a delicate peritrophic membrane which is
composed of chitin fibrils in a protein-carbohydrate matrix (Fig. 3).
Immediately posterior to the stomodaeal valve there is commonly a group
of diverticula, the gastric caeca. The number of these caeca varies in
different species. In bugs, the midgut is divided into two, three, and four
distinct regions.
The midgut epithelium of most insects is composed of three basic
cell types: columnar digestive cells with microvilli forming a striated
border regenerative cells and endocrine cells. The basal plasma
Digestive System
f 1 05
connective
tissue
secretory ---...::: /
cells
striated o.
border
pentrophic
membrane
\
1�1///·,1�1t-'=-= �=
_
B
Fig. 3. (A) Transverse section of midgut. (B) A section of midgut more highly magnified.
membrane
of digestive
cells
is
characteristically
infolded,
and
mitochondria are associated with these folds. These cells are involved in
the synthesis of digestive enzymes and absorption of digested food. At
the bases of the midgut epithelial cells are small regenerative cells, or
replacement cells. These cells replace the actively functioning gut cells
that die or that degenerate as a result of holocrine secretion.
The peritrophic membrane (peri=around; trophic=food) surrounds
the food bolus and may protect the epithelial cells from possible
abrasion by food particles. It is permeable to enzymes and the products
of digested food. The bugs, which are fluid-feeders, lack a peritrophic
membrane.
The plant bugs in order to obtain adequate quantity of nutrients
ingest large amount of sap. In them, the gut is modified to provide the
rapid elimination of the excess of water taken in to avoid excessive
dilution of the haemolymph and to concentrate the food to facilitate
enzyme activity. In leaf hoppers and aphids, the rapid removal of water
to the rectum is achieved by the anterior midgut forming a large
thin-walled bladder which is closely bound to anterior hindgut and
Malpighian tubules by its own basement membrane. The chamber
formed within this fold is called the filter chamber (Fig. 4). Water
passes directly from the midgut to the hindgut along an osmotic
gradient and there may be no significant flow of fluid through the
lumen of the gut.
106 1
Digestive System
convoluted
ventriculus Ill
anterior
instestine
filter
chamber
A
anterior midgut
c
Fig. 4. Filter chambers of bugs. (A) Filter chamber with straight ventriculus, (B) Filter
chamber with convoluted ventriculus, (C) T.S. filter chamber.
[ III] Hindgut or proctodeum
The hindgut (Fig. 1) is composed of cuboidal epithelial ceHs and is lined
by a layer of cuticle which is thinner and more permeable than that of the
foregut. It commences with the pylorus, which is associated with a variable
number of typically slender, elongate excretory structures, the Malpighian
tubules, and which usually contains a valvular structure, the pyloric valve.
The hindgut is divisible into an undifferentiated tubular anterior intestine,
just posterior to the Malpighian tubules, and a highly muscularised,
enlarged rectum, which terminates with the anus. The anterior intestine
Digestive System
[ 1 07
may be differentiated into an anterior ileum and posterior colon. The
rectum usually contains a number of pads, or papillae (usually six), that
project into the lumen. These structures receive an extensive supply of
tracheae and are metabolically very active. They play an especially
important role in the excretory system.
The hindgut is the major region of the insect involved in recycling.
Here needed materials are "reclaimed" while excess or waste materials
are ' 'trashed. ' ' Functions of the hindgut include the following: (i) water
absorption from urine and faeces, (ii) ion absorption from urine
and faeces, (iii) cryptonephridial system for water conservation,
(iv) pheromone
production, (v) respiration in larval dragonflies, and
(vi) modifications in structure for housing symbiotic microorganisms
(e.g., termites).
S alivary Glands
Although there may be glands associated with the mandibles (e.g., silver
fishes, termites, queen honey bee), maxillae (e.g., proturans, spring-tails),
and hypopharynx (e.g., worker honey bees), salivary glands are typically
associated with the labial segment. The salivary glands or labial glands
(Fig. 5) are paired structure, lie ventral to the foregut in the head and
thorax and occasionally extend posteriorly into the abdomen. Depending
on the type of food eaten and the insect species involved, salivary glands
vary in size, shape, and the type of secretion produced. Two basic types of
salivary glands exist, acinar and tubular. Orthoptera and Dictyoptera have
the acinar type while Diptera, Lepidoptera, and Hymenoptera have the
tubular type. In the acinar type, each acinus, bears a tiny duct that
communicates with other similar ducts, eventually forming a lateral salivary
duct. Lateral salivary ducts run anteriorly and merge as the common
salivary duct, which empties between the base of the hypopharynx and the
base of the labium. This region is called the salivarium and in some
sucking insects forms a salivary syringe that "injects" saliva into whatever
is being pierced. The lateral salivary ducts may communicate with salivary
reservoirs, as in the cockroaches. The secretory products of the salivary
glands are generally clear fluids that serve a variety of functions in different
insects: (i) they moisten the mouthparts and serve as a lubricant, (ii) they
act as a food solvent, (iii) they serve as a medium for digestive enzymes
and various anticoagulins and agglutinins, (iv) they secrete silk in larval
Lepidoptera (caterpillars) and Hymenoptera (bees, wasps, and relatives),
(v) they are used to "glue" puparial cases to the substrate in certain flies,
(vi) they serve for the production of toxins, and (vii) they secrete
antimicrobial factors (e.g., in certain blow fly larvae).
Amylase and invertase are the most common enzymes found in
saliva of insects, however, the saliva may also contain lipase and
1 08 J
Digestive System
hypopharynx
common efferent duct
common glandular duct
A
reservoir
glandular part
reservoir duct
glandular part
Fi g. 5. (A) Sahvary glands of cockroach, (B) grasshopper and (C) red cotton bug.
protease. Aphids secrete a pectinase that aids their mouthparts in the
penetration of plant tissues. The spreading factor, hyaluronidase, which
attacks a constituent of the intercellular matrix of many animals, has
been found in the assassin bug.
Blood-sucking
(haematophagous)
insects
contain
various
antihaemostatic (anticoagulant) agents. Current evidence, al least for
mosquitoes, is that these various salivary components mainly increase the
chances of the female locating a blood vessel.
Production and secretion of saliva in the dragonflies, grasshoppers,
and cockroaches are regulated by nervous innervation from both the
stomatogastric nervous system and the subesophageal ganglion, whereas
in the Diptera (e.g., the adult blow fly) these glands are controlled by
Digestive System
[ 1 09
an unidentified neurohormone. Salivation has been shown to be
controlled by phagostimulation of external chemoreceptors on the
mouthparts. This same stimulus probably also activates the salivary
pump.
Physiology of Digestion
In fluid feeders, digestion may begin before the food is ingested through
the injection or regurgitation of enzymes on to the food, or in foregut but
in general most digestion occurs in the midgut where most of the enzymes
are produced. In insects having biting and chewing type of mouthparts,
food is masticated not only in the buccal cavity but also in the
proventriculus. This not only facilitates passage through the alimentary
canal but increases the surface area for enzymatic action. Digestion takes
place by a series of progressive enzymatically catalysed steps, each
producing a simpler substance until molecules of absorbable size or nature
are produced. For example, polysaccharides are broken down into small
chains, disaccharides, and finally into simple, absorbable monosaccharides
(e.g., glucose); proteins are broken down into peptones, small polypeptides,
dipeptides, and finally into amino acids, which are absorbable.
There is some correlation between the kinds of food material eaten
by and the kinds of enzymes present in a given insect. Thus, cockroach
which is omnivorous secretes more enzymes than tsetse fly, that feeds
primarily on blood. In addition, different enzymes may be secreted by
different parts of the midgut epithelium.
[ I] Extra-intestinal digestion
Digestion of the food taking place outside the alimentary canal before the
food is ingested is known as extra-intestinal digestion. It happens with fluid
feeders where salivary enzymes are injected onto the food (e.g., house fly),
or into the host in predatory or parasitic insects, for example, assassin bugs
inject saliva into the prey which histolyses the contents before ingestion.
[ II] Intestinal digestion
In general, most of the digestion occurs in the midgut where enzymes are
secreted, however, some digestion also takes place in foregut, particularly
in crop, where midgut enzymes are regurgitated into it. In locust, the major
proportion of digestion takes place in crop.
The enzymes synthesised in the midgut depend upon the diet as
given in the table. For example, insects feeding protein diet proteases
are important, whereas a nectar feeding butterflies they are absent.
Aphids feeding on phloem sap having no polysaccharides or proteins
lack amylase and proteinase but have invertase (Table 1 ) .
110 J
Digestive System
Table I. The m idgut enzymes secreted by insects with different diets (+ and - indicate
presence and absence of enzymes).
I·-
Diet
Cockroach
omnivorous
Stick insect
phytophagous
Moth and butterfly
Larvae
Adults
Adults
Flesh fly larvae
Flesh fly adults
Tse tse fly
phytophagous
nectar
non-feeding
meat
sugar
blood
Protease
+
Lipase
+
Amylase
+
Invertase Maltase
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
?
+
+
+
weak
+
1. Digestion of carbohydrates. Carbohydrates are generally absorbed
as monosaccharides so that, before they are absorbed, disaccharides and
polysaccharides must be hydrolysed to their component monosaccharides.
(a) Polysaccharides. Starch, glycogen, chitin and cellulose are the
major polysaccharide food to be digested by different insects. Starch
(amylose) is hydrolysed to maltose, and glycogen to glucose by the
action of amylase, which specifically catalyses the hydrolysis of
1 :4-a-glucosidic linkage in polysaccharides. The major portion of the
food of phytophagous and xylophagous insects contains cellulose, only
few insects (Ctenolepisma, Schistocerca and some psocids) are able to
secrete cellulase. The insects unable to secrete cellulase, either cellulose
is excreted as such or they harbour micro-organisms (bacteria,
flagellates) to secrete cellulase.
Other polysaccharides, viz., chitin, lignocellulose and hemicellulose
are digested by chitinase, lignocellulase and hemicellulase, respectively.
(b) Disaccharides. The common disaccharides in the food are
maltose, trehalose, sucrose, cellobiose, melibiose and lactose that contain
a glucose residue which is linked to a second sugar residue by either
a-linkage or �-linkage. In the hydrolysis water molecule is the typical
acceptor for the sugar residues as follows :
Maltose+H20� G lucose+ G lucose [enzyme : maltase]
Trehalose+H20�Glucose+ Glucose [enzyme : trehalase]
Sucrose+H2 0 � Glucose+ fructose [enzyme : sucrase]
Cellobiose+H20 � Glucose+ G lucose [enzyme : cellobiase]
Melibiose+H20� Galactose+Glucose[enzyme : melibiase]
Lactose+H20� Galactose+Glucose [enzyme : lactase]
2. Digestion of proteins. Insects possess a series of proteases. A
trypsin like proteinase is secreted in the midgut which hydrolyses
protein to peptones and polypeptides. The products are then broken
down by peptidases. The carboxypolypeptidase attacks peptide chain
Digestive System
·
[
Ill
from the - COOH end and aminopolypeptidase attacks the chain from
the
NH 2 end). Some of these occur in the gut lumen, but most of
them are found in the intestinal epithelium. It indicates that most of the
polypeptides are absorbed before being further digested to amino acids.
Certain insects are able to digest ordinarily stable proteins. For example,
chewing lice and a few other insects are able to break down keratin, a
protein that occurs in hair and feathers.
3. Digestion of lipids. Many insects secrete lipases which hydrolyse
fats to fatty acids and glycerol. Wax moth (Galleria) is able to digest
beeswax (a mixture of esters, fatty acids and hydrocarbons). The insect
is known to produce not only the lipase, but also lecithinase and
cholinesterase with the help of bacteria.
Midgut pH (typically pH 6-8), buffering capacity, oxidation-reduction
potential, and temperature are important factors in the digestive process.
These factors vary from species to species and may also vary from one
region of the midgut to another within the same insect.
-
[ Ill] Absorption of the digested food
The midgut is the major site of absorption. In hindgut only reabsorption of
urine components occur while in foregut no absorption takes place. All the
substances are absorbed in solution and no phagocytosis of food particles
occurs. There are three major factors that affect the absorption of digested
food materials: (i) the presence of microvilli, which increase the surface
area for absorption; (ii) the functional differences in membrane
permeability of various regions of the digestive tract; and (iii) the presence
of a counter-current. Absorption may be active or passive. Passive
absorption (diffusion) takes place from the higher concentration inside the
lumen of the gut to lower one (inside the gut epithelium). Active
absorption depends on some metabolic process for movement of a
substance against a concentration or electrical gradient.
1. Carbohydrates. Carbohydrates are mainly absorbed as monosa­
ccharides that diffuse down concentration gradients between the midgut
lumen and haemolymph. The diffusion of simple sugars like glucose and
fructose is enhanced by the rapid conversion of these sugars to
trehalose in the fat body, a process called facilitated diffusion that
maintains a concentration gradient across the gut epithelium. Some
insects are able to absorb disaccharides as such.
2. Proteins. Protein& are absorbed as amino acids after hydrolysis
mainly in the midgut and caeca. Some amino acids in urine are also
reabsorbed in hindgut. Insects are unique in that they maintain rather
high levels of free amino acid stores in the haemolymph, thus many
amino acids have to be actively absorbed against a concentration
gradient. Some insects are able to absorb peptide fragments or even the
(Z-57)
112 l
Digestive System
protein as such, e.g., midgut cells of a haematophagous bug Rhodnius
absorb haemoglobin as such. Active absorption of amino acids varies
among insect species and depends on the composition of the . diet and
the haemolymph.
3. Lipids. Like some disaccharides and proteins, lipids are also
sometimes absorbed unchanged. The products of wax are absorbed in a
phosphorylated form while cholesterol is esterised before absorption.
The midgut caeca appear to be particularly active in lipid absorption,
but in few insects like adult Hymenoptera, lipid is absorbed in hindgut.
4. Water. Water is absorbed mainly in midgut and also in hindgut
either by diffusion or active transport depending upon the need of the
insect as insects regulate the salt-water balance very precisely. As the
amount of food is very poor in the contents of phloem and xylem,
insects feeding on them, e.g., plant bugs, in order to obtain sufficient
amount of amino acids and other nutrients, they possess various
mechanisms for concentrating the necessary nutrients from a dilute food
source by eliminating water. The filter chamber, present in the
Cicadoidea and Cercopidae (order Homoptera), is a modification of the
anterior midgut, which in combination with the Malpighian tubules
facilitates water removal and concentration of the desired nutrients prior
to absorption.
5. Inorganic ions. Inorganic ions are absorbed in the midgut and
reabsorbed in the fluid in the rectum. Even in the midgut, there are
specified cells that absorb particular ions, e.g., F/+
and cu++
All
+
+
the three ions Na , K
and c1 - are absorbed actively as their
concentration is very high in haemolymph than the gut lumen.
The active transport of Na+ may play a key role in the diffusion of
other molecules. When Na+ molecules are pumped from the midgut
cells into the haemocoel, they are replaced by Na+
diffusing into the
midgut cells from the lumen. The movement of Na+ across the cells
tends to produce a water gradient between the lumen and the cells
concentrating water in the lumen. Hence, water would diffuse into the
cells which, in tum, tend to concentrate other molecules that would
then diffuse down gradients into the cells. It implies that the work
necessary to produce the gradients for diffusion (a passive process) of
water and other absorbable molecules would be the active transport of
Na+
[ IV] Regulation of the alimentary system
Regulation of the alimentary system in insects involves control of food
movement, control of enzyme secretion, and control of absorption. The
alimentary canal is regulated in part through the action of the
stomatogastric nervous system. Food is ingested by the actiolls of the
(Z-57)
Digestive System
[ 11 3
mouthparts, cibarium, and pharynx, and is typically stored in the crop. It is
then released gradually, via the stomodaeal valve, into the midgut, where
digestion and absorption occur. In most insects that have been studied,
stretch receptors associated with the crop provide information to the brain
(via
the
frontal
ganglion)
regarding
crop
distension
and
help
prevent
overfilling of this organ. In some insects, stretch receptors in the abdominal
wall have a similar role.
The destination of ingested food may vary with the kind of food.
For
example,
in
female
mosquitoes
sugar
meals
(flower
nectar)
are
directed to the diverticula and blood meals are directed to the midgut.
Sensilla in the roof of the cibarial pump, acting via the frontal ganglion,
are thought to
be
involved
in
this
so-called
"switch mechanism."
In
other blood-feeding insects, such as tsetse flies, ingested blood goes to
the crop first.
Control
of passage of food from the crop to the midgut (rate of
crop emptying)
americana.
has
been
studied mainly
in
the cockroach,
Periplaneta
Passage of food from the cockroach crop is inversely related
to the osmotic pressure of the food, i.e.. the higher the concentration of
food, slower the passage. Osmotic receptors have been identified in the
wall of the cockroach pharynx.
Two mechanisms for the control of enzyme secretion in the insect
gut
have
material
been
suggested
:
may
stimulate
enzyme
secretogogue
hormonal
control
control
is
is
more
Secretogogue
an
(a substance in the ingested
secretion)
immediate
related
to
and
response
developmental
hormonal.
to
food,
and
The
whereas
environmental
effects. Nervous control is highly unlikely because the midgut is sparsely
innervated or not at all.
Absorption appears to be controlled by the availability of absorbable
molecules,
release
of food
material
from
the
crop
being
so regulated
that digestion and subsequent absorption occur at an optimal rate for a
given circumstance.
Many insects ingest foods with a very high water content. Some of
these insects (e.g., butterflies and many true flies) store the dilute food
in the impermeable crop and pass it gradually to the midgut. In others
(e.g.,
many
blood-feeding
insects)
food may
go
to
the midgut where
excess water is rapidly absorbed in the haemolymph and then excreted
via the Malpighian tubules. B oth mechanisms probably prevent extensive
dilution
of the
haemolymph,
and
removal
of water concentrates
solid
food, increasing the efficiency of digestion.
Movements of the alimentary canal (mainly foregut and hindgut) that
complement the actions of the digestive enzymes and help absorption are
under neural or neurosecretory control in some insects. In others, having no
(Z-57)
114 J
Digestive System
neural connections, gut movements are assumed to be myogenic. Hormonal
stimuli may also have a great deal to do with the rate of gut movements.
[ V] Insect nutrition
Like other animals, insects also require a balance diet having appropriate
amount of proteins, amino acids, carbohydrates, lipids, vitamins, minerals
etc. The dietary requirement of the insect is species specific. For the
proper development and growth, the insects derived most of the nutrients
either by taking food, or from the stores inside the body (e.g., fat bodies),
or as a result of synthesis (by the insect itself or through associated
micro-organisms). Certain moths do not feed as adult, and the food
accumulated during larval stages is used for their metabolic processes. All
insects are able to synthesise nucleic acids, however, only some insects are
able to synthesise vitamins, non-essential amino acids .
1. Amino acids. Amino acids are the building blocks of protein
making the tissues and enzymes. Different insects have different
requirements, depending upon which amino acids they are capable of
synthesising. In addition to essential amino acids, few insects need
glycine (e.g., flies) or alanine (e.g., Blatella), however, in these cases
methionine is not essential.
2. Carbohydrates. Carbohydrates are not considered to be essential
nutritive substances for most insects, but they are probably the most
common source of chemical energy utilised by insects. However, many
insects (e.g., many moths) do, in fact, need them if growth and
development are to occur normally. The carbohydrates may be
converted to fats for storage, or to amino acids.
3. Lipids. Lipids or fats, like carbohydrates, are good sources of
chemical energy and are also important in the formation of membranes
and synthesis of steroid hormones. Most insects are able to synthesise
lipids from carbohydrate and protein sources. However, some insect
species do require certain fatty acids and other lipids in their diets. For
example, certain Lepidoptera require linoleic acid for normal larval
development. All insects need a dietary source of sterol (cholesterol,
phytosterols, or ergosterol) for growth and development.
Carotenoids
are necessary in the diets of all insetcs as the visual pigment retinene is
derived from the food.
4. Vitamins. Vitamins are unrelated organic substances that are
needed in very small amounts in the diet for the normal functioning of
insects
as they cannot be synthesised. They provide structural
components of coenzymes. Vitamin A (fat soluble) is required for the
normal functioning of the compound eye of the mosquito. Insects
principally require water-soluble vitamins (e.g., B complex vitamins and
(Z-57)
Digestive System
ascorbic
{ 115
acid).
In
the
absence
of
ascorbic
acid
(vitamin
C) locusts
undergo abortive moults and dies.
5. Minerals. Like vitamins several minerals are required in traces by
insects for normal growth and development, e.g., potassium, phosphorus,
magnesium,
cobalt,
sodium,
nickel and
calcium,
zinc.
manganese,
copper,
The aquatic larvae
iron,
chlorine,
of mosquitoes
iodine,
are
able to
absorb mineral ions from the water through the thin cuticle.
6. The nucleic acids. Nucleic acids (DNA and RNA) constitute the
genetic material. Like other animals, insects are also able to synthesise
them. However, dietary nucleic acids (e.g., RNA) have been shown to have
an influence on growth of certain fly larvae.
7. Water. Like all animals, insects require water. Insects fulfil their
water reqmrements from food, by drinking, from absorption through the
cuticle
(in
aquatic forms), or from a by-product of metabolism. Insects
vary greatly
rice weevil
with respect to amounts
(Sitophilus oryzae),
of water needed.
Some,
like the
can survive and reproduce on essentially
dry food. Others , for example, honey bees and house flies, require large
amounts of water for survival. The excrement of the rice weevil is hard
and
dry,
with
almost
all
the
water absorbed
by
the insect,
while
the
excrement of bees and house flies contains large amounts of water.
[ VI] Microbiota and nutrition
Some
insects
bacteria,
and
require
the
protozoans)
presence
of
for normal
certain
growth
microbes
and
(yeasts,
development
fungi,
and,
in
some instances, for survival. In some insects the microbes are housed in
specialised cells,
mycetocytes, and the tissues
composed of these cells,
mycetomes, are associated with the gut, fat body, or, appropriately, the
gonads. The presence of microbes in the gonads ensures the infection of
any
egg
produced,
thus
transferring
the
microbes
in
next
generation.
Microbes are commonly found in the alimentary canals of insects, often in
the various diverticula of the midgut. The microbes probably benefit the
insect in various
ways, e.g., fix
atmospheric
nitrogen, synthesise protein
from nitrogenous waste materials, and supply vitamins
(B group), sterols,
and amino acids in addition to digestion of cellulose.
The association of microbes with the insects may either be casual or
constant. The microbes are almost present in food and are ingested by
the
insects
microbes
during
are
feeding,
important
fermentation
chamber
content
microbes
of
in
in
is
the
e.g.,
locusts.
Such
the
nutrition
of
hindgut
retained.
in
The
which
casual
dung
association
beetles
decaying
insects
may
that
food
have
with
have
with
its
constant
association with the microbes, e.g., insects feeding on wood, dry cereal,
feather, and hair.
116 J
Digestive System
Feeding Behaviour
[ I] Types of feeding
The food of the insect is intimately associated with the habitat where it is
found. On the basis of food (biological source of nutrients), insects may be
classified
as
herbivores
(phytophagous:
plant-eating),
carnivores
(zoophagous : animal-eating), omnivores (eating a wide variety of food),
detritivores
(saprophagous
:
debris-eating),
microphagous
(feeding
on
micro-organisms) and mycetophagous (fungus-eating).
1. Herbivores. Plants are the main source of food for the largest
number of insects,
them.
and the insects developed a variety of way to take
Grasshoppers,
locusts,
bugs,
moths,
butterflies,
thrips,
fruit flies, several beetles, wasps, bees are herbivorous.
kind
of food the herbivores
may
termites,
On the basis of
be either monophagous
feeding
on
plants belonging to a particular species or genus, (e.g., mustard sawfly,
cabbage butterfly), polyphagous feeding on a variety of plants distantly
related taxonomically
(e.g.,
desert
locust),
or
oligophagous
feeding
on
plants of same family or superfamily (e.g., pumpkin beetles). Insects are
able
to
feed
roots,
stems,
woody
parts,
buds,
flowers,
and
fruits.
However, a given insect is usually rather specific for the part of the
plants it eats. Thus there are leaf rollers, leaf miners , rootworms, root
borers, stem borers, fruit borers, and so on.
Detritivorous
2. Detritivores.
materials,
e.g.,
leaf
litter,
insects
decaying
feed
flesh,
on
dung,
decaying
and
so
organic
on.
Such
materials are supposed to be basic food source for the most primitive
·
insects living on the forest floor. These insects are important in the
progressive
degradation
of
decaying
organic
material.
For
examples,
dung beetles, carrion beetles, and many of the soil-inhabiting apterygotes
(e.g., Collembola).
3.
Microphagous.
Several
insects
are
dependent
upon
micro-organisms closely associated with decaying organic material, e.g.,
the fruit fly,
Drosophila melanogaster.
Its
maggots
cannot
live
on
a
sterile culture medium.
4. Mycetophagous. Many insects feed upon fungi and to obtain it
they
grow
fungi
on
specially
prepared
substrates.
Such
a relation
is
mutual. The fungi and the insects both derive benefit, e.g., few species
of ants
(Acromyrmex, Atta)
and
termites
(Macrotermes, Odontotermes).
The fungus is eaten in small amount by the workers and is fed to some
of the larvae.
5. Carnivores.
Insects that use living ·animals as a source of food
are called carnivores. They are either predators, parasites or parasitoids
(insect parasiting insects).
Digestive System
[ 11 7
(a) Predators.
Predatory insects capture and eat many living
animals (prey, usually other insects), e.g., praying mantids, dragonflies,
robber flies, ladybird beetles, tiger beetles, sphecids, predatory wasps,
antlions, lacewings etc. Predatory insects usually possess speed and
agility. Adult robber flies and dragonflies capture their prey in mid-air.
The praying mantid relies on stealth, extremely accurate visual targeting
and a lightening strike to trap its prey by the raptorial forelegs.
Certain insects, such as antlion larvae literally trap their prey forming a
shallow cone-shaped pit · in a sandy area and bury itself just beneath the
surface at the bottom of the pit. The aquatic larvae of certain species
of caddisflies construct silken nets in which they are able to catch small
organisms. The eggs of aphidophagous insects are placed in proximity to
prey species, e .g., hover flies. G iant water bugs (Belostoma) feed upon
snails.
(b) Parasites. The parasites live on or within the hosts which are
other than insects, particularly vertebrates and do not kill the host but
may spread diseases into them. Ectoparasitic insects obtain all or part
of their food from the host and live entirely on its external surfaces,
e.g., Pediculus (human lice), Xenopsylla (rat flea), Cimex (bedbug) etc.
Some of these insects remain on the host throughout their life-cycle,
some live on the host only during a particular stage of the life cycle,
and others visit the host intermittently during a particular stage of the
life cycle and are otherwise free-living. Parasitic insects that remain on
the host (e.g., the lice) are host specific. Adult fleas live on the host
In case of
while their larvae are free-living and detritivorous.
mosquitoes adult females visit host only to take a blood meal and
exhibit varying degrees of host specificity. Endoparasitic insects live
inside host and feed the tissues of the host, e.g., Gasterophilus (live
inside the gut of cattle).
(c) Parasitoids. The parasitoids are those insects whose larvae live
and feed within (endoparasitoid) or on the host (ectoparasitoid) and
adults are free-living. The majority of parasitoids belongs to the order
Hymenoptera. Some parasitoids exhibit a high degree of host specificity,
attacking a single or very few species of insects. A single host may
support the development of only one parasitoid (solitary parasitoid, e.g.,
aphid parasitoids) or several parasitoids (gregarious parasitoids, e.g.,
Apanteles - a braconid wasp). The parasitoids do not kill the host
immediately, but the hosts are ultimately killed after the development of
the larvae to pupae.
Some parasitoids attack other parasitoids
(hyperparasitoids), e.g., a cynipoid wasp Alloxysta pleura/is parasitises a
braconid wasp Binodoxys indicus which in turn parasitises plant lice.
Several parasitoids have been used in biological control of insects pests.
118 1
Digestive System
6. lnquilines. The inquiline
residents
in
the
insects
live
shelters of other insects
and
(both
feed
social
as
permanent
and non-social).
For example, certain galls made by one species are shared by larvae of
another species without any apparent damage to the original owner, e.g ..
ants of different species share common colony.
7.
The
Trophallaxis.
trophallaxis
exchange of food bet�een
adults
is
and
the
their
phenomenon
larvae,
e.g.,
of
an
mutual
ant
when
.
feeds a larva, it receives from the larva a drop of fluid (secreted by
salivary glands or integument or exudatoria) which is highly acceptable
to their nurses.
This mutual exchange of food is
the
basis
of social
(Vespula)
system in insects. Trophallaxis also occurs in certain wasps
but
in them its function is the disposal of excess water produced by larvae.
The rectal symbionts in termites are passed from one generation to the
next by means of anal trophallaxis.
[ II] Location of food sources
In many
herbivorous insects, the female parent oviposits on or in the
vicinity of a food source for her offspring. The same is true for most
parasitic insects. In these cases location of oviposition site, host location,
and food location are not distinctly separate activities.
Visual
and olfactory
stimuli
are
probably
the main
ones
used
in
food location by insects in general. Honey bees and butterflies also use
colour, form and movement of the flower as stimuli for food location.
Dragonfly
and prl!-ying rely heavily on
visual stimuli in prey capture.
Visual stimuli are important in host location in mosquitoes, tsetse flies,
and
parasitoids.
ammonia
Olfactory
present
dung-beetles
and
in
stimuli
the
food)
certain
flies
(volatile
are
(e.g.,
chemicals
involved
flesh
in
like
food
flies).
skatol
and
iocation
Secondary
in
plant
substiu1ces (volatile oils) are used by the butterflies for food location,
however, similar substances may also act as repellents to some insects.
Carbon dioxide, steroids, amino acids, and other volatiles
characteristic
of vertebrates are involved in their location by the mosquitoes. Contact,
thermal,
and
hygrostimuli
are
also
involved
in
host
location
by
the
ectoparasites of warm-blooded vertebrates, e.g., human body lice which
·prefer rough-textured materials. Backswimmers and whirligig beetles use
vibrations produced by their potential preys for their capture.
Once potential food is located, specific stimuli (phagostimulants) are
involved in the induction of feeding. Many of the volatile oils (secondary
plant
substances)
phagostimulants,
characteristic
e.g.,
the
of
cabbage
certain
aphid,
species
of
plants
Brevicoryne brassicae,
are
is
induced to feed even on an abnonral host when the substance sinigrin,
a
material
extracted
from
brassica
and
other
plants,
is
present.
Digestive System
{ 1 19
Maj ority of insects use sucrose, a common sugar in the food of many
insects, as phagostimulant. In addition, amino acids, glucose, certain
proteins, ascorbic acid, and several other chemicals have also been
shown to be feeding stimulants.
Important Questions
l.
2.
3.
4.
5.
Describe the anatomy of various parts of the alimentary canal of insects.
What do you mean by digestion ? How does it takes place in insects ? Describe.
Write an essay on "insect nutrition".
Give an account of feeding behaviour of insects.
Write short notes on : (i) Peritrophic membrane;
(iii) Trophallaxis.
(ii)
Salivary glands
and
7
Circulatory System
The insects have an open circulatory system as the blood bathes the
internal organs directly in the body cavity or haemocoel which is not a true
coelom. Almost the entire haemocoel is formed from the epineural sinus.
Dorsal and sometimes a ventral diaphragm divide the haemocoel into
perivisceral sinus and perineural sinus respectively. The dorsal longitudinal
and pulsatile vessel is the only conducting tube comprising a posterior
heart and an anterior aorta. The blood of insects is commonly called
haemolymph. In vast majority of the insects, blood does not contain
haemoglobin as it does not perform the transportation of gases.
D orsal Vessel and
Accessory Pulsatile S tructures
[ I] Dorsal vessel
The dorsal vessel (Fig. 1) is the principal organ for blood circulation. It
lies along the dorsal midline of the insect body, extending from the
posterior region of the abdomen to the head. Mostly the dorsal vessel is a
simple straight tube but sometimes it may have bulbar thickenings along its
length. The wall of the dorsal vessel is contractile and is composed of
mainly circular muscles, but it may also have semicircular, oblique, helical,
or longitudinal fibers. On the outside it is usually covered by connective
tissue. Fine elastic fibers, which arise from the dorsal integument,
alimentary canal, somatic muscles, and other structures, are used to
suspend the dorsal vessel. A network of tracheoles is often associated with
the dorsal vessel.
Circulatory System
[ 121
alary muscle
dorsal
diaphragm
x
ostium
I
r
I
j4----- aorta �--1...-����---=c_:_�
brain I oesophagus
_,
_
_
_____
perineural
sinus
Fig.
l.
Circulatory
system
of
a
generalised
pericardia!
sinsus
msect.
x-x's
in
top
two
figures
indicate
cross-sectional plane of bottom figure.
1. Heart. The dorsal vessel is divided into two major parts : a
posterior heart and anterior aorta. The heart is often restricted to the
abdomen, but may extend up to prothorax in cockroaches and is closed
posteriorly. It consists of a number of paired aRd usually lateral
openings, or ostia, both incurrent (allow haemolymph to enter the heart)
and excurrent (through which blood leaves the heart).
(a) The incurrent ostia. The incurrent ostia, usually nine pairs in
abdomen and three pairs in thorax, are vertical, slit-like openings in the
heart wall (Fig. 2) . There is a tendency of reduction in incurrent ostia
so that there are only five pairs in wasps and three pairs in house flies.
The valvular ostium permits the flow of blood into the heart at diastole,
but prevent its outward passage at systole.
122 J
Circulatory System
va1ve
diastole
�
-�·
systole
2
0
0
(B)
(A)
Fig.
valve
(A) Diagrammatic representation of the incurrent ostial valves seen in horizontal
section of the heart. (B) As seen in transverse section of the heart.
(b) Excurrent ostia. In certain insects, e.g., grasshoppers and silverfishes
there are paired (2 pairs thoracic and 5 pairs abdominal in grasshoppers)
ventro-lateral and non-valvular excurrent ostia (Fig. 3). These ostia expands
and contract during cardiac systole and diastole, respectively to allow the
blood flow from heart to pericardia! or perivisceral sinuses.
The heart may be constricted between successive ostia (e.g.,
cockroaches), giving it a chambered look, but its lumen is nearly continu­
ous throughout. On either side of the heart, in a segmental arrangement,
are the alary muscles which are attached laterally to the body wall (Fig. 1).
Lateral segmental vessels associated with the heart have been identified in
several msccts, for example, many cockroach species where excurrent ostia
are absent. At the origin, these vessels are valvular permitting the outward
flow of blood from the heart.
2. Aorta. The aorta extends anteriorly from the heart and opens
behind or beneath the brain. This is a simple tube without ostia but in
some insects it may be thrown into one or two vertical diverticula. These
cuticle -��r;;:iiti�W���¥��o:;:;:�
epidermis
heart
--��-
P'ti�?E�suspensory
cells
trachea
phagocyt1c
tissue
penv1sceral
sinus
valves of
ost1um
Fig. 3': Transverse section of the heart showmg the excurrent os1ta opening directly to the
penv1sceral smus
Circulatory System
[ 1 23
diverticula are often connected with the pulsatile organs. In silkworm, the
aorta have dilated portions along its length.
[ II] Accessory pulsatile organs
Besides the dorsal blood vessel, there are certain other accessory pulsatile
organs which are associated with the haemocoel and regulate the blood
circulation through the appendages (Fig. 4) . In most of the insects pulsatilc
organs are located in the meso- and metathorax that pump the blood into
the wings. In moth the dorsal blood vessel itself loops up to the dorsal
surface of the thorax forming the pulsatile organ. Grasshoppers and
cockroaches possess a small ampulla at their antenna! base. The ampulla
communicates the haemocoel by a valvular opening and continues as a
vessel into the antenna. During ampullar expansion the blood is drawn into
it from haemocoel and during contraction the blood is pumped into the
antennae.
reservoir
cuticle
epidermis
Fig 4 Sagittal section of · the mesothorax showmg pulsatile organ.
[ Ill] Sinuses, diaphragms and alary muscles
The haemocoel is usually separated into two and sometimes three cavities.
or sinuses by one (dorsal diaphragm) or two (dorsal and ventral
diaphragm)
fibromuscular horizontal septa. D orsoventral movement of
both the diaphragms help in the circulation of blood.
1. Dorsal diaphragm. The dorsal diaphragm, or pericardia! septum
typically consist� of two layers, which enclose the alary muscles
associated with the heart (Fig. I ). Usually it is perforated through which
haemolymph can readily pass. This diaphragm divides the dorsal
pericardia! sinus (around the heart) from the perivisceral sinus (around
the visceral organs).
124 l
Circulatory System
2. Ventral diaphragm. The ventral diaphragm is present in certain
insects like dragonflies, grasshoppers and wasps. It is located just above
the nerve cord and is ventral to the gut (Fig. l ). Like the dorsal
septum, it is usually perforated around its periphery. Laterally it is
attached to the sternum. It separates the perivisceral sinus from the
perineural sinus (around the nerve cord).
3. Allary (aliform) muscles. Alary muscles are closely associated
with heart. These extend from one side of the body to the other just
below the heart and fan out from a restricted origin on the tergum, the
muscles of each side meeting in a broad zone at the midline (Fig. l ).
The number of alary muscles varies in insects being ten abdominal and
two thoracic in grasshoppers and from four to seven in certain plant
bugs. A part of these muscles form a network that extends to the heart
wall. The alary muscles also form an integral part of the dorsal
diaphragm.
[ IV] Circulation
Figure 5 shows the general pattern of circulation in insects which can be
described as follows.
1. Course of circulation. B lood enters the heart through the
incurrent ostia. Thereafter, the blood is pumped forwardly through the
dorsal vessel at systole, flowing out of the heart through the excurrent
ostia and from the aorta. The valves of incurrent ostia prevent backward
movement of the blood through these openings. Blood usually returns to
general circulation in the head. With the help of movements of the
diaphragms, action of the accessory pulsatile structures, movement in the
alary muscles and body movements due to tergo-sternal muscles, blood
circulates throughout the haemocoel and appendages. Blood passes into
the perineural sinus supplying the nervous system with the help of the
ventral diaphragm. It then returns to the pericardia! sinus via
heart
ost1a
ventral
diaphragm
Fig. 5.
Generalised
course
of circulation of haemolymph.
Circulatory System
[ 125
perivisceral sinus through the openings m the dorsal diaphragm and
then it enters the heart.
2. Heartbeat. Systole, the contraction phase of the heartbeat cycle
in which heart volume decreases, results from the contraction of the
heart muscles which begins posteriorly and spread forwards as a wave .
Diastole, the relaxation phase of the heartbeat cycle in which heart
volume increases, results from relaxation of the heart muscles helped
by the contraction of alary muscles. A period of rest or diastasis occurs
between successive beats.
(a) Regulation of heartbeat. The heart of insects usually lacks
neural supply except few insects (e.g., cockroaches) where the heart is
supplied with nerves from the corpora cardiaca and segmental ganglia
(motor fibres). Therefore, it seems quite logical that the insect heart is
myogenic. It also lacks a pacemaker. The available evidences suggest the
existence
of
neurohormonal
control
of
heartbeat.
Both
a
cardioaccelerator neuropeptide and proctolin are myotropins that act on
the heart. In nymphalid butterfly, during larval periods, the heartbeats
alternate between forward peristalsis and backward peristalsis; but
during adult emergence the heartbeat rate increases only forwardly. This
forward movement facilitates the movement of the haemolymph into the
wing haemocoel and help expansion and wing unfolding. Reflex cardiac
responses to external stimuli are also probably important, for example,
stimulation of either a tarsal or labellar sensillum ensued a rapid change
in cardiac response of blow fly, Calliphom.
Several factors including intensity of activity, ambient temperature,
metabolic rate, developmental stage, the presence of biologically active
chemicals, insecticides, drugs etc. affect the rate and amplitude of the
heartbeat in insects. Though the accessory pulsatile structures are
independent of the dorsal vessel, are influenced by the same factors.
The frequency of heartbeat varies in insects from 14 beats/minute (larva
of stag beetle) to 1 50 beats/minute in flies. The heartbeat of larva
( 1 00- 1 30 beats/minute) is slower than the adult ( 1 50 beats/minute) in
mosquitoes. The heart of younger pupae sometimes stops beating and in
old pupae no beating is observed. In beetles, average blood pressure is
generally quite low. In soft-bodied insects (e.g., caterpillars), tonic
contraction of the body musculature raises the blood pressure so that
the haemolymph maintains body shape hydrostatically.
Haemolymph : Blood of Insects
Haemolymph is usually a clear colourless fluid, however due to presence
of ceretain pigments in few insects it may be slightly green, yellow or red.
Certain midges (Chironomus larva), backswimmers and the horse bot fly
(Gasterophilus) contain haemoglobin. It makes up about 5%-40% of the
126 J
Circulatory System
total body weight of an insect. Blood pH is usually slightly acidic (between
pH 6 and pH 7) but may be slightly alkalme, pH 7 and pH � .5 in few
insects . The specific gravity of haemolymph typically lies between 1 .0 1 5 and
1 .060 and is subject to increase during periods of moulting. The total
molecular concentration in the haemolymph is fairly high. Free amino
acids, organic acids, and other organic molecules play significant roles as
osmolar effectors rather than inorganic anions and cations. The insect
blood consists of a fluid plasma in which nucleated cells (haemocytes) are
suspended. In addition, it also contains several nonhaemocytic components
such as muscle fragments, free fat body cells, oenocytes, free crystals.
spermatozoa, various parasitic organisms, tumour cells, etc.
[ I] Chemical composition of the haemolymph
The chemical composition of the insect haemolymph varies considerably
both qualitatively and quantitatively. Even within species it varies
depending upon the physiological state, age, sex and food of the insect. It
means that the haemolymph is a dynamic fluid that changes with diet,
environmental changes, and different life stages.
1. Water. Water constitutes 84%-92% of the internal body fluid of
insects. In certain insects, however, its content may be much lower, even
less than 50%.
2. Inorganic constituents. Insect haemolympt: contains sodium,
potassium, calcium, sulphur, magnesium, phosphorus, chloride, and
carbonate as major inorganic materials. In herbivore insects, magnesium
and/or potassium replace sodium as major cations. The carnivorous
insects, which commonly take in large amounts of dietary sodium, do
not show high levels of magnesium and potassium. Copper, iron,
aluminum, zinc, manganese, and other metallic elements have been
found in very small amounts in insect haemolymph.
3. Nitrogenous wastes. The nitrogenous wastes are metabolites of
proteins and amino acids such as uric acid which occurs in a very high
concentration in the haemolymph. It is synthesised mainly in the fat
body and is usually excreted by the Malpighian tubules. Other
nitrogenous wastes are urea, aUantoin, allantoic acid, and ammonia, the
last being formed mainly in aquatic insects.
4. Organic acids. The haemolymph contains several organic acids
like succinate, malate, fumarate, citrate, lactate, pyruvate, a-ketoglutarate,
and several organic phosphate compounds. These acids are important in
balancing the cations in the blood.
5. Carbohydrates. The carbohydrates are extremely important to the
insect as the major energy source, as identification tags in cellular
recognition and in protein translocation, and in cold stress metabolism. ·
Trehalose, a disaccharide composed of two glucose molecules (thereby
Circulatory System
{ 1 27
containing twice as much energy as a single glucose molecule), is the
major blood sugar in most insects. Glucose, fructose, ribose, and others
have also been found as blood sugar in certain insects. Two
neurohormones, the hyperglycemic decapeptide and the hypoglycemic
neurohormone maintain the sugar level in the haemolymph. Glycogen
has been found in very small amount in the haemolymph. Certain
glycoproteins (lectins) found in haemolymph of some ir.sects serve as
scout molecules that continually survey the haemolymph for non-self
molecules, dead tissues, and/or foreign organisms. The haemolymph also
contains glycerol and sorbitol which are natural antifreeze or
cryoprotectant chemicals that protect the insects from cold stress.
6. Lipids. Lipids are usually found in insect haemolymph as
lipoproteins (lipophorins) which help transport of digested fats (i.e., fatty
acids) from the gut to various tissues and help in transportation of
cholesterol, carotenoids and possibly xenobiotics (any foreign chemicals
not normally found within an organism, such as pesticides). In addition,
lipophorins may be involved in the insect defense system. Vitellogenin a
very-high-density glycolipoprotein, somehow transports lipids into the
eggs for use by the developing embryo. Lipophorins also transport
hydrocarbons from their sites of synthesis to the cuticle.
7. Amino acids. TI1e free amino acids are found m very high
concentration in the haemolymph which may be either dietary or
synthesised by the insect and are major osmotic effectors. They may
represent an excess, derived from the diet, which is stored in the
haemolymph prior to excretion; or they may serve as a reservoir of raw
materials for the synthesis of protein needed in the construction of new
cells during periods of growth and metamorphosis, e.g., tyrosine plays a
key role in the sclerotisation of the cuticle. Proline serves as the major
flight energy source in some insects.
8. Proteins. A considerable amount of proteins are present in the
haemolymph either as storage molecules (e.g., calliphorin, which
functions as a storage depot for nutrients during larval development in
Ca/liphora), enzymes, antibactericidal proteins, carrier or binding
proteins, and antifreeze nucleators. Some of the enzymes found in the
haemolymph are trehalase, juvenile hormone esterases, lysozyme and
phenoloxidases.
9. Pigments. Several pigments both respiratory and non-respiratory
have been identified in insect blood. Haemoglobin is found in
Chironomus larvae as respiratory pigment, kathaemoglobin (derived from
the blood meals taken by the blood-sucking bug, Rhodnius ), carotene,
flavines and xanthophyll in herbivorous insects, protoaphin (in aphids);
riboflavin and fluoroscyanine, insectoverdin (green) and mesoviliverdin
(Z-57)
128 J
Circulatory System
(blue) found in locusts. In insects with thin transparent cuticle, blood
pigments may determine body coloration.
10. Gases. Both 02 and C02 may occur in the haemolymph, usually
in very low concentrations, however, in Chimnomus larva that contains
haemoglobin as respiratory pigment in the haemolymph, the amount of
0 2 is very high.
[ II] Haemocytes
Hacmocytes are the suspended cells found in the haemolymph. All
haemocytes are of mesodermal origin, do not contain any pigment, and do
not appear to enter the dorsal blood vessel or heart. The haemocytes are
able to recognise self from damaged self/non-self cells.
1. Origin of haemocytes. All the haemocytes develop from
mesodermal
prohaemocytes in haemocytopoietic organs
(blood-cell­
producing organs) where they multiply and/or differentiate. The
haemopoetic organ is known to occur both in developing stages and in
the adults in case of exopterygotes, but is absent in adult
endopterygotes where the major supply of blood cells arises from
circulating haemocytes. Developmental status, metamorphosis, stress of
starvation or overcrowding, wounding, and infection by foreign organisms
considerably affect the number of haemocytes.
2. Number of haemocytes. The total number of haemocytes per unit
volume of insect haemolymph varies from 10 to 1 ,67,000/
µ/
(micro-litre) and averages about 20,000/ = µ/ (micro-litre) depending
upon species, develQpmental stage, various physiological states, and so
on. Certain dipteran larvae (e.g., house fly and Chironomus ) have no
haemocytes in circulation. The number of haemocytes tends to increase
during the larval instars, decline in the pupal stage, and increase initially
and then decline in the adult stage.
3. Types of haemocytes. A number of morphologically distinct cells
have been identified in the haemolymph of insects (Fig. 6). Some of
these haemocytes are found in all groups of insects; others are less
common, and some are quite rare, found only in a few or even a single
species.
Following types of haemocytes have been recognised:
prohaemocytes (small rounded cells with relatively large nuclei and
basophilic cytoplasm, also give rise other haemocytes), plasmatocytes
(most abundant, variable in form, phagocytic with basophilic cyoplasm),
coagulocytes (granular cystocytes having small nucleus and a pale and
hyaline cytoplasm contammg hlack granules), granular phagocytes
(phagocytic with acidophilic granules in cytoplasm), oenocytoids (usually
large, thick, basophilic cytoplasm having canalicuh ) , spherule cells
(round, oval and filled with large, non- refringent, usually acidophilic
=
(Z-57)
Circulatory System
[ 129
prohaemocyte
oenocytoid
cystocyte
spherule cell
adipohaemocyte
Fig. 6. Types of haemocytes.
inclusions) and adipohaemocytes (spheroidocytes,
droplets).
having refringent fat
[ III] Functions of haemolymph
Haemocytes, either acting alone or in conjunction with the haemolymph,
have been found to provide protection against invading parasites, inanimate
particles, pathogens, and cell fragments in the following ways:
1. Transport and storage. Insect haemolymph transports nutrients
(amino acids, sugars, fats, etc. absorbed through the gut wall or released
from cells that store such materials), metabolic wastes, possibly
hormones, and 02 and C02 usually over very short distances. The
haemolymph also serves as an important storage pool for the raw
materials used in the building of new cells.
2. Phagocytosis. Plasmatocytes and granulocytes of the haemolymph
ingest foreign particles, bacteria, and cellular debris. Phagocytic
haemocytes are also important during periods of moulting and
metamorphosis, when many tissues are in a state of disintegration
{histolysis) and fragments of cells freely fall on the haemolymph.
3. Nodule formation. Higher number of bacteria and protozoans are
usually cleared from the haemolymph by nodule formation. Nodules are
aggregates of entrapped foreign material surrounded by plasmatocytes.
4. Encapsulation. When foreign organisms are too large for either
phagocytosis or nodule formation, are destroyed by encapsulation. In
this process foreign body is randomly contacted by a granulocyte which
recognises the foreign body. The granulocyte degranulates and material
sticks to foreign body, which is followed by additional granulocytes
attacking to foreign body. Lysis of granulocytes releases a haemocytic
recognition factor that attracts and recruits plasmatocytes that attach
(Z-57)
130 1
Circulatory System
foreign
body.
Plasmatocytes,
then,
flatten and spread
over the foreign
body surface increasing the number of layers around the foreign body
until it is no longer recognised as foreign.
5. Detoxification. Some haemocytes in the haemolymph
are capable
of making toxic metabolites and certain insecticidal materials nontoxic to
insects.
6. Wound healing.
Several
haemocytes
(e.g.,
plasmatocytes
and
spherule cells) tend to accumulate at sites of injury where they may be
phagocytic,
promote
coagulation
by
granulocytes,
and
form
protective
sheets all of which help healing.
7. Haemostasis, coagulation, and plasma precipitation. Haemocytes
prevent
loss
of
haemolymph
at
a
wound
site
by
plugging
and/or
promoting coagulation or plasma precipitation. Mechanical plugging may
be a nonspecific function of haemocytes that settle out at a wound site,
however, coagulation and plasma precipitation are functions of specific
haemocytes, the granulocytes which induce the rapid formation of a fine
granular
precipitate
or
of threadlike
networks
that
enmesh
the
cells
allowing
easy
around them.
8. Lubricant.
Haemolymph
serves
as
a
lubricant,
movement of the internal structures relative to one another.
9.
Hydraulic
medium.
Like
the
other
fluids,
haemolymph
is
incompressible. Thus, forces that reduce the blood volume in one part
of
the
body
(e.g.,
compression
of
the
abdomen)
are
transferred
hydrostatically through the haemolymph to other parts of the body and
influence a change there. For example, house flies ready to emerge, pop
the
lid
from the
end
of the
puparium
by
means
of the
hydrostatic
extrusion of the bladder-like ptilinum from the anterior portion of the
head.
Ecdysis
also
involves
hydrostatic
pressure,
and
newly
emerged
adult insects expand the wings hydrostatically.
10. Heat transfer. In most insects, the haemolymph transfers body
heat from one region to another.
11. Protection. Unlike vertebrates, insects do not have the classical
antigen-antibody
defensive
system.
mechanisms
However,
against
they
possess
pathogens
or
diverse
against
physiological
damage
to
the
cuticle. The term insect immune system simply refers to the mechanisms
that
permit
the
insect
·
to
resist
infection
by
micro-organisms.
The
humeral (haemolymph-borne) immune factors found in the haemolymph
consist of two basic types:
noninducible (do not require
synthesis
of
RNA and protein) and inducible. Inducible factors in the haemolymph
include antibacterial proteins (e.g., cecropin) · and lysozymes, whereas the
factors
are
the
lectins
(haemagglutinins)
and
the
noninducible
phenyloxidases.
The
phenyloxidase
system,
once
activated,
cascade of chemical events that ultimately kill the invader.
(Z-57)
produces
a
Circulatory System
12.
Secretion and
involved
in
epidermal
muscles.
l 131
the
cells
In
prothoracic
formation of other
formation
and
some
glands
of the
possibly
the
basement
sheath
insects,
haemocytes
before
moulting.
tissue.
material
were
Some
Haemocytes
membrane
underlying
are
the
surrounding
observed
haemocytes
to
the
the
also be
activate
may
involved in the formation of the fat body.
Important Questions
1.
2.
3.
4.
Describe in detail the structure and function o f dorsal vessels and pulsatory structures
in insects.
Give an account of composition of haemolyffiph.
What are the functions of haemolymph in insect life ? Describe.
Write short notes on : (i) Heart of insects; (ii) Allary muscles ; (iii) Regulation of
heartbeat; (iv) Haemocytes and (v) Pulsatile organs.
8
Respiratory System
The respiratory system or tracheal system is involved in gaseous exchange
in the insect with the environment. In this system there is a system of
internal tubes, the tracheae that directly transport · the oxygen to the parts
of the body. Therefore, it does not use the circulatory system as the vehicle
for gaseous exchange. The trachae open outside through segmental pores
called spiracles having some system of closing and opening. Except
proturans and some collembolans all insects possess tracheal system for
respiration. These insects live in moist habitats where gaseous exchange
takes place directly with the environment via the integument.
S tructure of the Tracheal S ystem
[I] Tracheae
The tracheae, ectodermal in ongm, are tubes that communicate with the
outside by spiracles. From each developing trachea, branches are given off
to the various organs including the wings. The tracheae are circular or
somewhat elliptical in cross section.· Histologically the tracheae are similar
to the integument, being composed of a layer of epithelial cells that secrete
a cuticular layer, the intima (Fig. 1 -A, B). Chitin is absent in the smaller
tracheal branches. The intima is thrown into a series of usually spiral folds
around the lumen called taenidia. The taenidia provide strength to the
tracheae and protect it against collapse with changes in pressure. The
intima is shed along with the old integument during each moult.
Although tracheae are resistant to compression in a transverse
direction. they can he stretched longitudinally to some extent. It helps
Respiratory System
I 133
taenidium
mitochondrion
tracheal cell
cuticulin
nucleus
basement
membrane
r������
A
plasma
membrane
taenidium
chitin and
protein layer
B
muscle
tracheoblast
c
D
Fig. I Tracheal structure. (A) a portion of trachea, (B) histology of trachea, (C) tracheoles
in close contact with muscles and (D) air sacs in the honey hee.
the insects in which the abdomen becomes greatly distended with food,
e.g., blood-sucking insects.
[ II] Tracheoles
The tracheoles are intracellular smallest branches of the tracheal system,
ranging in size from 0.2 µ to 1 .0 µ in diameter, and are the place where
gaseous exchange takes place. The very fine taenidia ( 10-20 mµ) are
retained during moulting unlike tracheae. A trachea typically ends with a
tracheal end cell, the tracheoblast, which gives rise to several tracheoles
that are all a part of this cell (Fig. 1 -C). The tracheoles are very intimately
associated with the tissues or organs that have a high metabolic rate and
high oxygen demand. These tissues or organs include the flight muscles,
ovaries, fat body, gut epithelium, Malpighian tubules, and rectal papillae.
Collectively the tracheoles provide a huge surface area for gaseous
exchange. A fifth instar silkworm larva has 1.5 million tracheoles.
134 1
Respiratory System
[ Ill] Air sacs
Air sacs (Fig. 1-D) are thin-walled tracheal dilations of varying size,
number, and distribution found mainly in flying insects. The taenidia are
absent or very poorly developed, therefore, air sacs are quite distensible
and collapsible. Whenever, the demand of 02 increases, the air sacs
collapse and pump air in and out of the tracheal system to meet out
the demand.
Air sacs perform several functions. The major function is to
increase the volume of the tidal air (the air that is inspired and
expired). The presence of large air sacs in the body cavity of a
terrestrial insect help in flight by reducing the specific gravity and of
aquatic insects it gives some degree of buoyancy. Air sacs also provide
space for growth of internal organs. It helps in heat conservation in
large insects that generate high temperatures for flight. It improves the
haemolymph circulation in the flight muscles. Air sacs also form the
tympanic cavity of the hearing organs of various insects (i.e., tymbal of
the cicada).
[ IV] Spiracles
Spiracles are the external openings of tracheae. They are lateral in
position, usually on the pleura. Usually a pair of spiracles are present in a
body segment.
1. Number and distribution of spiracles. With the exceptions of
some dipluran insects, the maximum number of spiracles found in
insects is ten pairs, two thoracic and 8 abdominal. On the basis of the
number and distribution of spiracles, the respiratory system is classified
as :
(a) Polypneustic. At least 8 pairs of functional spiracles.
Holopneustic - 10 pairs of spiracles ( 1 mesothoracic, 1 metathoracic and
8 abdominal), e.g., cockroaches; Peripneustic- 9 pairs of spiracles
( 1 mesothoracic, 8 abdominal), e.g., some fly larvae; Hemipneustic- 8 pairs
of spiracles ( 1 mesothoracic, 7 abdominal), e.g., some fly larvae.
(b) Oligopneustic. 1 or 2 pairs of functional spiracles. Amphipneustic 2 pairs of spiracles ( I mesothoracic, 1 post abdominal), e.g., larvae of
moth flies; Metapneustic- 1 pair of functional spiracles (post abdominal),
e.g., mosquito larvae; Propneustic- 1 pair of spiracles (mesothoracic),
e.g., dipteran pupae.
(c) Apneustic. No functional spiracles, e.g., chironomid larvae.
2. Types of the spiracles. The spiracles are of two basic types:
simple and atriate. The simple type of spiracle (Fig. 2-A) is only an
opening to the tracheal system. The atriate type is formed as a result of
the invagination of the primitive spiracular opening. Thus, in the fully
Respiratory System
spiracle
[ 135
integument
atrium
trachea
filter apparatus
trachea
B
A
c
Fig. 2. Types of spiracles. (A) simple non-atriate type, (B) atriate type with lip closure
mechanism and (C) atriate type with filter apparatus and valve closing mechanism.
developed atriate spiracle (Fig. 2B, C) the tracheal opening lies at the
bottom of a spiracular chamber, or atrium. In this type the external
opening is called as the atrial aperture or orifice.
3. Structure and closing/opening of spiracles. The tracheal system
readily allows the passage of water and due to this the insects may lose
water very rapidly. To prevent the loss of water, the insects have
evolved various types of spiracular closing mechanisms. Principally two
types of closing mechanisms are observed, lip type (folds of the
and valvular type (two
integument form opposing lips, Fig. 2-B)
movable valves lies at the inner end of the atrium, Fig. 2-C).
Irrespective of the type of mechanism, closure is carried out by
contraction of the associated muscles (Fig. 3). Most often the atrial wall
;--.--+- posterior valve
----'-- sclerotised rod
ventral orifice
process from--'"--'
scterotised rod
wall of thorax
Fig. 3. Interior view of the first thoraci1: spiracle of locust.
136 J
Respiratory System
closer muscle
Fig. 4. Longitudinal section of the spiracle of a louse showing the
dust catching spines.
is lined with tiny hairs, which form a felt chamber that filter out dust
(Fig. 4). In flies, beetles and moths the spiracle is covered by a sieve
plate having large numbers of fine pores that not only prevents the
entry of dust but also the water (in aquatic insects) in the tracheal
system. The spiracle is also associated with certain glandular tissue that
secretes lubricants for the movable parts of the spiracular closure
mechanism. The lubricants may prevent water from entering the
tracheae, and may improve the seal of the closure mechanism.
[ V] Types of the tracheal system
Except most collembolans, many proturans, and certain endoparasitic wasp
larvae other insects possess tracheae. Tracheae··along with spiracles, air
sacs, and tracheoles compose the respiratory or ventilatory system.
The organisation of tracheae may be comparatively simple, as in
some sprin
ls in which tracheae arise from each spiracle, but do not
connect to any other tracheae. However, tracheal organisation in most
insects is mote complex (Fig. 5-A). There is typically a pair of lateral
longitudinal trunks into which the spiracles open, a similar pair of
dorsal longitudinal trunks, and often a pair of ventral longitudinal
trunks. The dorsal, lateral, and ventral trunks are connected by more or
less dorso-ventrally oriented tracheae, and the longitudinal trunks on
either side are connected by transverse tracheal commissures (Fig. 5-B).
Although the basic pattern of tracheation is genetically determined,
new tracheae and tracheoles can be induced to develop if an insect is
reared in an atmosphere with a very low oxygen content. New tracheae
and tracheoles do not develop between successive moults, but on
demand changes in the distribution can occur at the time of moulting.
Major tracheal branching patterns are species-specific and are often
very similar among members of a given family or order. Based on the
presence or absence and functional or nonfunctional nature of spiracles,
guu
Respiratory System
I 13 7
thoracic
air sacs
abdominal
spiracles
ventral ventral
branch tracheal
trunk
�....,..�..._
abdominal
spiracles
'-.l;;���dorsal diaphragm
wing branch
alimentary canal -!-����-­
lateral tracheal trunk
thoracic spiracle
salivary gland _......,...,..__-+< ,
=:;"-.,.> -��-==-__::::-.
-ventral diaphragm --...,.
ventral commissure-===��������>c::-;;;,
;:; tracheal trunk
ventral
thoracic ganglion
dorsal tracheal
trunk
c
Fig. 5. Representative types of tracheal system. (A) dorsal and lateral views of the open
type, e.g., grasshopper, (B) cross-section of the thorax showing the major tracheal branches,
(C) open type with two spiracles, e.g., mosquito larva, and (D) closed type wtth no
functional spiracles, e.g., mayfly nymph
1 38 J
Respiratory System
there
are
principally
two
types
of tracheal
systems,
open
and
closed,
with a variety of modifications within each type.
1. The open tracheal system. Most of the insects have open tracheal
system which is characterised by the presence of one or more pairs of
functional spiracles (Fig. 5-A, C).
2. The closed tracheal system.
insect
larvae
do
not
possess
Many aquatic and endoparasitic
functional
spiracles
exchange occurs directly through the integument, e.g.,
and
the
gaseous
Chironomus
larva,
mayfly nymph (Fig. 5-D).
[ VI] Mechanism of gaseous exchange and ventilation
Two types of gaseous exchange are observed in insects: diffusion or passive
ventilation and active ventilation.
1. Diffusion or passive ventilation. Simple diffusion from the outside
of smaller insects and from well-ventilated air sacs in larger insects can
supply
sufficient oxygen
to the body
tissues to maintain
life.
It is a
passive form of ventilation in which the gases are not pumped in the
tracheae and tracheoles. Diffusion is also regulated by the opening and
of the spiracles. The spiracles respond to decreased 02 or
closing
increased
Spiracular
C02 in the air by remaining open for longer periods of time.
opening
and
closing
are
under
both
neural
and
hormonal
control.
2. Active ventilation. In larger and active insects passive ventilation
does
not
insects,
bring
adequate
amount
of
oxygen
to
the
tissues.
In
these
air sacs, if present, and larger tracheae are often ventilated by
rhythmical pumping movements of the body which is called as active
ventilation.
Peristaltic
waves
over
the
abdomen,
telescoping
or
dorsoventral flattening of the abdomen, and, in some, movements in the
thorax
or
even
protraction
and
retraction
inspiration and expiration of gases
the
gut
movement,
assist
the
head
cause
the
6). In addition, heartbeat and
(Fig.
ventilation
of
by
pressing
against
adjacent
tracheae. Both tracheae that are oval in cross section and air sacs are
collapsible and hence can serve to increase the volume of tidal air.
�
.j)
�
�\-�,t
'
',
A
...
t
... .
.. __
.. _
..
,'
B
•
�I
I
I
D
c
...
_-:::'."'-�---
Fig. 6. Diagrammatic representation of types of abdominal ventilatory movements. Dashed
lines indicate the contracted position, arrows the direction of movement. (A) and (8) in
transverse section and (C) m the longitudinal section.
Respiratory System
[ 139
3. Elimination of C02• In tissues C02 diffuses about 35 times more
rapidly than 02. Because of this, C02 is much more likely to be
eliminated from the body through the tracheal linings and integument
than is 02 to be absorbed along the same routes. Thus, although most
of the C02 produced by respiration is eliminated via the tracheae and
tracheoles, some of it may escape through the general body surface of
soft-bodied insects and the intersegmental membranes of hard-bodied
insects.
In some insects C02 is not continuously eliminated through the
spiracles, but in regular bursts, while 02 consumption remains constant.
Between these bursts the spiracles remain fully closed or half-closed.
The spiracles open completely during a C02 burst. As oxygen is
removed from the tracheoles and tracheae by respiration, at least a
portion of the carbon dioxide produced presumably goes into solution as
bicarbonate in the haemolymph. A C02 burst probably indicates the
previous buildup of C02 (in the haemolymph and tracheae) to a
threshold above which complete spiracular opening occurs. The ability to
release C02 periodically allows an insect to keep its spiracles partially
or entirely closed most of the time and hence is thought to be an
adaptation that favours the conservation of water by diminishing the rate
of transpiration.
[ VII] Respiration in aquatic insects
Many insects spend all or part of their lives in an aquatic environment.
These insects must either be able to utilise 02 in solution or have some
means of tapping a source of undissolved 02 whether it be at an air-water
interface or from aquatic vegetation.
1. Use of dissolved 02 in water. Aquatic insects with closed
tracheal systems depend entirely upon the diffusion of dissolved 02
through the integument (cutaneous respiration). These insects -obtain 02
in a variety of ways. Mayfly and damselfly nymph possess tracheal gills
(Fig. 7-A, B), which are integumental evaginations covered by a very
thin cuticle and are well supplied with tracheae and tracheoles. Such
gills are usually abdominal. Other aquatic insects with closed tracheal
systems possess spiracular gills (e.g., pupae of some dipterans), or
cuticular gills (Fig. 7-C).
2. Use of aerial 02• Aquatic insects having open tracheal systems
obtain 02 at the surface of water and for this they come at the surface
of the water periodically. Some aquatic insects may remain submerged
for an indefinite period of time and have certain .;tructures that help
them in obtaining aerial 02.
(a) Respiratory siphon. The larvae of
mosquitoes and Eristalis
possess posterior spiracles on siphon that penetrates the water surface
140 J
Respiratory System
trachea
tracheal
gills
B
cuticular
gills
{
Fig. 7. Respiratory structures in aquatic insects. (A) lateral abdominal tracheal gills in a
mayfly nymph, (8) tenrunal abdominal tracheal gills in a damselfly nymph and (C) cuticular
gills on the thorax of a black fly pupae.
and get atmospheric 02 . In Eristalis the siphon is telescopic and may
extend to a length of 6 cm or more (Fig. 8).
(b) Hydrofuge structures. Hydrofuge structures are usually made up
of hairs and are resistant to wetting by water, e.g., in Notonecta. Thus,
when an insect approaches the surface, the cohesive properties of water
cause it to be drawn away from the hydrofuge areas. These structures
are generally associated with particular spiracles (Fig. 9). Certain
dipterous larvae have peristigmatic glands that secrete fatty substances in
the immediate neighbourhood of the spiracle and make it hydrofuge.
Respiratory System
[ 141
spiracles
"=::==:::-=::::-=::=�:::====-===-==---1k:� surtace
---
--
-_
_::::: - water
t
...
· ' .
,'
"
:
=---= -
'
telescopic
respiratory
siphon
'-<..._ substratum
Fig. 8. Partly extended respiratory siphon of Eristalts
(Diptera)
hairs close over
spiracle, preventing
entry of water
larva.
haJrs separated by
surtace tension forces,
spiracle exposed
Fig. 9. Diagram to show the movements of hydrofuge hairs surrounding a spiracle when
the insect is submerged and at the surface.
Hydrofuge structures also serve to keep water out of the tracheae when
the insect is submerged.
(c) Air stores. Many aquatic bugs and adult beetles carry air stores
in the form of bubbles or films into which spiracles open. These air
stores are often held in place by a pile of erect hydrofuge hairs, but in
some insects the body is so shaped that it forms a storage area without
the use of hydrofuge hairs. Films or bubbles of air would obviously be
a temporary source of 02 if the insects were forced to remain
submerged. To replenish air stores water scorpions (Hemiptera,
Nepidae) use a caudal siphon, a long hollow tube extending from the
rear of the body.
(d) Physical gills. Many aquatic insects that carry stores of air are
able to replenish the 02 without surfacing. This is performed by the air
store acting as a physical gill. As the 02 in reserve is used up, a point
is reached where the partial pressure of oxygen is less in the air store
than it is in the surrounding water. At this point oxygen diffuses from
the water into the air store. Nitrogen in the air store does not readily
diffuse into the water and hence tends to keep the air store from
collapsing. Air stores usually make an insect positively buoyant and may
play a role in hydrostatic balance.
(e) Plastron. Insects that are able to remain submerged for a longer
period usually possess a structure known as a plastron. A plastron is a
Respiratory System
142 J
vertical cuticular
support
Fig. IO. Plastron of a crane fly larva.
very thin layer of gas held firmly in place by tiny hydrofuge hairs or
other very fine cuticular networks (Fig. 10). The latter are typically
associated with spiracular gills. Hair plastrons are found on adults of
certain aquatic beetles (e.g., Elmis ), nymphs and adults of the aquatic
bug Aphelocheirus, and adult females of the wingless moth Acentropus.
Plastrons composed of cuticular networks are found in the larval and/or
pupal stages of certain beetles and flies. Unlike typical air stores, the
gas layer held by a plastron cannot be displaced by water. The spiracles
open into the plastron, and it functions in a manner similar to a
physical gill except that it does not require repletion by a visit to the
surface.
(j) Use of plant surface. Insects obtain Oz from submerged
vegetation in a variety of ways. Many are able to hold bubbles on the
surface of plants by means of hydrofuge structµres. Others penetrate the
tissues of submerged plants by biting into them or by inserting a
specialised ventilatory structure into the intercellular air spaces, e.g.,
certain mosquito larvae, other flies, and some beetle larvae.
[ VIII] Respiration in endoparasitic ins ects
Most of the endoparasitic insects are parasitic only in the immature stages,
e.g., parasitic wasps. The environment of these insects presents problems
similar to those of the aquatic insects. In several insects, the tracheal
system is nonfunctional and respiration is cutaneous, gaseous exchange
occurring directly between the tissues of the parasite and body fluids of the
host. Some endoparasitic insects have tracheal gills. The larvae of Cotesia
(Hymenoptera, Braconidae) possess caudal vesicle which is an everted
structure of the hindgut. The wall of the vesicle is very thin and is
associated with the heart so that 02 passin g in is quickly carried round the
body (Fig. 1 1 ). Others depend, at least partly, on aerial Oz, obtaining it
either by tubes or other structures that communicate with the tracheal
system and that extend out of the host to the atmosphere.
Respiratory System
c::::::DA
[ 1 43
caudal vesicle
proctodaeum everted
to form vesicle
Fig. 1 1 . (A) Caudal vesicle of Cotesia Jarva,
(8) Longitudinal section of the vesicle.
[ IX] Respiratory pigments
The
haemolymph
does
not
generally
function
as
an
oxygen
carrier.
H owever, the haemolymph of certain chironomid larvae (bloodworms),
( Gasterophilus) ,
the endoparasitic bot fly larva
(Anisops )
and the backswimmers
contain haemoglobin as a respiratory pigment. Under conditions
of high oxygen tension this haemoglobin is saturated with oxygen and
thus does not serve as a carrier.
However,
under conditions of low
oxygen tension the haemoglobin is unsaturated and hence available to
carry oxygen.
[ X] Central nervous control of respiration
The ventilation involves the muscular system and the nervous system for
activation of and coordination over the muscular movements. As ventilation
is an oscillating system, it is regulated by specific ganglia that contain the
essential
neural
net-work
and
mechanisms
for
generating
coordinated
rhythmic outp}lts of the motomeurons that stimulate the muscles expressing
desired behaviour. These motor output programmes are usually instinctive,
stereotypic, and species specific. It is believed that each ganglion, which
registers
some
input
to
the
muscles
that
produce
either
ventilatory
movements or spiracular closure, has its own local control center; but one
of these
may
serve
Schistocerr:a gregaria
as
the
pacemaker.
The
ventilatory
pacemaker
in
appears to be in the metathoracic ganglion, whereas
in dragonflies nymph it resides in the last abdominal ganglion.
Important Questions
1.
2.
3.
Describe tracheal system o f insects.
Give an account of respiration in aquatic and endoparasitic insects.
Write short notes on : (i) Plastron; (ii) Physical gills; (iii) Spiracles and (iv) Air sacs.
(Z-57)
9
E xcre tory System
The function o f the excretory system i s to maintain a constant internal
environment (homeostasis) which is largely determined by the haemolymph
as it surrounds the visceral organs of the insects. Thus, the excretory
system maintains the uniformity of the haemolymph which is achieved by
the elimination of nitrogenous metabolic wastes and the regulation of salt
and water. The Malpighian tubules (named after their discoverer, Marcello
Malpighi, a seventeenth-century Italian scientist) are concerned in the
excretion whereas the rectum is involved in reabsorption of salts and water.
Both excretory substances and salts and water pass into the rectum.
Nitrogen is usually excreted as uric acid with minimum of water and thus
conserves water.
Excretory Organs
[ I] Malpighian tubules
Malpighlan tubules lie in the haemocoel and are attached to the gut at the
junction between the midgut and hindgut. Each tubule is usually long
slender blind tube and may open directly into the rnidgut or hindgut or
more commonly into a dilated ampullar structure. These tubules are
commonly convoluted and are usually free in the body cavity. Their number
varies depending on species, from 2 (in scale insects) to 250 or more (in
orthopterans) with large surface area. In Periplaneta, with 60 tubules, their
total surface area is about 1 320 cm2. Certain insects lack Malpighian
tubules, e.g., springtails and aphids.
1. Association of the Malpighian tubules with -the gut. At least two
types of arrangement of Malpighian tubules and posterior part of the gut
(Z-57)
Excretory System
{ 145
midgut
-�__,� malpighian
tubule
hindgut
hindgut
rectu m
A
B
midgut
malpighian
tubule
hindgut
hindgut
rectu m
c
D
Fig. l. Major types of Malpighian tubule-hindgut system. (A) orthopteran type,
(B) hem1pteran type, (C) coleopteran type, (D) lepidopteran type. Arrows indicate the
direction of movement of substances in and out of the tubule lumen.
are observed: gymnonephridial (free kidney) and cryptonephridial (hidden
kidney) arrangement.
(a) Gymnonephridial a"angement. The distal ends of the Malpighian
tubules are lying freely in the body cavity. This type of Malpighian
tubules are of two types : orthopteran and hemipteran types.
(i) Orthopteran type. Histologically the Malpighian tubules are alike
throughout its length and are only secretory in nature (Fig. l A).
(ii) Hemipteran types. Histologically the basal absorptive region of
the Malpighian tubules differs from the distal secretory region (Fig. l B ).
(b) Cryptonephridial a"angement. The distal ends of the tubules are
embedded in the tissues surrounding the rectum. Such an arrangement is
concerned with improving the uptake of water from the rectum. This type of
Malpighian tubules are also of two types: coleopteran and lepidopteran types.
(i) Coleopteran types. Similar to orthopteran type, the Malpighian
tubules are alike throughout its length and are only secretory in nature
(Fig. I C).
(Z-57)
146 1
Excretory System
(ii) Lepidopteran types. Similar to hemipteran type, the basal absorptive
region of the Malpighian tubules differs from the distal secretory region
(Fig. lD).
2. Histology of Malpighian tubules. Except silverfishes, earwigs and
thrips, muscles are associated with the tubules which produce serpentine
movement in the Malpighian tubules that helps in propelling the contents
of the lumen toward the opening into the alimentary canal, mixing of the
luminal contents, and exposure of the tubules to more haemolymph. The
tubules are usually well tracheolated and are one-cell thick with one or a
few cells encircling the lumen. These cells rest on a tough basement
membrane. The cytoplasm of these cells varies in appearance and is usually
colourless. It is generally filled with various refractile or pigmented
inclusions and sometimes contains needlelike crystals but may be nearly
clear. In some insects the free margins of the cells of more distal parts of
the tubule are produced into cytoplasmic filaments and packed very close
together, forming the so-called honey-comb border (Fig. 2) and is secretory
in nature. The more proximal cells have a typical brush border. This too,
is formed of cytoplasmic filaments, but these are separated from each
other by their own width and are concerned with absorption through active
transport. The tubular cells contain a very large number of mitochondria.
lumen of tubule
hOl}e}'COffib
border
mitOchondria
A
haemocoel
8
Fig. 2 . Transverse section of a cell from ( A ) the distal end of a Malpighian tubule
showing the regular cytoplasmic filaments of the honeycomb border, (B) the proxirnl region
showing the irregular filaments of the brush border. Arrows indicate the direction of
secretion.
(Z-57)
Excretory System
[ 147
[ II] Nephrocytes
Nephrocytes occur singly or in groups in several parts of the body. Their
size varies with insect species. They are large in dipterous larvae whereas
are small and may be multinucleated in others. They are closely associated
with pericardium and hence are also known as pericardia} cells. In
dragonfly the nephrocytes are scattered throughout the fat body. The
nephrocytes transform the original waste materials into a form that enter
routine metabolic pathway later on. It is supposed that nephrocytes also
take part in protein metabolism and regulation of heartbeat.
[ III] Excretion by rectum
The Malpighian tubules of Periplaneta do not contain uric acid, but the
granules of it are found in the wall of the rectum and in the faeces
suggesting that the hindgut may have an excretory function. There are
typically six rectal pads in Periplaneta each is a longitudinal folding of
cuticle containing thickened patches of epithelium and many tracheal
brancqes. In ammoniotelic insects, ammonia passes directly into the gut
without involving Malpighian tubules. In certain aquatic insects ammonia is
secreted directly into the rectum.
[ IV] Other excretory organs
Springtails that have no Malpighian tubules, and larvae of wasps and bees
and oriental cockroaches in which Malpighian tubules do not excrete uric
acid, uric acid is eliminated through other organs given below :
1. Labial glands. In springtails, labial glands are supposed to be
involved in excretion which consist of an upper saccule followed by a
coiled labyrinth and have a gland opening into the outlet duct (Fig. 3).
2. Utricular glands. In cockroaches (Blatta, Blatella), uric acid is
stored temporarily in the utricular glands (male accessory glands) and
then is poured out over the spermatophore during copi,Jlation. Recent
Fig. 3. Labial gland of a springtail.
148 J
Excretory System
studies demonstrated that it provides an alternate source of nitrogen to
the embryo. Also, the female's own uric acid stores could be passed
onto her embryos. Thus, it is suggested that both sexes can make a
parental investment in the offspring. Uric acid in embryo is hydrolysed
by the enzyme uricase produced by micro-organisms (in mycetocytes of
fat body) and serves as a nutritional nitrogen source.
3. Fat body. In the oriental cockroaches, uric acid is also stored in
the urate cells of fat body. It is possible that the uric acid in urate cells
provide a store of nitrogen (storage excretion) for use in the production
of new tissue or that after reduction it supplies adenine for
nucleoprotein synthesis. Uric acid stored in the fat body of larvae may
be the end product of metabolism of the individual cells and is
subsequently, in pupa, it is transferred to the � alpighian tubules and
excreted with meconium.
4. Other tissues. Epidermis of Rhodnius also accumulates uric acid
and during each moulting it is removed. Uric acid produced during
pupal stage may also be stored in scales of wings in butterflies.
Nitrogenous Excretion
[ I) Excretory products
Nitrogenous products of various types are usually accumulated in the
haemolymph as a result of protein, amino acid, and nucleic acid
metabolism. These materials are usually of no use to an insect and may be
toxic and it must either be excreted or stored in an inert state until they
can be used for another function or be excreted. Insects excrete
nitrogenous wastes in the form of ammonia, urea, uric acid, allantoin,
allantoic acid, amino acids and even protein. Table I shows the distribution
of nitrogen in the excreta of insects.
The habitat of the insects usually determines the type of excretory
end products. Like other animals, most terrestrial insects are uricotelic
(excrete uric acid), whereas most aquatic insects are ammoniotelic
(excrete ammonia) (Table I). The aquatic larvae produce ammonia as
its major excretory product while the terrestrial adults produce uric
acid. Uric acid, however, is the major waste product and excreted,
making up 80% or more of the nitrogenous end products observed in
the urine of most terrestrial insects as it does not need a large amount
of water for its elimination being less soluble in water. Excreting uric
acid the insects also conserve water. On the other hand, ammonia is the
major nitrogenous waste produced by aquatic insects as ammonia is
highly soluble in water. The red cotton bug (Dysdercus) excretes a large
amount of allantoin but no uric acid, although the latter is present in
the haemolymph. The meconium of moths and butterflies contains
Excretory System
[ 149
Table 1. The distribution of nitrogen in the excreta of insects. Values are expressed
percentage of the total nitrogen in the excreta.
Insects
Habit/habitat
Uric
acid
Urea
Rhodnius
blood feeder
(terrestrial)
90
+
blood feeder
(terrestrial)
42-47
8- 1 2
Mosquitoes
NH3
Sialis larva
entomophagous
(aquatic)
Bombyx larva
herbivore
(terrestrial)
Dysdercus
herbivo�e
(terrestrial)
larva
Amino
acids
Protein
+
6-10
larva blood feeder
(aquatic)
Lucilia
Allantoin
as
4-5
90
9- 1 1
10
90
86
12
13
61
6
allantoic acid. Urea is commonly present in the urine of insects in very
small quantity. In tse tse fly
histidine,
from
absorption.
The
the
blood
(Glossina) ,
of
allantoin,
two amino acids, arginine and
the
host
are
allantoic
acid,
and
excreted
urea
are
unchanged
after
produced
from
the breakdown of uric acid.
[ II] Mechanism of excretion
Materials in excess in the haemolymph are basically filtered through the
Malpighian tubules which are highly permeable to small molecules.
They
enter the lumen of a tubule either by simple diffusion (e.g., sugars, amino
acids, urea, certain ions) or linked with active transport of potassium ions,
which generates fluid flow (e.g., uric acid). Figure
4 shows the movement
of
the
ions,
water
and
other
molecules
between
haemolymph
and
Malpighian tubules and the hindgut and the haemolymph in a generalised
insect. In some insects, e.g.,
Rhodnius ,
instead of uric acid, potassium urate
is secreted.
Either the whole of the tubules or more distal parts of the
tubules
secretory
are
(Fig.
1). The substances that are needed by the
insects are reabsorbed into the haemolymph either in the proximal portion
of the tubules (hemipteran and lepidopteran types of Malpighian tubules)
and/or in the rectum.
These reabsorption processes may also
transport or simple diffusion.
be active
A continuous flow of water down the tubules
to the rectum carries the uric acid with it so that ultimately the nitrogenous
waste is excreted with the faeces through the anus. The rate of movement
of
K+ and, and hence of water, is proportional to the concentration of K+
150 1
Excretory System
. . . . ....... _.
mode of movement unknown
••••� active movement
� passive movement
-----•
suggested movement
proposed l inkage
between K and
• bulk flow
H20 transport
....... I. .
.
sugars
amino
acids
urea
Na
ph 6.8 to 7.5
K
other
ions
···
�
t
superior
tubule
,.
amino acids and
,, sugars to haemolymph
midgut
to exterior
rectum with
papillae
H2o Na
Fig. 4. Diagramatic representaion of the movement of ions, water, and organic molecules
between the haemolymph and Malpighian tubules and the hindgut and the haemolymph in
a generalised insect.
in the haemolymph as the movement of water is linked with the movement ·
of
K+
.
The
K+
movement is also correlated with the rate of secretion.
[ III] Salt and water balance ( osmoregulation)
Different environmental conditions pose different salts and water problems
for i�sects. Terrestrial forms are constantly faced with the tendency to lose
water through evaporation and are generally dependent on ingested food
for needed water and salt. Depending on the water content of their diet,
the faecal material may be quite watery (e.g., plant sap-feeding insects that
take in an excess of water), or a dry powdery pellet (e.g., insects that feed
on
materials
of very
low
water content
such
as
cereals).
Similarly,
freshwater insects in which a large amounts of water is absorbed through
the integument and by the gut along with ingested food must excrete
water and at the same time must conserve the inorganic ions. Marine
insects, similar to terrestrial insects, must constantly conserve water or
utilise metabolic water. They also take high amount of salt with the food
and the excess is eliminated in the urine after regulated resorption from
the rectum.
Excretory System
[ 151
Active transport is probably not always involved in rectal absorption.
Dysdercus,
For example, in
absorption is entirely passive and occurs only
when the rectal fluid is hypotonic relative to the haemolymph.
Certain
other
factors,
e.g.,
spiracular
control,
integument
permeability, food selection, and habitat selection are also involved with
the
regulation of salt and water in insects. In certain aquatic insects
·
(e.g., mosquito larvae), chloride ions are taken into the haemolymph by
way
of
papillae
occurring
against
surrounding
very
the
anus.
This
is
an
active
process,
high
concentration gradient. In addition,
+
+
papillae are also responsible for Na , K , and water uptake.
these
Some insects are able to absorb water from a drop on the cuticle.
Periplaneta
The cuticle of
is asymmetrical with regard to the passage of
water since water passes in more quickly than it passes out.
r IV] Dietary problems and excretion
Insect diets considerably affect the excretory systems to enable the insect
to encounter the problems created by the type of food ingested. Insects
feeding on vertebrate blood must actively conserve sodium by reabsorption
of Na+ from a food source low in that particular ion, whereas herbivore
+
and
insects face a different problem (i.e., the food is high in both K
++
Ca
). Thus, they must excrete the excessive amounts of both ions to
maintain homeostasis of the haemolymph.
In
addition
to
the
problems
associated
with
differences
in
ionic
concentrations between the food sources and the haemolymph, herbivore
insects face an additional problem, i.e., the toxic phytochemicals. Though
such insects hav� ·ability to detoxify these chemicals, but if absorbed into
the
haemolymph,
the
detoxified chemicals
and/or
the
toxic
chemicals
themselves must be excreted. In such a situation following ingestion of
plants
containing
Malpighian tubules
the
toxicant,
the
transport
mechanism
of
the
facilitating rapid excretion of the
'
toxin from the insect' s haemolymph. However, some insects retain and
sequester these
e.g.,
Z.Onocerus
is induced,
toxicants
to
thus
their
benefit rather than
to excrete them,
(a grasshopper) .
[ V] Control of diuresis and gut motility
Diuresis, or the production of urine, in insects is controlled by diuretic or
antidiuretic hormones. These substances have been isolated from the pars
intercerebralis
of the brain,
the
corpus
cardiacum,
and
various
ventral
chain ganglia, including the sub-oesophageal ganglia. A diuretic peptide
(DP)
from
Malpighian
Locusta
tubules
and
an
antidiuretic
of the house cricket,
hormone
(ADH)
effecting
Acheta domesticus,
the
have been
isolated. Similarly, a chloride transport stimulating hormone (CTSH) has
Excretory System
152 1
been isolated, which has been shown to regulate both ions and water
balance in the rectum of the locust. Proctolin, a neuropeptide that was
isolated from the hindgut of
Periplaneta americana
is widely distributed in
the insect nervous system, and functions as an excitatory neurotransmitter.
It produces a myotropic effect on the visceral muscles of the hindgut.
Important Questions
I.
2.
3.
Give an account of excretory organs o f insects.
Describe the excretory physiology of insects.
Write brief notes on : (i) Malpighian tubules, (ii) Excretory products, (iii) Salt and
water balance in insects, (iv) Nephrocytes.
IO
R eprodu ctive System
Insects usually reproduce sexually and are dioecious, i.e., sexes are
separate. The purpose of the male reproductive system is (i) to produce,
store, and to deliver the sperms (spermatozoa); (ii) to produce the seminal
fluid, which nourishes and provides an appropriate environment for the
sperms; and (iii) to induce the female for oviposition. The female system
produces and stores eggs, provides the eggs with the necessary nutrients
for embryonic development, receives and stores sperms, is the site of
fertilisation, deposits eggs and may provide additi'onal protection to the
embryos. The reproductive system of both male and female insects usually
consists of a pair of gonads connected to a median duct that opens
exteriorly through a gonopore. Some accessory glands are often associated
with the reproductive systems for secondary sexual purposes, - e.g., in the
formation of spermatophore in males and egg-cases (otheca, eggpod etc.)
in females.
Male Reproductive System
The male reproductive system is located in the abdomen and typically
consists of paired testes connected by ducts (vas deferens, seminal vesicle
and ejaculatory duct), which ultimately open into the intromittent organ
(aedeagus or penis). Accessory glands of various sorts are usually
associated with these ducts (Fig. 1 ).
·
154 1
Reproductive System
vas deferens
accessory glands -=:!E---L..L---..:!.­
seminal vesicle J
ejaculatory duct
8
A�-��;:--- connective
tissue sheatn
epithelial sheath
A
c
Fig. l. Male reproductive S)Stem of a generalised insect. (A) Principal organ S)Stem,
(B) Detailed structure of a testis, and (C) Section of a testis to show its histology.
[ I] Testes
Testes (sing. testis) are paired structures lie in the abdomen. Basically each
testis is composed of a number of sperm tubes or testicular follicles, e.g.,
there is only one follicle in certain beetles, two follicles in lice and more
than 100 in grasshoppers. The wall of the follicle is formed by a layer of
epithelial cells lying on a basement membrane, which are supposed to
absorb
nutrients
from
the
haemolymph
for the
germ
cells
(Fig.
2).
Sometime, the two testes are close together and may fuse each other, e.g.,
locust, moth.
Usually
contains
the
the
distal
germ
end
cells
sperms, the process is
of
each
{spermatogonia)
testicular
that
follicle
divide
to
(germariurn)
give
rise
to
known as spermatogenesis. This process usually
occurs during the last larval instar or pupal stage and in some species
it continues in the adult stage. Each follicle contains a large apical cell
or complex of cells that nourish the spermatogonia which are associated
with other cells forming a cyst (Fig.
2A). Three zones of development
are usually recognised below the germarium that represent the different
stages of spermatogenesis: i. the growth zone or zone of spermatocytes
where
the
spermatogonia
(diploid)
undergo
several
mitotic
divisions,
Reproductive System
[ 155
germarium
epithelial sheath
zone of growth
sperm cyst
zone of
maturation
and reduction
B
spermatids
spermatozoa
spermatozoa
3-layered
wall
zone of
transformation
A
c
2. (A) Diagramatic internal structure of a testicular follicle, (B) Spermatophore of
cockroach, and (C) Longitudinal section of a spermatophore.
Fig.
forming primary spermatocytes
primary
spermatocytes
spermatids;
and
spermeogenesis
iii.
and
the
where
flagellated sperms.
efferens
(diploid)
vas
(diploid);
undergo
basal
the
and
the maturation
meiosis
and
transformation
spermatids
When the cysts
deferens
ii.
rupture,
then
zone
become
housed
the
in
or
haploid
zone
transformed
sperms
the
zone where
produce
of
into
enter the vas
seminal
vesicles.
Commonly, when the sperms are released into the ducts, they remain in
bundles
(spermatodesms)
held
together
by
gelatinous
material.
The
sperms of most insects studied are filamentous with very narrow heads.
The head and tail of the sperms are of approximately the same diameter.
The movement of sperms within the male reproductive system is due to
contractions of the muscles associated with each vas deferens and the
ejaculatory duct.
[ II] Vas deferens
A very small duct, the vas efferens, communicates each testicular follicle to
a lateral duct, the vas deferens. The vasa deferentia from each testis unite
in middle to form the ejaculatory duct. However, in primitive insects like
mayflies,
each
vas deferens opens
separately
to the exterior.
The
vas
deferens is covered with a layer of muscles and connective tissue. In few
insects, each vas deferens dilate distally to form seminal vesicle where
sperms are stored for a while.
156 J
Reproductive System
[ III] Ejaculatory duct
The vas deferens of either side unite to form the ejaculatory duct that ends
in the penis or aedeagus at the gonopore. It is ectodermal in origin and is
lined with cuticle and helps in the propulsion of semen. The wall of the
duct is usually muscular and contractile. In insects where spermatophores
are formed, the ejaculatory ducts are very complex, e.g., locusts and
cockroaches and may also have glandular functions.
[ IV] Accessory glands
Several accessory glands are associated with the vasa deferentia or the
seminal vesicle or the ejaculatory duct, however, 'these are absent in
silverfishes and flies. In Locusta there are 15 pairs of accessory glands
whereas in Periplaneta there are a large number of glands (mushroom
glands or utricular glands). The accessory glands are formed either as
evaginations of the vasa deferentia (mesodermal origin) or the ejaculatory
duct (ectodermal origin). In some insects the portion of vasa deferentia or
ejaculatory duct may also have glandular functions. The secretions of the
glands (the seminal fluid) usually contain chemicals that activate the
sperms and also in the production of spermatophores. In addition, the
secretions of accessory glands may stimulate mated female for oviposition,
accelerate oocyte maturation and stimulate contractions of the genital ducts
that help in sperm movement, and inhibition of subsequent insemination by
formation of vaginal plugs or be exerting an effect on the female' s
behaviour.
There is evidence for hormonal control of seminal fluid production
and growth of the accessory glands. Removal of corpora allata from
young males of several species retards growth of accessory glands.
[ V] Spermatophores
In few insects the sperms are not transferred directly into the spermatheca
of female but are passed in specialised gelatinous capsules known as
spermatophores in which sperms are held together by secretions of the
male accessory glands (Fig. 2B). Spermatophores are common in the
primitive insects (e.g., the apterygotes, cockroaches, grasshoppers, crickets)
and rare or absent in neopterans, e.g., Hymenoptera. Two or more layers
may be visible in the capsule having one or two sacs containing sperms.
Spermatophores are formed either before the male pairs with female (e.g.,
crickets, longhorn grasshoppers) or during the copulation (cockroaches).
However, in certain moths it is produced within the female genitalia.
Female Reproductive System
The female reproductive system is located in the abdomen (Fig. 3A). It
typically consists of paired ovaries connected by lateral oviducts to the
Reproductive System
[ 157
terminal
filament
terminal filament
germarium
ovariole
ovary
vitellarium
spermathecal
gland
."'
spermatheca
pedicel
A
B
Fig. 3 . Female reproductive system of a generahsed insect. (A) Pnncipal organ system, and
(B) Detailed stucture of an ovariole.
common or median oviduct. The common oviduct opens posteriorly into a
genital chamber which sometimes form a vagina. The vagina often
developed to form a bursa copulatrix which opens to the exterior and
receives the penis during copulation. Attached to the genital chamber of
vagina are a spermatheca for the storage of sperms and a pair of accessory
glands.
[ I] Ovaries
The ovaries are bilaterally located, mesodermal organs that produce eggs.
Each ovary is composed of a number of functional units or ovarioles which
are covered by a layer of epithelial cells along with muscles and large
number of tracheae. The tracheae provide necessary oxygen to the
developing follicles for the maturation of eggs.
The number of ovarioles per ovary is normally constant for a
species but varies greatly in insects, from 1 in the tse tse flies, and
some aphids to over 2000 in the queens of certain termite species.
Normally larger species have more ovarioles than smaller ones.
In most insects, each ovariole at its distal end is produced into a
long terminal thread that joins those of its neighbours, forming a
terminal suspensory filament. Sometimes the filaments of two sides
merge into a median filament, which attaches to the dorsal diaphragm.
Reproductive System
158 J
At the base of each ovariole is a small duct or pedicel, which joins
those of the other ovarioles in a bulbous calyx.
Oogenesis includes all those processes that ultimately lead to the
development of a . mature ovum, capable of being fertilised, and
development of the nutritive capacity to support embryonic development.
1. Histology of ovariole. Each ovariole is divided into zones that
contain germ cells or oocytes in various stages of development and
maturation (Fig. 3B). There are two broad zones, the apical gennarium
and the basal vitellarium covered by outer ovariole sheath. The
germarium contains the oogonia that divide mitot1cally and become
primary oocytes. The germarium also contains prefollicular tissue which
forms follicular epithelium in the vitellarium that uptakes the nutrients
(yolk) from the haemolymph and deposit it in the mature egg. The yolk
or vitellin is synthesised in the fat body, released into the haemolymph
and taken up by the oocyte through the endocytosis.
2. Types of ovariole. There are 2 major types of ovarioles, based on
the method by which yolk deposition occurs: panoistic, and meroistic.
(a) Panoistic ovari.ole. It is the most primitive type of ovariole and
has no trophocytes or nurse cells (Fig. 4A). Each developing oocyte is
surrounded by the follicular epithelium and follicular plugs are found
terminal filament --..J.
.
nutritive
cords
oocytes
nutritive
cells
chorion
egg
A
B
c
Fig. 4. Longitudill al sections of different types of ovarioles. (A) Simple panoistic ovariole
havmg only oocytes and follicular epithelium; (B) Polytrophic ovariole having oocytes,
follicular epithelium, and nutritive cells; and (C) Telotrophic ovariole having nutritive cells
connected to oocytes by nutritive cords.
Reproductive System
[ 159
between adjacent oocytes. Panoistic ovarioles are found in the
apterygotes, grasshoppers, crickets, termites, dragonflies, stoneflies, fleas,
and some beetles.
(b) Meroistic ovariole. This type of ovariole has nurse cells.
Depending upon the site of the nurse cells within the ovarioles, the
ovarioles may be either polytrophic or telotrophic. Polytrophic ovarioles
(Fig. 4B) are characterised by the presence of nurse cells which are
directly associated with each developing oocyte, e.g.� ant-iions, moths,
butterflis, true flies, wasps, bees etc. Telotrophic ovarioles (Fig. 4C) are
characterised by the presence of nurse cells in the terminal germarium
(e.g., heteropteran bugs and most of the beetles) and the nurse cells are
connected to the various oocytes by means of cytoplasmic strands or
nutritive cords.
3. Growth of the oocytes. Growth of the oocytes takes place in two
phases: a slow phase and a rapid phase. During slow phase, oocytes
and trophocytes both grow at almost the same rate and various essential
nutrients (RNA, DNA, protein, lipids, some carbohydrates) are passed
into the oocytes by the trophocytes in meroistic ovarioles. Oocytes may
also synthesise some nutrients. During rapid phase yolk deposition
(vitellogenesis) takes place.
4. Vitellogenesis. Yolk deposition into the oocyte occurs in the
lower parts of the ovariole that results in a very rapid increase in size,
e.g., oocyte volume of Drosophila increases about 1 ,00,000 times during
vitellogenesis. The rate of vitellogenesis determines the rate of ovulation.
Yolk deposition in oocytes may occur during late larval or pupal
stages not only in insects which do not feed as adults (e.g., certain
moths, mayflies) but also in insects that feed as adults (e.g., parasitic
wasps). In this situation the adults do not need preoviposition time and
begin to deposit eggs just after emergence. However, majority of insects
require a period of maturation before the deposition of eggs. This
period varies from few days to weeks.
The yolk in oocytes may be a protein yolk which is a
protein-carbohydrate complex, lipid yolk and polysaccharide yolk. The
protein yolk is most abundant and forms the richest deposit of protein
in the oocyte. The different types of yolk are derived from different
sources.
S. Paedogenesis. In most of the insects, oogenesis occurs in the last
larval instar or in the pupa or in the adult stages. However, in some
species immature stages are capable of producing mature oocytes that
may commen<>e and even complete embryogenesis. This phenomenon is
known as
paedogenesis, e.g., Micromalthus debilis (a beetle) and
Miastor species (midges).
(Z-57)
Reproductive System
160 1
6. Ovulation. The passage of the oocyte into the oviduct is known
as ovulation. It involves escaping of the mature eggs from the follicular
epithelium and the breakdown of the epithelial plug at the entrance to
the pedicel . In grasshoppers, all the ovarioles ovulate simultaneously, but
in
viviparous
insect eggs
subject
prevent
glands
to
fl ies,
are
dehydration.
water
may
they
usually
loss.
help
function
The
Coatings
in
alternately
deposited
water
or
outside the
chorion
serves
of
eggs
the
in
sequence.
parent
as
a
protective
secreted
conservation.
Some
Because
female, they
by
the
eggs
laid
are
coating,
accessory
in
moist
situations are capable of absorbing water from their surroundings.
7. Eggs. The
shape
and
size
of the
mature
insect eggs
vary
in
different group of insects (Fig. SA-I) but typically they are elongate and
oval
in
longitudinal
polylecithal
located
proteins
located
and
centrally
in
in
addition
around
section
(Fig.
centrolecithal,
the
the
to
51). In most instances the eggs are
i.e.,
oocytes.
the
The
carbohydrates
periphery
of
the
amount
of
yolk
yolk
contains
and
lipids.
yolk
The
(periplasm
is
high
high
amount
cytoplasm
or
cortex).
and
of
is
A
typical egg is a bilaterally -symmetrical cell and is encased by two layers:
0
I
'
'
'
'
'
F
G
E
nucleus
chorion
yolk spheres
periplasm or
cortex
J
Fig . 5 Eggs of msect�
( A ) Collembolan, ( B ) Head louse, (C) Malanal mosquito,
( D ) Grasshopper, (E) Damselfly. (F) Cloth moth, ( G ) Lacev.mg, ( H ) Ichneumon wasp,
( ! ) H ouse fly. and (J 1 Sagmal section of a typical egg
(Z-57)
Reproductive System
[ 161
the outer covering is a tough membrane, the chorion or eggshell and
the
inner
is
a
delicate
the
vitelline
are
by
the
follicular
cells.
present,
is non-chitinous,
and
may be
smooth
it
secreted
membrane,
membranes
membrane.
When
or
a
These
chorion
sculptured in
is
a
variety of ways. There are several minute pores called as micropyles in
the membrane that permit sperms to enter it and affect fertilization of
the egg.
[ II] Oviducts
The pedicel or calyx opens into the lateral oviduct. The lateral oviducts
join to form the common oviduct which serves as a communicating tube
between the lateral oviducts and the bursa copulatrix or vagina. The bursa
copulatrix, when it occurs (e.g., moths), is a · pouch-like expansion of the
vagina
which
receives
the
aedeagus.
The
common
oviduct
and
bursa
copulatrix or vagina are ectodermal in origin and, therefore, are lined with
a modified cuticle.
[ III] Spermatheca
In most of the insects, there is usually. a single baglike structure associated
with the common oviduct, bursa copulatrix or vagina in which sperms
are stored prior to fertilization. In flies, the number of spermatheca is two
or three. In grasshoppers, the spermatheca opens into the genital chamber
independently
a
of
spermathecal
the
oviduct.
gland,
or
the
The
spermatheca
epithelium
of the
is
associated
with
spermatheca
itself
becomes glandular. These glands secrete spermathecal fluid that nourishes
the sperm.
[ IV] Accessory glands
There are generally one or two pairs of accessory glands, which usually
open into the apical portion of the bursa copulatrix, vagina, or common
oviduct. These glands vary in structure and function. They usually secrete
adhesive materials to cement eggs to the substratum or hold them together
in ootheca (e.g., colleterial glands of cockroaches). In many aquatic insects,
mayflies
and
stoneflies,
accessory
glands
secrete
gelatinous
masses
surrounding the eggs. The secretion of accessory glands of female tse tse
flies provides nutrition for the incubating larvae (tse tse fly is a viviparous
insect).
The
secretion
of
accessory
glands
in
hunting
wasps
is
used
to
paralyse the prey and in honey bees it is used in a defensive manner.
In ants the accessory glands secrete trailing pheromones.
Insemination and Fertilization
In insects internal fertilization takes place which is generally brought about
by the act of copulation. Most of the female insects are polyandrous, i.e.,
(Z-57)
162 J
Reproductive System
mate several times in her life. However, certain parasitic wasps, bees, flies
and some bugs mate only once in life (monandrous). Males of the most of
the species are polygynous, i.e., mates with several females. During the act
of copulation the semen (sperms plus various glandular secretions)
produced in the male reproductive system is transferred to an appropriate
site in the female reproductive system. Seminal transfer may involve the
passage of either free semen or, in many insects, one or more
spermatophores from the male to the female.
[ I] Insemination
Free semen is usually deposited in the bursa copulatrix or vagina, bat in
some species it may be deposited in the common oviduct, in the lateral
oviducts, or even directly into the spermatheca (direct insemination). Male
dragonflies and damselflies deposit semen in a specialised organ on the
venter of the second abdominal sternite. A portion of that organ is then
placed in the female's vagina, and seminal transfer is accomplished. Many
species in the Cimicoidea (e.g., bed bugs) transfer the sperms into the
haemocoel of the female by perforating its vaginal wall with a spine at the
tip of the aedeagus (haemocoelic insemination). The seminal fluid and
many of the sperms are phagocytosed by the female, while some of them
reach the ovaries.
Spermatophores are usually deposited by the male somewhere in the
female reproductive system: the bursa copulatrix, vagina, or, rarely, the
spermatheca. However, in the apterygotes (e.g., silverfishes), the male
deposits a spermatophore on the substrate and the female picks it up
and deposits it within herself. Various mechanisms are involved for the
release of the sperms from the spermatheca. In most insects the sperms
are either forced out by pressure applied by the female or by the
mechanical perforation of the spermatophore. In some insects the
spermatophore may be digested away, causing the release of the sperms.
After release from the spcnnatophore or deposition in the form of free
semen, the sperms ultimately move to the spermatheca.
[ II] Fertilization
The processes involved in fertilization may be divided into three parts :
(i) release of sperms from the spermatheca, (ii) entry of the sperm into the
egg, and (iii) formation and fusion of the male and female pronuclei.
In monandrous insects, sperms stored in the spermatheca survive for
several months or years and are used to fertilise ovulating eggs in the
median oviduct, bursa copulatrix or in vagina. In polyandrous species,
the storage of sperms in the spermatheca may only be for a short
period of time as it is replenished by subsequent insemination whenever
exhausted.
(Z-57)
Reproductive System
{ 163
mechanisms
The
for
release
of
sperms
from
the
spennatheca
are
not clearly understood. In most of the insects, the spermathecal duct at
its opening are surrounded by sphinctor muscles and that these muscles
are
regulated
embedded
in
by
neurosecretions
these
mechanisms are
muscles.
very
Hymenoptera
precise
by
from
the
monandrous
to
reproduce
parthenogentically
In
prevent
sperm
eggs
the
cells
regulatory
wastage. Many
arrhenotokously,
unfertilised
neurosecretory
insects,
i.e.,
while
sons
daughters
female
develop
develop
by
fertilised diploid eggs. Consequently, a mother is able to manipulate the
sex
of
each progeny during oviposition
by regulating the fertilization.
Several factors (temperature, host-complex, host size, paternal age, host
distribution
etc.)
influence
the
decision
of
female
regarding
the
regulation of sperm release. Information about how this is accomplished
has
important
consequences
in
implementing
control
strategies
using
various parasitic wasps as biological control agents.
Following ovulation, the egg is oriented in the reproductive tract in
such a way that the micropylar region is in close proximity to the site
of sperm release. The sperms migrate to the micropylar region of the
egg, possibly responding chemotactically, and one or more enter the egg
through the micropyle. In the vast majority of insects more than one
sperm
�nters
the
egg,
but
only
one
fuses . with
the
egg
pronucleus.
Excess sperms usually degenerate without disrupting the development of
penn
the zygote. Shortly following the entry of s
into the egg, the egg
nucleus undergoes meiotic division, forming the female pronucleus. The
sperm that will fuse with this female pronuleus loses its tail, becoming
the male pronucleus. The fusion of the two pronuclei forms the zygote
and signals the commencement of morphogenesis.
Oviposition
Most insects are oviparous, i.e., they lay eggs. On the other hand, many
insects are viviparous, i.e., they deposit different developmental stages. The
terms
larviparous
(e.g.,
pupiparous (e.g.,
few
larvae,
and
nymphs,
few
flies)
flies),
nymphiparous
are commonly
pupae,
(e.g.,
used to
respectively.
The
aphids),
refer to
term
and
viviparous
ovoviviparity
is
sometimes used, and it refers to instances where an egg with a chorion
develops, but the egg hatches within the parent before it is deposited.
The
favorable
female
insect
for
survival
the
typically
deposits
of
the
progeny.
insects deposit their eggs on
the
surface
eggs/larvae
Most
of the
of
in
the
leaves,
situations
phytophagous
often
on the
underside to protect from extreme heats and natural enemies. The eggs
are
cemented
to
the
surface
by
an
adhesive
secretion
of
accessory
glands. The locusts secrete a frothy material that encases an egg mass,
which
is
deposited
in
the
ground
whereas
many flies
oviposit
in the
Reproductive System
164 1
surface
of a freshly
deposited cow dung
by
making
a hole
with its
ovipositor. Sawflies deposit their eggs inside plant tissues by using their
ovipositor.
hosts.
Endoparasitic
Aphidophagous
aphids.
Dung
subterranean
culicine
beetles
chamber
mosquitoes
wasps
hover
lay
eggs
constructed
(Culex
deposit
flies
their
deposit
inside
by
the
the
eggs
eggs
inside
near
the
mounds
of
female.
Aquatic
appropriate
patch
the
of the
dung
insects
in
like
sp.) construct a raft of 1 50-300 eggs which
lie flat on the surface. The eggs float in an upright position.
Anopheles
lay eggs singly on the water surface. Each egg is provided ventrally with
air-filled float. The cockroaches deposit their eggs
in
oothecae.
Insects
that parasitise vertebrates often attach their eggs to hair or feathers of
their hosts.
External
genitalia
of male
and
female
insects
are
described
in
chapter 5.
Important Questions
1.
2.
3.
4.
Describe the reproductive system of male insects.
Describe the female reproductive system of insects.
Give an account of insemination and fertilization in insects.
Write short notes on : (i) Spermatophores, (ii) Types of ovariole, (iii) Accessory
reproductive glands, (iv) Spermatheca.
1 1
Post-embryonic Development
The insect larva after the complete development emerges out from the egg
by rupturing the eggshell or chorion. It passes through several stages called
as instars. Each instar is separated by a moult. The growth from larva to
adult involves some degree of morphological changes or metamorphosis.
Thus, egg hatches into first instar larva which later moults to the second
instar larva and so on until at a final moult the adult or imago emerges.
No further moults takes place after attaining adulthood except in the
apterygote insects.
Growth
Since the exoskeleton of insects is relatively inexpansible, insects have
evolved a mechanism that allows for increase in size. This mechanism is the
process of moulting. Moulting involves the periodic digestion of most of
the old cuticle, secretion of new cuticle (usually with increased surface
area), and shedding of undigested old cuticle (the exuvia) which is
commonly referred to as ecdysis. As a typical insect progresses from the
newly hatched immature form, it goes through a series of moults, generally
increasing in size with each one. Each developmental stage of the insect
itself is called an instar, and the interval of time passed in that instar is
referred to as a stadium.
In many insects, especially those with a small number of instars
(e.g., mosquitoes), it is possible to determine exactly the instar of a
given
individual larva
by characteristic morphological characters.
However, in others, different instars do not vary morphologically other
than growth, which may vary considerably with the availability of food
166 1
Post-embryonic Development
and other environmental
factors.
The
final
instar,
during
which sexual
maturity is reached and functional wings (in pterygote insects} appear, is
the adult, or imago.
The
having
number of
between
constant;
in
instars
varies
among insect
species,
the
majority
2 and 20. In some insects the number of instars is
others
it
may
be
variable
in
response
to
environmental
factors (e.g., availability of food and temperature). In some species the
number 0f male instars may be different from the female instar. The
more specialised insects tend to have fewer instars.
Growth in insects occurs as the result of an increase in the number
of cells by mitotic division and/or an increase in cell size. The increase
in
weight
between
a
newly
hatched
immature
and
a
fully
grown
immature is usually quite pronounced, e.g., a fully grown larva of the
carpenter moths
weighs 72,000 times its first instar weight, and it takes
three years to attain this growth.
Metamorphosis
The developing stages (larva, pupa) are morphologically different from the
adult. The degree of difference varies from slight to extreme, with many
intermediates. The developmental process by which a first-instar immature
stage is transformed into the adult is called metamorphosis, which means
literally change in the form. This process may take place. gradually, with
the immature being in general appearance comparatively similar to the
adult (e.g., cockroaches), or it may be quite abrupt, the immature instars
being drastically different from the adult (e.g., butterflies).
Types of metamorphosis
Depending upon the degree of metamorphosis, insects may be grouped as
ametabolous
(apparently
paurometabolous
(gradual
no
metamorphosis
metamorphosis),
or
ametamorphosis),
hemimetabolous
(developing
stages more or less are similar to adult, no pupal stage) or holometabolous
(complete metamorphosis with pupal stage).
1.
Ametamorphosis. Apterygote insects like
silverfishes
do
not
undergo any change in form, the immature instars differ from the adults
only in size, gonadial development and external genitalia (Fig. 1 ). The
insects
are
known
as
arnetabolous.
Both
developing
stages
and
the
adults live in the same habitat.
2. Paurometamorphosis. In certain exopterygote insects like termites,
grasshoppers, cockroaches, most of the bugs, the development is gradual
and the change in form is slight. The immatures resemble the adults in
many respects, including the presence of compound eyes, but they lack
wings,
gonads,
and external . genitalia
(Fig.
2). During the course of
development the wings become externally apparent as
wing pads.
The
Post-embryonic Development
8
[ 167
N1 - - - - N
2
C
-
- -A -
N1 - - - - - N - 2
-
I
-
.
-
-
-
D
L1 - - - - L2 -
-
-
-
-
E L1 - - - L2 - - - - - L:3- - - - P - - Fig. 5. Types of metamorphosis. (A) Ametamorphosis (ametabolous insect, silverfish),
(B) Hemimetamorphosis (hemimetabolous insect, damselflies), (C) Paurometamorphosis
(paurometabolous insect, leafhopper), (D) Holometamorphosis
(holometabolous
insect,
house fly), and (E) Hypermetamorphosis (hypennetabolous insect, oil beetle) (N l ...Nn:
number of nymphal instars; Ll ..L3; number of larval instars; P: pupa and A: adult).
1 68 J
Post-embryonic Development
immature instars in this group of insects are commonly known as
nymphs, although they may also be correctly referred to as larvae. The
insects are known as paurometabolous. Both developing stages and the
adults live in the same habitat like ametabolous insects.
3. Hemimetamorphosis. In certain exopterygote insects like mayflies,
dragonflies and stoneflies, the immature instars pass in an aquatic
habitat while the adults enjoy either terrestrial or aerial habitat. The
immatures appear to be quite different from the adult stage (e.g.,
immature stoneflies, with highly specialised ventilatory gills) (Fig. 3). The
immature stages are called naiads. The insects are known as
hemimetabolous.
In recent terminology, paurometamorpho"sis has been abandoned and
is merged under the heading hemimetamorphosis.
4. Holometamorphosis. This type of metamorphosis, also known as
complete metamorphosis, takes place in endopterygote insects in which
the immature instars (called larvae) are distinctly different from the
adults and generally are adapted to different environmental situations.
The larvae typically lack compound eyes and ;j.isuall have biting and
chewing mouthparts, whether or not they have this type of mouthparts
in the adults. There is a pupal instar between last larval instar and
adult (Fig. 4). The pupa is typically a resting stage protected in some
way (within cocoon, in a puparial case, C?tc.), but the pupae of some
insects are quite active (e.g., the pupae of mosquitoes). The insects are
known as holometabolous.
5.
Hypermetamorphosis
or
heteromorphosis.
In
most
holometabolous insects, all the larval instars are alike except for a few
minor morphological details, however, some holometabolous insects pass
through one or more larval instars that are distinctly different from the
others (Fig. 5). This phenomenon is called hyperme_tamorphosis or
heteromorphosis and has been described in certain species of ant-lions,
beetles, flies, wasps and in all species of Strepsiptera.
y
Types of larvae
The immature stages of insects have a wide variety of forms. In most
instances the nymphs of hemimetabolous species closely resemble the
adults, but in the holometabolous species, the larvae are often drastically
different from the adults. There are different names for the larvae of
certain group of insects, e.g., naiad (larva of dragonflies and mayflies),
nymph (larva of grasshoppers, cockroaches, termites, bugs), maggot (larva
of flies), wriggler (larva of mosquitoes), crawler (larva of lac insects), grub
(larva of beetles, wasps, bees), caterpillar (larva of moth, butterflies) etc.
There are many different larval forms amongst the holometabolous
insects. Some types of larvae are as follows :
Post-embryonic Development
[ 169
1. Protopod larvae.
The larvae represent a very early stage of
development in which little segmentation of the body has occurred and
cephalic and thoracic appendages are either absent or rudimentary.
These larvae are found among certain parasitic Hymenoptera that
larviposit in the haemocoel of other insects placing the embryo in the
only kind of environment possible for survival, e.g., larvae of Platygaster
(Fig. 6A).
2. Oligopod larvae. The larvae are characterised by the absence of
abdominal prolegs, however, thoracic legs are well-developed. Depending
upon the different forms, these larvae are of following types.
(a) Campodeiform larvae. The larvae resemble diplurans in the genus
Campodea , having flattened bodies, long legs, and usually long antennae
and cerci (Fig. 6B) (e.g., several beetles, Neuroptera, and Trichoptera).
(b) Carabiform larvae. The larvae resemble the larvae of carabid
beetles (ground beetles), which are similar to the campodeiform type
but have shorter legs and cerci (Fig. 6C) (e.g., several beetles).
(c) Elateriform larvae. The larvae resemble the larvae of click beetle
(Elateridae) and have cylindrical bodies with a distinct head, short legs,
and a smooth and hard cuticle (Fig. 6D) (e.g., certain beetles).
(d) Platyform larvae. The larvae have flattened bodies with or
without short thoracic legs (Fig. 6E) (e.g., certain Lepidoptera, Diptera,
and Coleoptera).
(e) Scarabaeiform larvae. The larvae are the grubs and have a
cylindrical body typically curled into a C-shape with a well-developed
head and thoracic legs (Fig. 6F) (e.g., several beetles).
3. Polypod or eruciform larvae. The larvae are typical caterpillars
or caterpillar-like larvae and have a cylindrical body with a
well-developed head, short antennae, and short thoracic and - abdominal
legs (prolegs) (Fig. 6G) (e.g., moths, butterflies, certain hymenopterans).
4. Apodous or vermiform larvae. The larvae have worm-like bodies
with no legs and eyes. On the basis of degree of sclerotisation of the
head they are of three types: eucephalous, hemicephalous and
(a) Eucephalous larvae. They have more or less well sclerotised head
capsule with relatively less reduction in cephalic appendages (Fig. 6H)
(e.g., larvae of mosquitoes and wasps).
(b) Hemicephalous larvae. The head capsule and its appendages are
reduced and can be retracted into the thorax (Fig. 61) (e.g., larvae of
certain dipterans).
(c) Acephalous larvae. The head capsule is absent but some cephalic
appedages may be present (Fig. 6J) (e.g., larvae of house flies.).
Types of pupa
All endopterygote insects pass through pupal stage during development in
between the last larval stage and adult stage. Based on the presence and
1 70 J
Post-embryonic Development
antenna
,,, �:;..�mandible
...lcephalothorax
--::;:?
Jprot��rac1c
appenaage
B
�
·. •
,
,
:
....
�
-
- -...
. -.. · _
'• •
:....:: · �
I
G
E
D
F
Fig. 6. Types of J.arvae. (A) Protopod (Platygaste�. (B) Campodeiform (alderfly) ,
(C) Carabiform (ground beetle), ( D ) Elateriform (click beetle) , (E) Platyform (aquatic
beetle), (F) Scarabeiform (twig borer beetle), (G) Eruciform (moth), (H) Eucephalous
(honey bee), (D Hemicephalous (Brachycera fly), and (J) Acephalous (house fly).
absence of the articulated mandibles that are used in escaping from a
cocoon or pupal cell, the pupae are classified as decticous and adecticous.
1 . Decticous pupae. These are of primitive type and have functional
mandibles. The appendages are always free, i.e., exarate (Fig. 7A) (e.g.,
pupae of Neuroptera, Trichoptera, Mecoptera, Coleoptera and certain
Lepidoptera.
2. Adecticous pupae. These pupae do not have functional mandibles,
e.g., Strepsiptera, Coleoptera, Hymeneptera, Diptera, and Siphonaptera.
Based on whether the appendages are free or adherent to the body,
adecticous pupae are classified into exarate and obtect types.
(a) Exarate pupae. The appendages are free and are usually not
covered by a cocoon (Fig. 7B), e.g., the pupae of Siphonaptera,
Coleoptera, Hymenopfera and Cyclorrhapha. The exarate pupae when
encased in the hardened cuticle of the next-to-last (penultimate) larval
Post-embryonic Development
{ 1 71
functional
mouth parts
• !:
" '
..
•,
.J,�:.
-.':·
A
··::
:
. .
...
.:'f
E
D
c
B
Fig. 7. Types of pupae. (A) Decticous exarate (beetle), (B) Adecticous exarate (ichnewmon
wasp), (C) Obtect (silkworm), (D) Coarctate (having puparium, house fly), (E) Puparium
removed (house fly . pupa).
instar, the puparium, is known as coarctate pupae, e.g., pupae of house
fly (Fig. 7C, D).
(b) Obtect pupae.
The appendages are adhered closely to the body
7E), e.g.,
of the pupae and are commonly covered by a cocoon (Fig.
pupae of moths and butterflies.
During
changes
the
transition
occur.
from
These
immature
changes
are
to
adult
many
comparatively
histological
gradual
in
hemimetabolous insects, being expanded throughout the nymphal instars,
however, more changes are recognised during the last instar than during
the earlier ones. In holometabolous insects, these changes occur mostly
in
the
tissues)
tissue
are
pupal
stage· by
and
his.togenesis
reorientation,
usually
removed
means
growth,
by
of histolysis
(reconstruction
and
of
(breakdown
the
differentiation).
the haemocytes.
adult
The
of the
tissue
histolysed
Holometabolous
larval
through
tissues
insects
have
developed a mechanism in which specific primordial or progenitor cells
are set aside in the larval stage for later use during the pupal period.
The masses of such undifferentiated cells are called as imaginal discs.
They
are
used
for
constructing
adult
organs
or
appendages.
From
a
developmental standpoint, it is an ideal evolutionary strategy for setting
aside the building blocks early in the life cycle of the insect, which are
carried by the insect through the embryonic and larval stages
and are
not u sed until the p upal stage.
Important Questions
l.
2.
3.
Give an account of the post-embryonic development o f jnsects.
Write short notes on : (iJ Types of larvae; (ii) Types of pupae.
Define metamorphosis. How does it differs from growth ? Describe the types of
metamorphosis found in insects.
12
Exocrine and Endocrine Glands
There are a diverse group of cells and tissues that secrete a wide variety
of substances with a wide variety of functions, called glands. Though all
cells are secretory to some extent, but secretion is the main function of the
glands. On the basis of the mode of discharge of their secretion, the glands
are classified into exocrine and endocrine glands. Exocrine glands release
their secretions through apertures or ducts into the external world or into
lumens of various internal organs, e.g., accessory reproductive glands,
salivary glands, silk glands, etc. whereas endocrine glands are typically
ductless, and release their secretions directly into the haemolymph.
Exocrine Glands
Morphologically, exocrine glands are either simple unicellular, simple
bicellular, simple multicellular, complex multicellular or compound glands.
A single secretory cell secretes toxic substances and usually contains an
intracellular ducteole (Fig. l A). Sometimes it is associated with another
cell, a ductule cell, that forms a duct for the release of the secretion
(Fig. lB). The simple multicellular glands are formed by invagination of a
large number of secretory cells and the secretion is stored in a common
lumen with a single aperture (Fig. lC). Sometimes a common duct is
associated with them (e.g., accessory reproductive glands, Fig. l D). In
addition, a separate reservoir is attached with the duct which stores a large
amounts of secretion (e.g., salivary glands of cockroach, Fig. 1 E ).
Externally, fine hairs may be associated with the opening of the gland that
help in the rapid dispersal of the secretory materials. Histologically, a
gland is composed mainly of the secretory epithelial cells which are often
Exocrine and Endocrine Glands
[ 1 73
reservoir
gland cells
D
Fig. 1. (A) Simple unicelllar gland, (B) Unicellular gland with ductule cell, (C) Simple
multicellular gland, (0) Multicellular gland with duct, and (E) Complex multicellular gland
with reservoir.
very large and elaborate in structure. Their nuclei may be ovoidal or much
branched and may contain polytene chromosomes (e.g., salivary glands of
maggots).
Most
of
the
exocrine
glands
are
ectodermal
in
origin
and
are
scattered over the insect body. The specific location of a given gland is
often correlated with its function. Some of the major kinds of exocrine
glands
are
wax
repugnatorial
glands,
gland,
reproductive
glands,
lac
glands,
attractant
salivary
cephalic
glands,
glands
glands,
poison
etc.
which
silk
glands,
can
be
glands,
accessory
grouped
as
follows.
[ I] Glands secreting structural materials
Certain
insects
secrete
substances
that
are
used
in
constructing
their
houses (e.g., beehive) or protecting them (e.g., lac or cocoon). Following
glands are recognised to secrete such substances.
1. Wax glands. These glands are mostly found amongst Homoptera
and
honey
bees
and
are
uni-
or multicellular structures
distributed
in
various parts of the integument. They are especially found in Coccoidea
and Aphidoidea in Homoptera and Apoidea in Hymenoptera. The wax
is secreted either in the form of powder or filament. In aphids wax (fat)
is discharged through plates
composed
of a ring
or in aggregation of
several large cells, each cell having a central wax chamber, within which
the secretion accumulates. The wax secreted by
dermal
glands located
ventrally
of the
fourth
the
between
seventh
honey
comb.
the
overlapping
abdominal
The wax
segments
is
sternal
of
plates
honey
secreted as
bees
fluids
is
used
to
through
construct
and exuded through fine
Exocrine and Endocrine Glands
1 74 J
_
cuticular pores and accumulating and hardening in the form of thin
plates which are removed by the hind legs.
2. Lac -glands. Lac is secreted by certain Coccoidea mostly
Tachardiidae and in particular by Kerria lacca
(= Tachardia lacca,
Laccifer lacca) which yields commercial lac. Lac is a resinous substance
produced in large amount by the female and in less amount by the
male insect as a protective covering. The secreting cells are distributed
throughout the integument. In addition to resin, lac also contains certain
amount of wax, pigments, proteins and some inorganic materials.
3. Silk glands. In Lepidoptera and Trichoptera the labial glands are
modified into organs of producing the silk utilised in the formation of
the larval shelters and cocoons. Other groups like, Neuroptera,
Hymenoptera, Siphonaptera larvae also possess silk glands. In the
silk.worms (Bombycidae and Saturniidae) silk is secreted by labial glands
(Fig. 2A) in the form of fibrinogen which undergoes denaturation on
extrusion to fOHB the tough, elastic protein fibroin, and is surrounded
by an outer layer of a water soluble gelatinous protein, the sericin.
[ II] Glands s ecreting defense materials
Insects secrete a variety of chemical substances that are used to defend
them from their natural enemies as they have a pungent smell and other
repellent properties. Following glands are recognised that secrete such
substances.
1. Repugnatorial glands. A large variety of noxious substances are
secreted by the repugnatorial glands which are variable in number,
location, and morphology. The secretion of these glands initiates the
escape response among the predators and other potential enemies. The
chemicals -are discharged from storage reservoir associated with the
apex of
spinneret
�liht' J ; ,t;'?r�-'chitinous
capsule
A
Fig. 2. (A) Silk gland of silkwc1nn, and (B) Repugnatorial gland of bombardier beetle.
Exocrine and Endocrine Glands
·
[ 1 75
poison gland
Fig. 3. Poison gland associated with sting apparatus
of honey bee .
glands by a variety of mechanisms. The scent (stink) glands located on
the dorsum of the abdomen in stink bugs, secrete a number of
odoriferous chemicals (hexanal, hexanol, acetic acid, hexyl acetate, etc.)
that repel their natural enemies. B ombardier beetles eject a hot spray,
containing quinones, from glands in the posterior part of the abdomen
with a distinctly audible explosion (Fig. 2B). Similarly, ants secrete
formic acid which is used as defense substance.
2. Frontal glands. The frontal gland is unpaired characteFistic organ
of termites particularly in the nasute soldiers. It is a sac like gland and
opens by means of frontal pore. It secretes a sticky defensive material.
3. Pygidial glands. Among certain beetles pygidial glands are
located at the posterior end of the body and open near the anus. It
secretes pungent or corrosive materials.
4. Poison glands. These organs are peculiar to bees and wasps
where they are modified accessory reproductive glands associated with
the ovipositor ·or sting (Fig. 3). The secretions are generally a complex
mixture of several substances like melittin (bees venom) and kinin
(wasps venom). Several lepidopteran larvae are provided with epidermal
poison glands associated with seta or spines which when broken allow
the discharge of a secretion causing urticaria in man.
5. Alarm pheromone glands. The aphids secrete mainly triglycerides
of hexanoic and other acids that are liberated through the comicles
when the aphid is attacked by a predator and act as alarm pheromone.
This causes the aphids in the vicinity to drop off the plants.
[ III] Glands secreting materials for communication
Many insects possess glands that secrete chemicals (infochemicals) that
serve as signals of various sorts for other members of the same
(Z-57)
Exocrine and Endocrine Glands
1 76 J
(intraspecific) or different species (interspecific ). lntraspecific substances
are called pheromones whereas interspecific substances are known as
semiochemicals.
1. Pheromone glands. The pheromones are concerned with the
co-ordination of individual in a population. Mostly, the integumentary
glands of the abdomen produce them. The pheromones are responsible
for attracting individuals of opposite sexes for promoting mating
particularly in moths and butterfly. The trail-marking pheromones are
secreted from the gut or epidermal glands in few species of social
insects (termites and ants) and laid on the ground by successful foragers
returning to the nests.
(a) Aphrodisiac scent glands (androconia). The androconia or scent
scales are found on the wings, legs or abdomen of butterflies and have
an elongated form and terminate in a row of processes or fimbriae
(Fig. 4A). At the base of the scale, glandular cells are present. In
moths, particularly in Bombyx mori, the pheromone glands are formed
by the invagination of epidermal glands lined by cuticle . and opens on
either side of the genital pore. The scent is emitted via the terminal
opening.
(b) Mandibular glands. The mandibular glands of the queen and
worker honey bees is sac-like with an epithelium of secretory cells lined
by a thin cuticle. The duct from the gland opens at the bast'. of the
mandible into a groove which runs into a depression on the inner face
of the mandible (Fig. 4B) and produces the "queen substance, " which
inhibits the workers from constructing queen cells and stimulates various
other behaviours in workers. These are small glands that open near the
ftmbriae
poison
reservoir
_
_
_._
lamina
mandible
dufour's
gland
A
accessory
disc
B
Fig. 4. (A) Aphrodisiac scent gland of butterfly, (B) Mandibular gland of honey bee, and
(C) Dufour's gland and poison gland of a worker ant.
(Z-57)
Exocrine and Endocrine Glands
[ 1 77
base of the mandible in silverfishes, termites, cockroaches, beetles,
wasps, ants, and bees. The mandibular glands of the larvae of moths
attain a considerable size and secrete saliva, the normal salivary glands
(labial) are modified for silk production in this ..gr.oup.
(c) Nassanoff's gland. These glands are mainly found in the worker
honey bees beneath the intersegmental membrane between sixth and
seventh abdominal tergites and secrete pheromones.
(d) Dufour's gland. Dufour' s gland in worker ant open into the
poison duct near the base of the sting. It is a small, simple sac with
their glandular walls and a delicate muscular sheath. The gland secretes
trail pheromone (Fig. 4C).
2. Pheromones as sex attractants.
As stated earlier, the
pheromones are employed by a large number of insects in bringing
sexes together for mating. Such pheromones are also known as sex
attractants. The pheromones are secreted mostly by the females, less
frequently by males. In some species both sexes produce pheromones.
(a) Pheromones attracting male insects. The pheromone glands of
females lie between posterior abdominal segments. Normally scent is
released at particular times of day which are characteristics of the
species. The sex attractants are commonly released after 1-2 days of
emergence from the adult females and continue until a successful
copulation. However, in certain species, pheromone is produced even
before the emergence. In such case, the males assemble nearby the
female occupancy, waiting their emergence for mating. After mating, the
attractiveness of the female decreases in species that mates once in life,
e.g., Bombyx mori, however, the species that mates several times the
release of pheromone may continue undiminished after mating.
The pheromones are perceived by olfactory receptors on the
antennae of the males and it is significant that the antennae of many
male moths are strongly pectinate. The effect of the scent is to excite
the male and to initiate the mating. Males are seen 'copulating' a cotton
swab soaked with the pheromone of the female.
The chemical natures of pheromones of several insects are known.
The principal component of the pheromone secreted by mandibular
glands of the queen Apis mellifera is 9-oxodecenoic acid. The bombykol,
the sex attractants of the Bombyx mori is an unsaturated alcohol having
empirical formula C16 H 28 O H .
(b) Pheromones attracting female imects. There are few examples
where
male
insects
produce
pheromone
to
attract
females
(Harpobittacus, Mecoptera). These insects are predatory and after the
male has seized his prey and started devouring, the pheromone is
released from two vesicles near the posterior abdominal tergite
(Z-57)
1 78 J
Exocrine and Endocrine Glands
attracting the females. On her arrival the male copulates with her and
presents her with the remains of his prey.
(c) Pheromones attracting both sexes. Only few insects secrete
pheromones that attract both sexes, e.g. the female of Dendroctonus and
males of lps and Lycus (beetles). The major function of the pheromone
of Lycus is to attract the population to form an aggregation nearby the
flower of Melilotus. The yellow colour of this beetle is distasteful and it
is believed that as a result of grouping its predators (birds) learn to
avoid them.
3. Pheromones of social insects. In social insects, the pheromones are
either concerned with communication between workers (e.g., ants, bees) or
with the maintenance of colony structure (e.g., termites).
(a) Ant trails. While foraging, the worker ants produce a series of
scent spots on the ground with its abdomen as it runs along. The scent
is released mostly by Dufour' s gland but in certain ants it is also
released by poison glands (Fig. 4C). These marking scent spots (trails)
are mostly species specific and guide the worker fellows to follow the
foraging route as well as safe return to the nest. In ants several other
pheromones are produced that induce gathering and settling of workers,
grooming other workers and food exchange (throphallaxis).
(b) Termite pheromones. In termites particularly in Kalotermes, the
caste differentiation is regulated by a series of pheromones. Whenever
king and queen die, they are replaced by the reproductives within a
week but in excess. Leaving only one pair, others are eaten away by the
pseudergates of the colony and it is believed to be controlled by
pheromones produced by the reproductives. Queen Kalotermes produces
a substance that inhibits the further development of female
pseudergates. In the absence of this substance any female pseudergates
that are competent to do so become replacement reproductives.
Similarly male produces a comparable pheromone which inhibits the
development of male pseudergates. These pheromones are secreted by
integumentary glands and perceived by the antennae of the
pseudergates, as a result, the colony is maintained with one male and
one female reproductive.
(c) Queen substance. Queen honey bees produce pheromones which
play in controlling the social structure of her colony. If a queen is
removed from the hive, her absence is very soon perceived by the
workers who become restless within about half an hour of her removal.
They begin to build emergency queen cells after few hours. In the
presence of queen bee this behaviour is inhibited by a pheromone,
known as queen substance. This substance is spread over the body of
the queen and is licked by the workers and is circulated among workers
(Z-57)
Exocrine and Endocrine Glands
by
licking
each
other
as
a
{ 1 79
result
the
pheromone
is
very
quickly
distributed round the colony.
[ IV] Glands secreting materials involved in physiology
Secretion of salivary glands and accessory reproductive glands are involved
in the digestive (secrete enzymes) and reproductive physiology (formation
of spermatophore and ootheca, activation of sperms, etc.)
of the insects,
respectively.
1. Salivary glands. Maxillary
principal
salivary
in
glands
the
glands
insects.
and
labial
glands
are
The major function
the
of these
glands is to secrete digestive enzymes that help in the digestion of food
materials.
The maxillary glands are found in Collembola
(a) Maxillary glands.
in the larvae or Neuroptera, and certain Trichoptera. Part
and Protura,
of the
salivary
gland
complex
Icerya
of
(Coccid
bug)
is
possibly
a
maxillary gland. They are also reported to occur in beetles.
Labial
(b) Labial glands.
situated
in
the
thorax,
on
glands
either
are
side
paired
of
the
structure
foregut.
generally
Their
ducts
combine to form a median salivary duct which normally opens into the
pre-oral food cavity near the base of the hypopharynx. In grasshoppers
and
cockroaches,
they
are
commonly
very large
and
composed
of a
number of lobes, each consisting of glandular acini; a salivary reservoir
is
also
present
in
many
species
in
relation
with
each
gland.
In
the
phytophagous bugs, the salivary glands not only produce saliva to digest
the food but also certain toxic' materials inj urious for plants.
In adult
moth, the labial glands form filamentous tubes whose secretion forms a
solvent for an enzyme cocoonase. The solution softens the silken cocoon
for the emergence of the adult moth.
In different group of the insects
also
in
function.
In
Panorpa
this
gland vary in structure and
(scorpionfly)
the
enlarged
labial
glands
secrete digestive enzymes, viz., amylase, invertase, protease, lipase. The
saliva
of
blood
sucking
material. Labial glands of
Diptera
contains
Drosophila
anticoagulants
and
toxic
larvae secrete a mucoprotein used
to cement the puparium to its substratum.
2.
are
Accessory reproductive glands. In female and male insects, there
generally
one
or
two
pairs
of accessory
glands
associated
with
genital ducts. For details see chapter 1 0.
Endocrine Glands
Endocrine glands, also known as ductless glands, secrete chemomessengers
called
hormones.
Hormones,
like
the
nervous
system,
regulate
the
physiological and behavioural responses of the insects. However, its action
on physiological
and behavioural
responses
is
much
slower than those
180 J
Exocrine and Endocrine Glands
oenocytes
neurosecretory cells
of nerve cord
corpus luteum
Fig. 5. Diagram showing the secretion sites of the main hormones.
mediated by the nervous system. In other words, nervous system primarily
regulate the body activities that require quick responses, e.g., muscular
activities. In contrast, long-term changes, e.g., development, growth,
reproduction, and metabolism are generally regulated by endocrine
systems. The major endocrine glands are the brain (neurosecretory brain
cells located in the protocerebrum), corpora allata, corpora cardiaca, and
prothoracic glands. However, neurosecretory cells of the oesophageal
ganglion, ventral glands, pericardia! glands, oenocytes, neurosecretory cells
of the ventral nerve cord, and corpus luteum also secrete neurohormones
or neuropeptides (Fig. 5). Like other animals, in insects neurosecretory
cells are modified neurons that secrete neurohormones and act as the link
between the nervous and endocrine systems. Neurosecretory cells are
characterised by the presence of electron-dense granules.
[ I] Neurosecretory cells of the brain
The pars intercerebralis of protocerebrum of brain contains two groups of
neurosecretory
cells
that
secrete
ecdysiotropin
(prothoracicotropic
hormone, PTTH) and transport it to the corpus cardiacum from where it
is subsequently released into the haemolymph. However, recently it has
Exocrine and Endocrine Glands
[ 181
been demonstrated that the brain hormone is also stored in corpus allatum
least in Lepidoptera. Brain hormone stimulates the secretory activity of
the prothoracic gland and play part in growth and metamorphosis.
Several factors influence the secretion of brain hormone, e.g.,
nutritional component of the diet and the abdominal distension resulting
from feeding in blood-sucking bug Rhodnius and stretch receptors in
the pharyngeal wall in the grasshopper Locusta .
at
,
[ II] Corpus cardiacum
The corpora cardiaca serve as neurohaemal organs, are a pair of small
glands located behind the brain in close association with the aorta. In
addition to containing intrinsic secretory cells, the corpora cardiaca receive
axons from the neurosecretory cells in the brain and serve as storage and
release sites for their secretions. The intnns1c secretory cells produce
hormones which are concerned with the regulation of the heartbeat
(Fig. 6).
lumen of aorta
Fig. 6. Transverse section through the posterior
parts of the corpora cardiaca of desert locust.
[ III] Corpus allatum
The corpora allata are glandular bodies, generally oval or elliptical in
outline, and · lie behind the brain or in the neck on the sides of the
oesophagous. These arc usually paired but in higher dipterans they may be
fused to a single body. Each corpus allatum is connected with the corpus
cardiacum of the same side by a nerve which carries fibres from the
neurosecretory cells of the brain. The corpora allata are hollow balls of
cells which are more or less uniform, reticulated, multinucleated and
contain numerous vacuoles. The size of the gland varies with the cycle of
its activity.
The corpora allata produce juvenile hormone (JH) that regulates
the metamorphosis and yolk deposition in the eggs.
182 J
Exocrine and Endocrine Glands
[ IV] Prothoracic or Ecdysial glands
The prothoracic or thoracic or ecdysial glands, found only in immature
insects with the exception of the apterygotes, are irregular masses of tissue
of ectodermal origin that are usually intimately associated with tracheae
(Fig. 5). They may or may not be innervated.
The glands show the cycles of development associated with
secretion. At rest the nuclei are small and oval, but in the active gland
they become enlarged and lobulated and the cell has more extensive
cytoplasm. They secrete the moulting hormone, ecdysone under the
influence of ecdysiotropin (P'ITH or prothoracicotropic hormone) from
neurosecretory cells in the brain.
[ V] Ring glands or Weisman's ring
In maggots of the higher Diptera (suborder Cyclorrhapha), there is a small
ring of tissue, supported by tracheae, called the ring gland, or Weisman's
ring. It is formed by the fusion of the corpora allata, corpora cardiaca,
and the thoracic glands. The ring gland is connected to the brain by a pair
of nerves.
Structure and Function of Hormones
The brain hormone or the neurohormone is a peptide whereas ecdysone is
a steroid hormone and is probably derived from chole<>terol or some
related steroid obtained with the diet. Phytoecdysones have been found in
certain plants. It acts on gene through a receptor-mediated process,
determining which genes are brought into action at a given time, and as a
result influences the kinds of protein synthesised.
At least five non-sterolic compounds are found that act as juvenile
hormone. The actual mode of action of the hormone is not properly
knowp but it is believed that they activate or repress target genes by
binding through its association with a specific receptor to a regulatory
sequence of the DNA. Like ecdysone," JH mimics are found in the
plants, e.g., terpene farnesol.
Specifically, hormones and neurohormones influence colour changes;
osmoregulation; control of heartbeat and amplitude; regulation of
metabolic activities, such as the maintenance of carbohydrate (trehalose)
levels in the haemolymph, synthesis of proteins, and metabolism of
lipids; control of selerotisation and melanisation of the cuticle; control
of growth and metamorphosis; (possibly) control of circadian rhythms;
determination of sexual receptivity; regulation of dormancy; pheromone
production and release; and regulation of migratory behavior.
Table 1 shows the list of selected hormones· and their biological
functions
Exocrine and Endocrine Glands
[ 183
Table I. List of insect hormones, their origin and functions.
Active Princ!I!.le
A. Immature insects
Eci;!rsone
T�et
Ori_&n
Functional Role
1. Non-neural hormones
ecclysial
�and
epidermis
initiates moult
Juvenile
hormone
corpora allata
epidermis
regulate
metamo�hosis at moult
Ecdysone
ovarian tissue
fat body
Juvenile hormone
corpora allata
fat body
Juvenile hormone
corpora allata
production
accessory reproduc- and
tive
and
andular secretion
Juvenile hormone
corpora allata
follicle cells
PTIH
protocerebrum
Bursicon
neurosecre- epidermis
Median
tory cells and thoracicoabdominal Kanglion
Eclosion hormone
brain of
moths
Allatotropin
brain
corpora allata
thoracic ganglion
regulate diuresis and fluid
Malpighian
tubules and rectum secretion
B. Adult insects
Diuretic
�
initiates
I production
induce fat body to produce
vitellogenin
�
hormone
pre-ecdysis abdominal ganglia
stimulates
production
ecdysone
regulates
and
and release of
stimulates
sclerotisation
and melanisation of cuticle
synchronisation of eclosion
with _JJhotoperiod
stimulate
JH
production
and release homeostasis
Mating inhibition
hormone
reproduc- brain
accessory
tive _glands of male
prevents remating
Oviposition
initiation hormone
accessory
reproduc- oviduct
tive _glands of male
initiates oviposition
Cardioaccelerator
hormone
brain I corpora
cardiaca
Proctolin
corpora cardiaca
myocardium
of
activates
potency
and
uptake of vitellogenin by
follicle cells
2. Neural hormones and�tide hormones
ecdysial gland
regulates
and
of vitellogenin
increase in frequency and
of
muscle
amplitude
contraction
contraction,
hindgut and poss- muscle
ible visceral muscle defecation, oviposition and
(heart and oviduct) heart beat
Control of Growth and Metamorphosis
Metamorphosis not only changes the developing stages to adults in most of
the insects but also prepares the insect for major changes in both ecology
and behaviour. The nymphal/larval stages of most of the h�mimetabolous
and almost all holometabolous insects are adapted for feeding and
184 1
Exocrine and Endocrine Glands
brain
1.
2.
3.
4.
5.
6.
�-
r
}�
egg maturation
accessory sex gland secretion
maintenance-of pupal diapause
maintenance of larval dlapause
mating
general metabolism
neurosecretory cells
-
corpus allatum
brain
hormone
',
',,
juvenile
hormone
\
Stirn
IJfation
rnaifl ; - - .... _ _
t n
ance - - ... ....
.... .... ... _
prothoracic
gland hormone
... .... ..
A/
/
prothoracic
gland hormone
�1 :tf':i.""�"'"
i� � �om�so �
��Jfk�
l
/
larva
Fig.
7.
l
'
pupa
protein synthesis
t
adult structures
adult
Principal endocrine glands in the silkworm moth that regulate the metamorpliosis.
accumulating nutrients while the adult forms concentrate on dispersal and
reproduction.
Following endocrine glands are involved in the control of growth
and metamorphosis in insects: 1 . the median neurosecretory cells
(MNSC)-secrete prothoracicotropic hormone, PTTH, 2. the corpora
cardiaca-stores PTTH, 3. the prothoracic glands (PTG)-secrete ecdysone,
and 4. the corpora allata-secrete juvenile hormone (JH) (Fig. 7).
1. Ecdysiotropin. As stated earlier, the MNSC in the pars
intecerebralis
region
of
the
brain
secrete
the
neurohormone
Exocrine and Endocrine Glands
{ 185
ecdysiotropin or P'ITH, which is stored in the corpora allata and is
subsequently released into the haemolymph. P'ITH stimulates the
prothoracic gland to secrete moulting hormone, ecdysone.
2. Ecdysone. The prothoracic glands (PTG) secrete ecdysone or
moulting hormone. This hormone is also secreted by other tissues in the
body, e.g., the ovaries of mosquitoes and the locusts but these sources
are not considered to influence moulting. Ecdysone initiates the. growth
and changes in the cells concerned with moulting.
3. Juvenile hormone. The corpora allata secrete juvenile hormone
(JH), which promotes larval development and inhibits development of
adult characteristics. During the larval instars both ecdysone and JH are
produced. However, during the last immature instar, JH is not produced
or decreases below some threshold. Due to this, the formation of adult
characters is not inhibited, and metamorphosis to pupal and then to the
adult stage occurs. In hemimetabolous insects there is a gradual changes
in the developing stages to the adult stage. In contrast, in
holometabolous insects the changes from immature to adult are drastic
involving an additional pupal stage. In this case also the JH is not
produced or reduces below some threshold during the last larval instar.
Therefore, there is no significant difference in the physiological
mechanisms controlling growth and metamorphosis in either of these
groups. Ecdysone is thought to activate a gene that directs the synthesis
of a key enzyme in the selerotisation process.
At least two other hormones are involved that regulate the growth
and development of some insects, eclosion hormone and bursicon.
3. Eclosion hormone. Eclosion hormone is a neuropeptide and has
been observed in several moths. It is secreted in the MNSC of the
brain. Photoperiod is believed to regulate its release. This hormone
influences several aspects of pupal-adult ecdysis (eclosion), in.eluding the
behaviour associated with ecdysis. Subsequent degeneration of the
abdominal intersegmental muscles takes place which are used in the act
of ecdysis.
4. Bursicon or tanning hormone. Bursicon or tanning hormone is a
neurosecretory hormone secreted and/or released from a variety of
tissues according to the insect species. It is commonly found in
neurohaemal organs (similar to the corpora cardiaca) related with the
ventral chain ganglia. Bursicon stimulate tanning and selerotisation of
the cuticle following ecdysis.
Regulation of Diapause
The ability of insects to survive during unfavourable conditions (e.g.,
adverse climatic conditions or lack of food) in a state of arrested
development is called diapause. The physiological, biochemical, and
186 1
Exocrine and Endbcrine Glands
behavioural adaptation related with diapause is regulated by hormones.
The moths having a pupal diapause, MNSC fail to secrete neurohormone
and thus no ecdysone is secreted by PTG. Inhibition of MNSC is believed
to be due to the secretion of another hormone secreted by corpora
allata. D iapause in the egg stage (e.g., silkmoth) is not dependent on
neurohormone but on temperature and photoperiod of previous generation.
These conditions, acting via the brain, stimulate or inhibit the neurosecre­
tery cells in the suboesophageal ganglion of the parent female. Upon
stimulation, the hormone acts on the eggs in the ovary so that when laid
they undergo diapause.
The adult diapause results from the inhibition of the corpora allata
which are normally responsible for the maturation.
Regulation of Polymorphism
The term polymorphism (poly=many, morph=forms) means many
morphological types of an insect species. According to this definition any
phenomena where different forms are found, are the examples of
polymorphism as differences between the sexes (sexual dimorphism),
differences between individuals in the social insects, differences between
individuals caused by the ecological factors (polymorphism in aphids),
differences between developmental stages (temporal polymorphism), and
so on. However, the more restricted definition (given by O.W. Richards,
1961) is as follows: Polymorphism exists when one or both sexes of a
species occur in two or more forms which are sufficiently sharply distinct
to be recognisable without a morphometric analysis; the occurrence is
regular or recurrent; the rarer of the two forms makes up a reasonable
proportion of the population (say, at least 5%) or, as in some social
species, the rarest type is at any rate essential to the survival of the species.
Polymorphism is observed in a variety of attributes in different insects, e.g.,
patterns of body colour in many butterflies; the presence and absence of
wings in aphids and other insects; chromosomal differences; the relative
size and development of different structures in social insects like ants,
termites and bees; and various integumental structures, including horns,
spines, and other protuberances. There are basically two types of
polymorphism: balanced and environmentally-induced polymorphism; anc�
both are regulated by genes. In the latter type of polymorphism, the
expression of form is determined by several ecological factors that
influence the endocrine system and ultimately the action of different genes.
The brachypterous (short wing) form of cricket Gryllus campestris is
due to result of a slight predominance of JH in the later stages. In
aphids winged (alate) and wingless (apterous) forms occur. The
integument in the apterous forms is less sclerotised due to excess of JH
than alate forms. In certain aphid species like Megoura, the high density
Exocrine and Endocrine Glands
{ 187
of individuals induces corpora allata to secrete high amount of JH that
inhibits formation of wings, in contrast, aphids kept in isolation, the
corpora allata are not induced and wings develop. In other species (e.g.,
Aphis ), crowding induce the development of wing by inhibiting secretion
of JH from corpora allata.
Low temperature also upsets the hormonal balance in favour of JH
whereas high temperature slightly favours ecdysone and may_ cause
increase or decrease of larval instars and development of wings inducing
neoteny (metathely, juvenile charatcers persist in adult) or paedogenesis
(prothetel)I, adult characters present in larva).
Nutrition, both in terms of quantity and quality, plays a significant
role in polymorphism in honey bees. Female larvae all have the
potential to develop into either workers or queens: if larvae are fed
royal jelly for only two or three days, they develop into worker adults,
but if they are fed royal jelly throughout the larval stadia, they develop
into queens.
Variations in the quantity of food produce polymorphism in insects
that grow heterogonically by inducing early or late pupation, e.g., the
mandibles in stag beetles (Lucanidae, Coleoptera).
Important Questions
1.
2.
3.
4.
5.
6.
Give an account of exocrine glands found in insects.
What are the functions of exocrine glands ? Describe in detail the glands that secrete
substances involved in communication.
Describe the glands that secrete pheromones acting as sex attractants and also mention
their functions.
Give an account of endocrine glands of insects.
Describe the hormonal regulation of growth and metamorphosis.
Write short notes on : (i) Repugnatorial gland, (ii) Pheromones of social insect s,
(iii) Juvenile hormone, (iv) Ecdysone, (v) Regulation of diapause, (vi) Hormonal
regulation of polymorphism.
13
Nervou s System
Like other animals, the nervous system in insects serves to coordinate the
activities of its various systems. The units of Uris system are elongated cells,
or neurons, which carry information in the form of electrical impulses from
external and internal sensilla (sensory cells) to appropriate effectors (e.g.,
glands, muscles), and special cells called glial cells, which protect, support,
and provide nutrition for the neurons.
Structure of the Nervous System
[ I] Neurons
The basic functional unit of the nervous system is the nerve cell, or neuron.
Typically, a neuron consists of a cell body (perikaryon or soma) and one
or more long, very thin fibres or axons that end in terminal arborisation.
Frequently the axon has collateral branch. Associated with the perikaryon
or near it there are tiny branching processes, the dendrites.
The neurons may be unipolar (monopolar). bipolar, or multipolar.
Unipolar neurons possess single stalk from the cell body and are more
frequent in insects (Fig. I A). Peripheral neurons are bipolar, the cell
body bears an axon and a single, branched or unbranched dendrite
(Fi.go IB). In hypocerebral and frontal ganglia, the neurons have an axon
and several branched dendrites, hence they are multipolar (Fig. 1 C).
The terminal arborisations of an axon come into extremely close
association with the dendrites or axon of another neuron or they may
end near a muscle (i.e., a neuromuscular junction). The association
between terminal arborisations and dendrites is caJled a synapse, and
Nervous System
[ 189
cell body
�
I
L�
axon
collateral
axon
terminal
arborisations
B
A
sensory
neuron
axon
terminal
arborizations
_,,,.
_
_
intemeurons
D
c
Fig. I . Types of neuron. (A) Monopolar or unipolar, (B)
(D) Relationship among sensory, ITIQtor and interneurons.
Bipolar,
(C) Multipolar,
the space betw.�en the arborisations and dendrites is called the synaptic
cleft. The perikarya lie within the ganglia (Fig.
Several
histological
components
of a
I D).
ganglion
can
be
identified
2). The entire nervous system is enveloped in a connective tissue
(Fig.
called
the
support
neural
for
the
sheath
central
or
neural
nervous
lamella.
system,
It
holding
provides
the
a
cells
mechanical
and
axons
together. Beneath the neural sheath, there is a thin layer of cells rich in
mitochondria called
as
perineurium
which
probably
secretes the
neural
lamella. Below this, regions containing the perikarya with associated glial
cells
are
found.
Indeed,
glial
cells
invest
the
neurons
and
serve
as
protective sheet and insulation. The glial cells also provide nutrition to
the
neurons.
There
is
a
central
region
consisting
of
intermingling,
synapsing axons encapsulated by processes of glial cells, the neuropile.
Between the glial cells are extracellular spaces with fluid. The fluid in
Nervous System
1 90 J
perineurium
neural lamella
{
tight junctions
region of
parikarya
glial cell
processes
. ..,...... longitudinal section
�
of axon
neuropile
cross sections of axons
Fig 2 Cross secl!on of part of the caudal ganglion of the cockroach. Darkly shaded areas
indicate extracellular spaces
these spaces contains higher concentrations of sodium and potassium
ions and a lower concentration of chloride ions than the haemolymph.
Maintenance of the proper ionic concentration of this fluid is critical to
neural function. The neural lamella, perineurium, and glial cells are
involved in maintammg the composition of this fluid as well as
transporting and storing nutrients used by neurons.
Nerves are bundles of axons invested in the neural lamella, and/or
the underlying glial cells that fonn the perineurium. The nerves provide
connection among ganglia and between ganglia and other parts of the
nervous system.
Neurons are usually classified in ways that relate to function, e.g.,
sensory or afferent that receive stimulus from the environment and
motor or efferent that carry the infonnation to glands or muscles;
excitatory or inhibitory; and cholinergic (acetylecholine as neurotrans­
mitter), glutaminergic (glutamic acid as neurotransmitter) etc.
Nervous System
Sensory
[ 1 91
neurons
are
usually
bipolar
with
peripherally
located
cell
bodies. The dendrite is associated with a sensory structure of some type;
the proximal process usually directly associated with a motor neuron or
more
with
one
or
more
intemeurons.
profusely branched over the
inner
Their
surface
distal
of the
processes
usually
integument or over
the alimentary canal, while their axons enter the ganglia of the central
nervous system.
Motor
neurons
are
unipolar
with
perikarya
lacking
dendrites
are
located in the periphery of a ganglion. The bundles of axons from the
cell bodies form the motor nerves that activate muscles.
Cell bodies of intemeurons and association neurons are also located
in the periphery of a ganglion and may synapse with one or more other
intemeurons,
sensory
neurons,
or
motor
neurons.
Some
being quite large (giant axons) having very large diameters
run
the
entire
americana).
length
These
of
axons
the
ventral
nerve
cord
(e.g.,
intemeurons
(45 µ) may
in
Periplaneta
serve as a rapid conduction system for alarm
reactions.
Based on anatomy, the insect nervous system is divided into three
major
parts:
the
central
nervous
system,
the
visceral
(sympathetic
or
stomatogastric) nervous system and the peripheral nervous system.
[ II] Central nervous system
The basic units of the central nervous system (CNS) (Fig.
the brain and
a double chain
3) are essentially
of ventral nerve cord having
segmental
ganglia joined by lateral and longitudinal connectives.
longitudinal
connectives
Fig.
3. Central nervous system of a generalised insect.
(Z-57)
Nervous System
1 92 1
optic lobe
-n.>.--"-'--
accessory lobe
antenna! lobe
Fig 4 Diagram showing major neuropile region (shaded) of the brain
connections between these regions. Black dots indicate location of perikarya.
and
some
1. ' Brain. The brain is very complex and is located in the head
dorsal to the oesophagus. The brain is connected behind to the
suboesophagcal ganglion by circumoesophageal connectives ventral to the
oesophagus. The insect brain (Fig. 4) is a very complex structure,
formed by the fusion of three anterior most paired segmental ganglia
during development, and thus three distinct Jobes from dorsal to ventral
are observed: protocerebrum, deutocerebrum, and tritocerebrum.
(a) Protocerebrum. The protocerebrum is the largest and most
complex part of the brain having following distinct cell masses and
regions of neuropile: optic lobes, ocellar centers, central body,
protocerebral bridge, pars intercerebralis and corpora pedunculata.
(i) Optic lobes. The optic Jobes are lateral extensions of the
protocerebrum and receive sensory input from the compound eyes. Each
optic lobe is oomposed of three neuropiles, viz., lamina ganglionaris,
medulla externa and medulla interna and associated perikarya and
connectives (chiasma). The axons of retinular cells of the compound
eyes pass into the lamina ganglionaris where they synapse with
monopolar neurons.
(ii) Ocellar centres. The ocellar centers are associated with the bases
of the nerves from the ocelli.
(iii) Central body. Centrally located central body is a neuropile and
connects the right and left lobes of the protocerebrum. It receives axons
from various parts of the brain and may be the source of premotor
outflow from the brain.
(Z-57)
Nervous System
{ 193
(iv) Protocerebral bridge. The protocerebral bridge or pons cerebralis
is a mass of neuropile located medially dorsal to central body. It is
connected with axons from many parts of the brain, except the corpora
pedunculata.
(v) Pars intercerebralis. The pars intercerebralis is located in the
dorsal median region above the protocerebral bridge. It contains two
groups of neurosecretory cells that transport neurosecretory material
(neurohormone) to the corpus cardiacum.
(vi) Corpora pedunculata. The corpora pedunculata (mushroom
bodies) are located at the sides of the pars intercerebralis. It is
composed of a central stalk that splits ventrally into a and P lobes and
capped dorsally by the calyx. The calyx is a mass of neuropile and
associated perikarya. The corpora pedunculata contain intemeurons,
which do not extend outside of these bodies, and terminal portions of
axons that enter from perikarya located in other parts of the brain. The
connections to the calyx and a lobe are mainly sensory; those
connecting with the P lobe are premotor axons, which in tum synapse
with motor fibres.
The protocerebrum is considered to be the location of the higher
centers in the central nervous system, which control the most complex
insect
behaviour.
The
fact that
the
corpora
pedunculata
are
comparatively large in the social Hymenoptera (ants, bees, and wasps)
and small in less behaviourally sophisticated insects (true bugs, flies,
etc.) strengthens this concept.
(b) Deutocerebrum. The deutocerebrum contains the antennal or
olfactory lobes. Each lobe is divided into dorsal sensory and ventral
motor neuropiles. The antennal lobes receive both sensory and motor
axons from the antennae. The two neuropiles are connected with each
other by a commissure. Tracts of olfactory fibres connect the antennal
lobes and corpora pedunculata of the protocerebrum. The antenna)
lobes are important as they are the centres for rece1vmg and processing
several kind of information related with host selection, mate location,
food finding, locating oviposition sites etc.
(c) Tritocerebrum. The tritocerebrum is a smallest part of the brain
and consists of a pair of lobes beneath the deutocerebrum. It connects
the brain to the stomatogastric nervous system via the frontal ganglion
and to the ventral chain of ganglia via the circumoesophageal
connectives. The tritocerebrum also receives nerves from the labrum.
The connecting nerves contain both sensory and motor elements.
2. Ventral nerve cord. In the thorax and abdomen there is typically
a nerve ganglion in the ventral portion of each segment. The ganglia of
adjoining segments are joined by paired connectives.
(Z-57)
Nervous System
1 94 J
(a) Suboesophageal ganglion. The first ganglion in the ventral chain is
the subesophageal which is composed of three fused ganglia
representing the mandibular, maxillary, and labial segments (Fig. 3). It
innervates sense organs and muscles associated with the mouthparts,
salivary glands, and the neck region. In many insects, it has an
excitatory or inhibitory effect on the motor activity of the whole insect.
(b) Thoracic ganglia. There are typically three segmental thoracic
ganglia behind the suboesophageal ganglion, each having the sensory and
motor centre for its respective segment. Two pairs of major nerves arise
from each ganglion supplying the legs and the musculature of each
segment. In winged insects, the mesothoracic and metathoracic ganglia
each give rise to a third pair of nerves supplying the wing musculature.
There is a tendency of fusion of thoracic ganglia in some insects
belonging to Hymenoptera, Diptera, and some Coleoptera (Fig. 5) .
(c) Abdominal ganglia. The largest number of a abdominal ganglia
occurring in larval or adult insects is 8 in the first 8 abdominal
segments in apterygote insects and many larval forms. The ll!st
abdominal ganglion is formed by the fusion of last 4 abdominal ganglia
(of segment 8- 1 1 ). However, there has been a tendency toward
reduction in the number of abdominal ganglia; e.g., 7 in dragonfly, 5 or
6 in grasshoppers and their relatives, and even l in several adult flies
which is partially fused with the large single thoracic ganglion (Fig. 5).
The last abdominal ganglion furnishes the sensory and motor nerves for
Fig. 5. Variation
of Diptera.
(Z-57)
m
the concentration of the thoracic and abdominal gangha of four species
[ 1 95
Nervous System
the genitalia and is, therefore, involved in the control of copulation and
oviposition. The other abdominal ganglia typically give rise to a pair of
nerves to the segmental muscles.
Although ganglia are associated with specific body segments, the
muscles of one segment may receive nerves from a ganglion associated
with a different segment.
[ III] Visceral nervous system
controls
optic lobe
antenna! nerve
circumoesophageal ---'!':'� 1
connective
...:;>._
...
___._
.
_
suboesophageal
ganglion
8
Fig. 6. Brain and stomatogastric nervous system of the grasshopper. (A) Anterior view and
(B) Lateral view.
Nervous System
196 1
and dorsal blood vessel. It is made up of three separate subsystems:
stomatogastric (stomodeal), ventral visceral and caudal visceral nervous
systems.
1. Stomatogastric nervous system. The stomatogastric nervous
system consists of a number of small ganglia and their associated nerves
(Fig. 6). It includes a frontal ganglion, which lies on the dorsal midline
of the oesophagus in front of the brain. The frontal ganglion connects
with the tritocerebrum of brain by nerves on either side. The recurrent
nerve arises medially from the frontal ganglion and extends beneath and
posterior to the brain. The recurrent nerve ends posteriorly in a
hypocerebral ganglion, which may give rise to one or two gastric nerves,
or ventricular nerves, which continue posteriorly and terminate with a
ventricular ganglion (Fig. 7) . Two endocrine glands, corpora cardiaca
and corpora allata are connected with nerves to the hypocerebral
ganglion. Sometimes suboesophageal ganglion is also connected with
hypocerebral ganglion by nerve.
The stomatogastric system regulates the swallowing movements and
possibly the labral muscles, mandibular muscles, and the salivary glands.
In Locusta the frontal ganglion also control the release of the secretion
by the corpora cardiaca.
2. Ventral visceral nervous system. Ven tral visceral nervous system
is associated with the ventral nerve cord and its ganglia. From each
segmental ganglion a single median nerve arises and divides into two
lateral nerves. These nerves innervate the muscles and regulate the
closing and the opening of the segmental spiracles. These nerves may be
absent in some insects.
3. Caudal visceral nervous system. The caudal visceral nervous
system is associated with the posterior segments of the abdomen. The
�
0LJ
[)L
� � \....
oesophagus
brain
corpus cardiacum
frontal ganglion
kontal '°""""'"
recurrent nerve
hypocerebral ganglion
corpus allatum
Fig. 7. Relationship between stomatogastric nervous system and endocrine glands.
[ 1 97
Nervous System
nerves of this system arise from the caudal ganglion of the ventral chain
and supply the posterior portions of the hindgut and the internal
reproductive organs.
[ IV] Peripheral nervous system
All the nerves emerging from the ganglia of the central and visceral
nervous systems comprise the peripheral nervous system. The dendrites of
sensory neurons within these nerves are associated with sensilla, whereas
the axons usually · synapse with neurons within a ganglion of the central
nervous system. Nerves contain motor fibres. The perikarya of these nerves
are located in the ganglia of the central nervous system and the axons
terminate in the muscles, glands, and other effector organs. The peripheral
nervous system continuously inform the insect about its surroundings by
receiving stimuli through sensory organs. These sensory structures are
located all over the body but are generally concentrated on the antennae,
tarsi, palps, labellum, ovipositor, and cerci. Sense organs such as the eyes
peripheral nervous system
sense organs in the integument
(proprioceptors, chemoreceptors,
and tactile hairs)
T
sensory fibers
central nervous system
neurohormones
haemolymph
to control
movement
of heart, gut,
malpighian
tubules,
and other
functions
interneurons
brain, nerve cord, & ganglia
motor fibers
muscles
wings, legs,
and other
mobile
structures
..---- sensory fibers --visceral nervous system
stomatogastric
I retrocerebral
'
complex)
neurohormones
r
ventral
sympathetic
r
caudal
sympathetic
motor fibers
(aod ne"roho�o""
foregut and
salivary glands
""'"own fuootioo)
spiracles
and heart
l
reproductive
organs and anal
appendages
Fig. 8. A mode l of the major interrelatlonslups of the insect nervous system.
198 J
Nervous System
and tympana also provide information. The information about the external
and internal environment is continuously carried from the sensilla to the
central nervous system where it is integrated in a way that appropriate
behavioural and regulatory changes are · made.
Figure 8 summarises the interrelationships among the various parts
of the nervous system.
Physiology of the Nervous System
External and internal stimuli may be perceived in a number of ways
depending on the nature of the stimulus and the specificity of the sensilla.
The conduction of nervous impulses (action potentials and often called
spikes) from a single sensillum to the central nervous system usually
consists of the following events: stimulus, reception and transduction of
stimulus to receptor potential,
receptor potential produced via
depolarisation of dendrite or cell body, action potential produced via
depolarisation in the axon of the sensory cell, release of chemical
neurotransmitter at the presynaptic membrane, numerous biochemical.
events at the postsynaptic membrane, receptor potential in the next neuron
(postsynaptic) in line and action potential, and so on. Any text book of
animal physiology may be consulted for the detail physiology of the above
mechanisms.
Like other animals, insects also perceive a variety of stimuli, e.g.,
physical, mechanical, chemical, electromagnetic, etc. Exactly how the
energy of a given stimulus is changed into the receptor potential is not
completely known, but a change in membrane permeability of the
dendrite is involved.
Important Questions
l.
2.
Give an account of central nervous system of insects.
Write short notes on : (1) Stomatogastric nervous system; (ii) Protocerebrum and
(iii) Mushroom body.
14
Sen se Organ s
Irritability, i.e., the ability to respond to stimuli, is one of the characteristic
features of a living body. It makes the organism aware about its
surroundings, therefore, all organisms have adaptation to collect
environmental (external as well as internal) information by having sense
organs or sensilla (singular sensillum; also known as receptor). Thus the
basic function of the receptors or sense organs (aggregation of large
number of receptors) is to receive stimulus from the environment and
transmit them to the effector organs (e.g., muscles, glands) initiate a chain
of events that ultimately results in a nerve impulse (response). It also
involves the conduction, coordination, and integration, by the nervous
and endocrine systems, of information received from the receptors of
the stimuli.
Types of Receptors
Depending upon the nature of stimulus that activate the receptor cell(s),
jnsects possess many kind of receptors that include mechanoreceptors
(tactoreceptors,
sound
receptors,
proprioceptors),
chemoreceptors,
photoreceptors, thermoreceptors, and hygroceptors.
[ I] Morphology of sense organs
The majority of sense organs are composed of two types of cells: receptor
cells and accessory cells. Receptor cells are usually bipolar neurons that
perform the actual detection of stimuli and generation of the nervous
impulse, which is ultimately transmitted to the central nervous system.
Accessory cells envelope the receptor cells and secrete the specialised
200 1
Sense Organs
cuticular structures that make up the most parts of a sense organ.
However, the multipolar receptor neurons associated with the muscles, the
gut and interior surface of the body wall never contact with the cuticle.
On the basis of the differences in associated cuticular: structures the
receptors are variously classified as trichoid sensilla, basiconic sensilla,
campaniform sensilla, chordotonal sensilla, etc.
1. Trichoid sensillum. Most of the external sensilla (except
photoreceptors) are derived
from
setae,
hence are homologous
structures. As each hair is formed by two cells, the hair-forming
trichogen cell and the surrounding socket-forming tormogen cell, the
addition of one to several bipolar receptor cells to this structure
produces the basic trichoid sensillum or sensillum trichodea or hair
sensilla (Fig. IA).
2. Basiconic sensilla. The basiconic sensilla or sensilla basiconica
have peglike or conelike process (Fig l B ) . These sensilla are mostly
chemoreceptors.
3. Coeloconic sensilla. The coeloconic sensilla or sensilla coeloconica
are found sunken in shallow pits (Fig. IC) and usually serves as
chemoreceptors.
4. Ampullaceous sensilla. The ampullaceous sensilla or sensilla
ampullacea are situated comparatively in deep pits (Fig. ID) and usually
serves as chemoreceptors.
5. Campaniform sensilla. The campaniform sensilla or sensilla
campaniformia do not have hairs, pegs, cones or bristles like
aforementioned sensilla. These sensilla are shallow round or oval pits
and in longitudinal section consist of a bell-shaped cuticular cap or
dome innervated by a single receptor cell (Fig. lE).
6. Placoid sensilla. Similar to campaniform sensilla, the placoid
sensilla or sensilla placoidea also do not have hairs, pegs, cones or
bristles (do not have hairs, pegs, cones or bristles. Placoid sensilla are
plate-like structures made up of a round or oval cuticular plate
surrounded by a narrow membranous ring (Fig. IF) and are innervated
by a number of receptor cells.
7. Chordotonal sensilla. The chordotonal sensilla, also commonly
known as scolophore or scolopidium consist of a bipolar neuron
invested by a scolopale cell, and an attachment cell (Fig. 1 G). These
scolopidia occurs in bundles forming chordotonal organs or scolophorous
organs which are usually stretched between two internal integumental
surfaces and usually serve to perceive vibrations.
A given morphological type of sensillum does not necessarily mean
a particular function. A given sensillum may have different functions in
the same insect or may contain two or more receptors, which collect
different nature of information, e.g., a single hair on the labellum of the
[ 201
Sense Organs
�
trichogen cell
.. .
· :.
.·
.
.
..
.
cuticular plate
.
epidermal cell
trichogen cell
cuticle
F
epidermis
tormogen cell
receptor cells
trichogen cell
nerve
D
G
Fig. I . Types of sens1lla: (A) Tricho1d, (B) Bas1conic, (C) Coeloconic, (D) Ampullaceous,
(E)-Campaniform, (F) Placoid, and (G) Simple chordotonal organ.
blow fly (Phormia) has four chemoreceptor cells and a mechanoreceptor
cell. However, certain sensilla have the same general function, e.g., the
campaniform sensilla are always mechanoreceptors, which is stimulated
by deformation of the cuticle. Similarly, the chordotonal sensilla are
associated with perception of sound, vibration, and stretch stimuli.
The gustatory receptors possess a single pore (uniporous) near the
tips of the hair whereas chemoreceptors have several such pores
202 J
Sense Organs
(multiporous). The mechanoreceptors do not have any such pores
(aporous).
Several sensilla with diverse functions may tend to be aggregated on
specific body regions or appendages called sensory fields. Head,
antennae, mouthparts, legs, wings, genitalia anal cerci, and the ovipositor
are the example of sensory fields.
[ II] Mechanoreceptors
Receptors or sense organs that are sensitive to the actions of stretching,
bending, compression, torque, and so on applied to the integument or
some internal organ are the mechanoreceptors or mechanosensilla. These
sense organs maintain the posture, stability during locomotion, and body
position with respect to gravity. In addition, many of them detect sound
waves or vibrations in a solid substrate. Insects possess following
mechanoreceptors: tactoreceptor, proprioceptor, and sound or vibration.
1. Tactoreceptors. The tactoreceptors (contact or tactile or touch
receptor) are typically trichoid sensilla or hair sensilla. Movement of a
hair triggers the associated bipolar receptor cell(s). The dendrite of a
receptor cell is in very close contact with the base of a hair sensillum
'·
and contains an array of microtubules, the tubular body. The
deformation of the tubular body initiates a nervous impulse.
Hair sensilla are commonly found on the legs, mouthparts, and
antennae as these organs frequently come into direct contact with
the surfaces. The hair sensilla on the anal cerci initiate an escape
response (e.g., in cockroaches). Hair sensilla on the anal papillae of
silkworm moths are used for finding suitable oviposition site.
2. Proprioceptors. Propriopceptors are associated with the maintenance
of the proper · orientation of the body parts with respect to one another or
of the entire body with respect to gravity in both the stationary and the
moving insect. These receptors provide the insect with continuous
information as to the position of the various body parts and the tensions of
the various muscles. The tactile and photoreceptors help the proprioceptors
in orientation.
A number of different types of sensilla function as proprioceptors,
e.g., hair plates, campaniform sensilla, stretch receptors, and chordotonal
organs (including Johnston's organ).
(a) Hair plates. The hair plates are very common in insects and
appear as clusters of tiny trichoid sensilla. They are found in
overlapping areas of the body, e.g., hair plates in the ant Formica are
found between the head and thorax (Fig. 2). When the head turns
sideways, one set of these hair plates receives more pressure than the
other. When the head is in the normal position, the pressure on both
hair plates is the same. By monitoring these pressures through the
Sense Organs
[ 203
Fig 2. Transverse sect10n of the head of the
ant to show hair plates on the prothorax.
nerves, the brain knows the position of the head. Other sites for hair
plates in the ants are between the first and second abdominal segments,
and the second and third abdominal segments. These hair plates in the
ant are then important in the maintenance of proper posture, whether
the insect is stationary or moving. Hair plates on the vertex of the head
of locusts (Schistocerca and Locusta ) sense airflow and are involved in
the regulation of flight.
(b) Stretch receptors. Campaniform sensilla serve as compression and
stretch receptors, and therefore, are located in areas where compression
and stretching occur as a result of muscular activity, e.g., in the legs,
wings, halteres, ·the bases of the mandibles, and ovipositors. Multipolar
neurons associated with muscles, the gut, and internal surface of the
body wall also act as stretch receptors. Whenever the tissue in which
they are embedded is subjected to change in length, these neurons
respond with a nervous impulse. They have been found in dragonfly
nymphs, grasshoppers, ants, bees, wasps, moths and butterflies.
(c) Chordotonal organs. The chordotonal organs are fqund in the
bundles of scolopidia which are usually stretched between two internal
integumental surfaces. They are found in the pedicel of the antennae,
mouthparts, wing bases, halteres, legs, and abdominal segments. They
are also
associated
with tracheae, pulsatile structures and
in
haemocoelomic cavities. Since the scolopidia are adapted to perceive the
vibration, many of them are auditory in function. However, those
scolopidia not associated with hearing serve as proprioceptors, e.g.,
sensation of body orientation, passive body movements, and muscular
movements. Their close associations with tracheae, pulsatile organs, and
the various haemocoelic cavities suggest that they may respond to
changes in intertracheal air pressure and in blood pressure.
(d) Johnston 's organ. The Johnston' s organ found in the pedicel of
antenna of all adult insects is a specialised chordmonal organ (Fig. 3).
The scolopidia are radially attached to the pedicellar wall and to the
204 J
Sense Organs
flagellum of antenna
Johnston's
organ
pedicel
organ.
membrane between the pedicel and the first flagellar segment. It is well
developed in mosquitoes (Culicidae) and midges (Chironomidae). In
these two families the pedicel is enlarged and houses the scolopidia. In
mosquitoes, the base of the antenna} flagellum forms a plate from which
processes extend for the insertion of the scolopidia. The latter are
arranged in two rings all round the axis of the antenna and in addition
there are three single scolopidia which extend from the scape to the
flagellum. Although the Johnston' s organ is known to function as a
proprioceptor in most of the insects, in mosquitoes and midges it is
adapted to perceive sound vibration and hence, are associated with
hearing mechanisms.
A good example of an insect in which it serves as a proprioceptor
is the honey bee. During flight, Johnston's organ responds the movement
of the antenna! flagellum and in this way provides the bee with a
measure of the stream of air passing over it. The amplitude of the
wingbeat is regulated on the basis of this measurement.
Aquatic bugs (e.g., Corixa) swim dorsal side upwards, whereas
others (e.g., Notonecta ) r.wim keeping dorsal side downwards. In either
case, the proper body orientation during swimming is maintained
because the insect is able to sense when its dorsum is up or down. This
is attained by the buoyant action of a small air bubble trapped between
the ventral part of the head and each antenna. Any change in the
position of the insect results in a change in the direction of the buoyant
force of the bubble relative to the insect and hence results in a
movement of the antennae, which in turn causes a change in the
sensory patterns generated by each Johnston' s organ.
Sense Organs
[ 205
3. Sound receptors. The perception of sound is important in a
number of ways. Many of the stimuli that strike on an insect from its
surrounding are in the form of sound. Some of these sounds are
produced by other insects of the same or different species and other
sounds come from a variety of environmental sources. The perception of
sounds informs the insects about the potential danger, a potential mate,
prey, other members of the same species and so on.
As mentioned earlier, only two basic types of sensilla are involved
in sound reception: trichoid sensilla and specialised organs composed
of chordotonal receptors. The organs composed of sound receptors are :
the tympanic organs, subgenual organs, and
Johnston's organ
(described above).
(a) Tympanic organs. Tympanal organs are specialised chordotonal
organs and adapted for hearing. Basically it is composed of a thin
integumental area called tympanum and a group of chordotonal sensilla
attached directly or indirectly to the interior surfaces of the tympanum.
A tracheal air sac is usually closely associated with the tympanum and
sensilla which serves to amplify certain frequencies of sound in male
cicada. The number of chordotonal receptors varies from l in Plea (a
bug) to 1 500 or more in cicadas.
Tympanic organs have been identified in a number of different
locations in a variety of insects. In long-horn grasshoppers and crickets,
they are found on the tibiae of the forelegs while in short-horn
cuticular rim
�----1"'1-- styliform body
..-..""""-....___,...__
_
elevated process
fusiform body
pyriform vesicle
auditory nerve
cut edge of tympanum
Fig. 4. Diagram to show the method of attachment of the auditory ganglion on the mner
surface of the tympanum of Locusta . The folded body, styhform body and elevated process
are cuticular structure. The orientation of the scolopidia are indicated by the arrov·s.
Sense Organs
206 J
grasshoppers on either side of the first abdominal segment. The
tympanic organs are located in the metathorax in noctuid moths and in
the abdomen in pyralid moths and in cicadas.
The tympanum in short-horn grasshoppers is surrounded by a
cuticular ring. Inner surface of each tympanum is attached with Muller's
organ (a number of scolopophores forming a swelling) connected by the
auditory nerve to the metathoracic ganglion (Fig. 4). Muller's organ is
assisted with two sclerotised processes and a pyrifonn vesicle filled with
a clear liquid. These structures probably transmit the tympanal
vibrations to the sensilla. The first abdominal spiracle, near the anterior
margin of the tympanum, gives off an air sac applied to the inner
surface of the membrane. Two additional air sacs arise from the ventral
tracheal trunk on each side of the second abdominal segment and lie
internal to and in close association with the other sac. The cell body
and axon of scolopophore cells are enclosed in a Schwann cell and a
fibrous sheath cell is wrapped around the basal part of the dendrite. A
total of 60-80 chordotonal units are arranged in 4 groups. The tympanal
organ also shows direction sensitivity to sound and are able to recognise
individual sound pulses when these occur at the rate of up to 90-300
per second, depending on the species. In these insects the tympana
receive sounds produced by other members of the same species, and
are involved with sexual behaviour.
In the noctuid moths, the tympanic membrane faces into a cavity
between the thorax and abdomen. Two scolopidia are attached to the
back of the tympanum and are supported by an apodeme ligament (Fig.
5). In these moths the tympanic organs are able to detect the ultrasonic
sound used by echolocating insectivorous bats. Detection of these sounds
stimulates avoidance behaviour.
suspensory ligament
tympanic membrane
Fig. 5. Section of a metathoracic tympanal organ of a noctuid moth.
Sense Organs
[ 207
epidermis
accessory cells
Fig. 6. Section of the tibia of an ant to show subgenual organ .
(b) Subgenual organs. Subgenual organs (Fig. 6) are groups of 1 0-40
scolopidia located in the basal portion of the tibia. They are not
associated with any JOmts. They vary considerably in degree of
development from group to group, being somewhat weakly developed in
the true bugs, and highly developed in the moths, ants, beetles and the
true flies. In ants, the processes from the accessory cells at the distal
ends of the scolopidia are packed together as an attachment body which
is fixed to the cuticle at one point, while the proximal ends are
supported by a trachea. These organs are specifically involved in the
perception of vibration.
Hawkmoths detect ultrasound by means of structure associated with
the mouthparts. The second palpal segment is bulbous and is composed
almost completely of an air sac. The medial region of this palpal
segment is bulbous and is composed almost completely of an air sac.
The medial region of this palpal segment rests against the distal lobe of
the pilifer, a small appendage associated with the labrum. Ultrasonic
vibrations are translated via the palps to the pilifer, which contains the
sensory transducer.
Certain tri<;hoid sensilla on exposed regions of the body in a
number of insects have been shown to be sensitive to airborne sounds.
Sensilla on the cerci of cockroaches are especially sensitive, and
stimulation by sound may elicit the characteristic alarm reaction
mentioned earlier. Johnston's organ in the antennae of some species of
mosquitoes and midges has been shown to be a sound receiver. The
males are able to detect the sounds produced by the rapidly beating
wings of the females.
(Z-57)
208 J
Sense Organs
[ III] Chemoreceptors
The chemoreception is the process by which the potential energy ex1stmg
'
in the mutual attraction and repulsion of the particles making up atoms is
detected. Thus chemoreceptors are responsive to direct contact with atoms
and molecules. Chemical cues from the environment are useful to insects
in several ways, e.g., food (host plant or animal, prey, decaying organic
material, and so on) location and obtainment, mediation of caste functions
in social forms, mate location, identification of noxious stimuli that are a
potential threat to survival, selection of oviposition site, and habitat
selection.
The chemoreceptors may be divided into olfactory receptors (distant
receptors
(contact
chemoreceptors)
and
chemoreceptors),
gustatory
general chemoreceptors.
1. Olfactory receptors. In general, thin walled basiconic pegs and
coeloconic pegs serve as olfactory receptors. These sensilla bear many
pores through which the chemical stimuli reach the nerve endings. In
grasshoppers the coeloconic sensilla consist of short pegs about 8 µm
long, resembling thick-walled basiconic pegs, but sunken into a cavity.
The cavity, about 20 µm in diameter, is broadly open to the outside
(Fig. 7A). The plate organs found on the antennae of the aphids
consists of oval areas of transparent thin cuticle. Below this layer there
is another layer of cuticle with pores in it so that a fluid-filled space
between the two layers is continuous with the vacuole formed by the
tormogen cell. A few neurons are associated with each sensillum and
the dendrites, each with a cilium extend towards the surface cuticle
through the perforations in the inner layer. The olfaction is mediated by
chemosensilla that are responsive to molecules or ions of a chemical in
the gaseous state at low concentrations. These sensilla are very sensitive
and show a high degree of specificity with regard to the kind of
chemical that elicits a response. The olfactory receptors have been
identified in the antennae and mouthparts of a variety of insects and m
the ovipositor of at least one.
Olfactory cues play a major role in the lives of insects (location of
habitat,
food,
mate,
prey,
host
etc.)
that
involve
chemical
communication. The distance chemoreceptive abilities of some insects
are fantastically acute. For example, the male silkworm moth, Bombyx
mori, reacts to the sex pheromone (bombykol) produced by the female
at a concentration as low as 1 00 molecules of attractant per cubic
centimeter of air. A single molecule of female sex pheromone is
sufficient to trigger an impulse in the male receptor cell. Considerable
interest is being shown in identification of various host odours (i.e., for
vertebrates and plants) in an effort to develop traps or better
understand how the insect locates its food source.
(Z-57)
Sense Organs
A
[ 209
B
Fig. 7. Chemoreceptors. (A) Olfactory, and (Bl Gustatory sensilla.
2. Gustatory receptors. The gustatory receptors (contact receptors)
are trichoid sensilla and basiconic sensilla on the legs and mouthparts of
most of the insects. They are 30 to 300 µm long. From the tip scolopale
is invaginated, and is confluent with one wall of the hair so that the
lumen of the hair is divided into two. The 5colopale extends down to
the level of the perikarya where its wall is invaginated so that the
dendrites are separated from each other. Four to six neurons are
associated with each sensillum. These chemosensilla usually have only
one or two pores located distally (Fig. 7B). Gustations mediated by
chemosensilla that are responsive to molecules or ions of a chemical in
solution at a high concentration. Generally, these sensilla are less
sensitive than the distance chemosensilla and are commonly associated
with feeding activities.
The gustatory receptors have been found in the mouthparts of all
insects, e.g., the tips of the maxillary and labial palps and in the buccal
cavity of the cockroaches; in the cibarium of mosquitoes; in the pharynx
of the house fly; on the tips of the antennae in honey . bees and wasps;
the distal portion of the tibia and tarsi of the forelegs of house flies,
butterflies and bees.
There are other kinds of contact receptors located on the ovipositor of
parasitic wasps by which the females judge the suitability of the host for
oviposition by sensing the internal environment of the host body. The aphid
parasitoid Binodoxys indicus (Hymenoptera: Braconidae) is able to
distinguish a parasitised host from a healthy host with its ovipositor.
3. General chemoreceptors. There are certain chemosensilla which
are responsive to relatively high concentrations of stimulating chemicals.
The receptors associated with the perception are usually thick-walled
basiconic pegs. These sensilla are distributed over the body of the
insects being particularly abundant on the antennae, maxillary and labial
(Z-57)
210 J
Sense Organs
palps, legs etc. Similar sensilla usually associated with an avoidance or
escape response of the insect by detecting irritant substances.
[ IV] Ther01oreceptors
Insects are poikilothermic and hence there is a little physiological control
of body temperature. However, their behavioural adaptations tend to
maintain the temperature as near to an overall optimum for metabolic
activity as environmental conditions allow. The insects develop only within
a limited range of temperature which is charatceristic of the species. To
perceive variation in ambient temperature, insects have specialised sensilla
or receptors. The thermoreceptors (temperature receptors) have been
found over the body in general but are concentrated on the antennae,
maxillary palps, and tarsi of many insects, e.g., in the blood-sucking bug
Rhodnius prolixus, the thick-walled trichoid sensilla on the antennae are
extremely sensitive to small differences in air temperature. Blood-sucking
insects (mosquitoes, lice, bed bugs, etc.), locate their hosts by detecting the
temperature gradient around the hosts' body. In grasshoppers, a series of
paired specialised area on the head, thorax and abdomen appear to be
thermoreceptors. These areas differ in cuticular structure and texture from
the surrounding epidermis. A few insects may also have cold receptors.
All insects will move away from high temperatures, but this is
probably a generalised sensitivity with no. particular sensilla being
involved. In addition, since the insects are poikilothermic, the central
nervous system itself is subject to temperature changes and the
spontaneous output from the ganglia varies with temperature. Some
insects are evidently able to perceive the radiant heat of the sun or
other light source, e.g., stink bugs tum their dorsal sides toward a light
source at low temperature and thus, receive the maximum possible
radiant heat.
[ V] llygroreceptors
Moisture may affect the metabolism and hence the rate of development of
insects. Few insects require low humidity (e.g., Tribolium ) while others
need high humidity (e.g., Locusta, Schistocerca).
Hygroreceptors are a special type of chemoreceptors that perceive
moisture in the air. The sensilla or receptors that are sensitive to
moisture have been found on the antennae and maxillary palps of some
insects. In Tenebrio they are thin-walled basiconic pegs and similar
structures in Tribolium are branched (Fig. 8A). In Pediculus the
hygroreceptors are tufts of four small hairs on the antennae innervated
by several neurons (Fig. 8B) while in tsetse fly the guard cells of the
spiracles are humidity sensitive. Springtails, like other small soil-dwelling
(Z-57)
Sense Organs
[ 2ll
A
8
Fig. 8. Humidity receptors. (A) Tuft organ from the antenna
(B)-Branched humidity receptor from the antenna of Tribolium.
insects, are very sensitive to moisture both in the
substratum. They are attracted to areas of high humidity.
of
human
air
and
louse.
the
[VI] Photoreceptors
Photoreception may be defined as the ability to perceive energy (light) in
the visible or near visible (near ultra-violet) range of the electromagnetic
spectrum. Different types of photoreceptors permit various insects to
perceive the form of objects, patterns, movement, distance, certain colours,
light intensity, the polarisation plane of light, light versus darkness, and the
length of the photoperiod. In insects, these receptors take the form of
dermal light response, compound eyes, stemmata (lateral ocelli), and dorsal
ocelli. These photoreceptors perceive light by means of a pigment that
absorbs light of a particular wavelength, and thus stimulates associated
neurons. Photoreceptors may be reduced or absent in cave-dwelling
(cavernicolous), burrowing, and other species that live in dark situations.
1. Dermal light response. Many insects apparently possess a light
sensitivity over the general body surface, e.g., totally blind cockroaches
continue to demonstrate a preference for dark situations. Similarly,
decapitated mealworm larvae (Tenebrio sp.) continue to avoid light.
2. Compound eyes. The paired compound eyes are the major organ
for photoreception in adult insects located on either side of the head.
Each is composed of a number of individual sensory units, or ommatidia
(Fig. 9A). Externally, these ommatidia are marked by hexagonal
cuticular· facets. The facets, and hence the ommatidia, vary in number
from a very few to several thousand, e.g., from 12 to 17,000 in some
Lepidoptera and from 10 to more than 28,000 in some Odonata.
(a) Structure of an ommatidium. Each ommatidium (Fig. 9B) has
two major components: a light gathering optical part (dioptric
apparatus) and a sensory receptor (receptor apparatus).
(i) The dioptric apparatus. It consists of the cornea, crystalline cone
and corneal pigment cells. The cornea is an external cuticular structure,
the cuticle being transparent and acts as biconvex or plano-convex lens.
It is continuous with the cuticle of the integument. Some insects have
Sense Organs
212 1
� cornea
facets of ommatidium
S"
=
crystalline cone
L primary
pigment cell
retinular cell
lrhabdome
secondary
pigment cell
B
A
Fig. 9. (A)
ommatidium
small
basement
membrane
nerve
Vertical
conical
section of part of compound eye,
projections
(corneal
nipples)
(B)
Typical structure of an
arranged
in
a
hexagonal
pattern on the outer surface of the cornea which act as an antireflection
coating. In addition to corneal nipples, some insects, such as the house
fly and the honey bee have interommatidial hairs that function as flight
control.
The cornea, like the rest of the cuticle, is secreted by the epidermal
cells, each lens being produced by two cells, the corneagen cells. These
corneagen cells are withdrawn to the sides of the ommatidium and form
the primary
pigment cells.
Beneath
the
cornea
lies
a crystalline cone
which is composed of a translucent material. The primary pigment cells
generally surround the crystalline cone, except in silverfishes, in which
they lie beneath the cornea.
(ii)
The
receptor apparatus.
underlies the ·optical unit.
The
sensory
receptor
component
It consists of six or seven retinular (nerve)
cells which are arranged in one or two layers. Like the crystalline cone,
the group of retinular cells is usually surrounded by darkly secondary
pigment cells that limit to varying degrees the light that may enter from
adjacent ommatidia.
Each retinular
that
a basement membrane
passes
contributes
through
to
the
formation
rhabdom.
The
called
rhabdomere.
a
contribution
packed
fingerlike
thought
that
rhodopsins
the
The
of
and
a
each
usually
gives
rise
to
and enters the
centrally
retinular
rhabdomeres
projections
microvilli
of
cell
located
an
retinal
to
the
are made
up
of tiny,
the
retinal
the
light-absorbing
pigment(s),
are
involved
directly
is
is
closely
from
that
or
rhabdom
contain
metarhodopsins,
It
and
rod,
cell
cells.
axon
brain,
generally
the
with
photoreception.
Groups of tracheal branches called tapetum lie below tht.� receptor
apparatus
close
to
the
basement
membrane.
It
forms
a surface
{rom
Sense Organs
I 2 13
which light that has traversed through the rhabdoms is reflected back.
along the rhabdoms.
Thus it
gives
a double
exposure of light to the
retinular cells helping to increase its light sensitivity.
Based
on
ommatidia and
the
association
thus
the compound
of
optic
and
receptive
eyes are generally
components,
categorised
into
apposition and superposition types.
The apposition type of ommatidium (apposition or photopic eyes) is
found
in
beneath
diurnal
the
insects
in
which
crystalline · cone
the
(Fig.
retinai
l OA)
and
cells
lie
there
is
immediately
little
or
no
movement of the pigment in the primary pigment cells in response to
changes
rather
from
light to dark or vice
uniformly distributed.
(superposition
insects
or
active
between
scotopic
during
the
retinal
eyes)
dusk
cells
versa. Thus
the
pigment remains
The superposition type of ommatidium
is
found
period,
and
in
the
in
which
nocturnal
or
there
a
crystalline
is
cone
crepuscular
clear
(Fig.
space
l OB)
and
pigments in the primary pigment cells move in response to a light-dark
change. In bright light the pigment tends to migrate in the pigment cells
proximally, producing the light-adapted condition (Fig. l OB left-side). On
the
other
hand,
in
dark,
the
pigment
migrates
distally
(dark-adapted
condition, Fig. l OB right-side).
In
(b) Image formation.
the compound
eye,
light
falling
on
the
ommatidium is focused by the cornea, then funnelled by the crystalline
cone
to
the
(rhodopsin
rhabdom.
or
relayed
directly
portion
of the
to
the
insect's
sensed by individual
or
mosaic
In
rhabdom
metarhodopsin)
view
of
brain.
the
results
Each
changes
in
in
sensory
ommatidium
surroundings.
The
visual
pigments
information
sees
only
combination
being
a
of the
small
images
ommatidia supposedly together forms a composite
the
external
environment.
The
compound
eyes
function differently when the light intensity varies markedly from bright
light to dim light.
(i) Image formation in bright light or apposition image.
apposition
eye
and
the
light-adapted
superposition
eye,
the
In
the
rhabdom
receives only light rays entering parallel to the long axis of an individual
ommatidium.
Oblique
rays
coming
from
the
adjacent
ommatidia
absorbed by the pigment in the primary pigment cells (Fig.
left-side).
l OA,
are
l OB
Such images are sharp and the eyes have a great resolving
ability.
(ii) Image formation in dim light or superposition image.
dark-adapted
cells
tends
superposition
to
move
eye,
distally
the
pigment
removing
the
of
the
optical
In
the
primary
pigment
isolation
between
adjacent ommatidia. Thus, the light rays that fall on the rhabdom of a
given ommatidium come through several adjacent ommatidia (Fig.
l OB
right-side). The superposition image is not sharp, as all the light rays
Sense Organs
214 J
I
\
I I
EHl-- rhabdom ---.l!!§�
retinular cell ----HI
A
Fig. 10. Image formation in compound eyes. (A) Apposition type of ommatidium
(apposition image formation), (B) Superposition type of ommatidium: left-side shows light
adapted image formation (apposition image formation), right-side shows dark adapted
image formation (superposition image).
entering through different ommatidia do not fall at exactly the same
point on the rhabdom.
(c) Perception of form, pattern and movement. Some insects are
capable to perceive simple pattern and form, e.g. , honey bees. However,
the bees perceive form on the basis of the brokenness of pattern.
Therefore, the bees tend to visit flowers that are being shaken by the
wind more readily than those that are not. It is well known that bees,
ants, and wasps are able to locate their nests on the basis of various
landmarks. In addition, flying bees are able to identify right from left,
before from behind, and above from below. The dragonfly nymphs
attack the prey only when it is moving.
(d) Distance perception. Insects possess the ability to judge the distance
of an object from itself, e.g., prey capture by praying mantids. To catch
prey, distance perception must be acute particularly for flying predators,
e.g., dragonflies. In this, binocular vision is involved.
(e) Colour vision. The range of the electromagnetic spectrum
perceived by insects is from near ultraviolet to approximately infrared.
Insects in general are particularly sensitive to the ultraviolet and
blue-green regions of the spectrum. In insects that do possess colour
vision, part of an eye may be colour sensitive and another part
colour-blind (e.g., in the water boatman). The colour sensitivity of an
insect also vary with its physiological state, e.g., cabbage butterflies
(Pieris brassicae) seem to prefer blue or yellow flowers; gravid females
ready to oviposit seem to prefer green and blue-green. The insects are
generally
red-blind,
however,
certain
butterflies
are
capable
of
recognising red flowers.
Sense Organs
[ 21 5
Fig. 1 1 . (A) Section of stemmata and (B) Section of dorsal ocellus.
(/) Polarised light perception. The honey bees and ants are able to
recognise the direction of polarisation of light. These insects detect the
polarisation pattern of the sky, which varies with the position of the sun
and enables them to determine direction which is important in finding
the hive or nest after a foraging or hunting trip, e.g., honey bees.
3. Lateral ocelli or stemmata. Stemmata are structurally variable
among insects. Some types are similar in structure to an individual
ommatidium of a compound eye, e.g., in caterpillars (Fig. l lA), each
eye consists of a cornea, a crystalline cone, and a cluster of retinal cells
that form a single rhabdom. Stemmata function in the manner of eyes.
Typically they are the only eyes found in holometabolous larvae. In
various insects they have been shown to be involved with colour, form,
and distance perception. Like compound eyes, they receive nerves from
the optic lobes of the brain.
4. Dorsal ocelli. In addition to compound eyes, many adult insects
also have photoreceptors that consists of a single cornea, a layer of
corneagen cells, which secretes the cornea, clusters of pigment cells, and
a group of retinular cells (may be up to 1000 cells depending on the
species) that forms several rhabdom (Fig. 1 lB). Commonly three ocelli
are arranged in a triangular pattern on the anterior part of the head.
Dorsal ocelli function as pigment cups that detect changes in light
intensity and not important in image perception. They are supposed to
increase the sensitivity of the compound eyes to light.
Important Questions
1.
2.
Give an account o f different types o f mechanoreceptors found i n insects.
Describe the structure of chemical receptors found in insects.
3.
Describe the structure of compound eyes of an insect. How image is formed; descnbe.
4.
Write short notes on :
(i) Muller's organ; (ii) Johnston's organ; (iii) Hair plates; ( iv) Subgenal organ;
(v) Chordotonal sensilla; (vi) Thermoreceptors; (vii) Hygroreceptors; (viii) Stemmata
and (ix) Colour vision in insects.
15
Bioluminescence and Sound Production
Bioluminescence
Insects that produce light at night or at dusk are particularly fascinating.
The luminescence (light production) occurs in a large number of insect
species. In many cases it is due to the bacteria, however, there are certain
insects (given in the following table) where true or self-luminescence
occurs, particularly in the order Collembola, Homoptera, Coleoptera and
Diptera. The luminous organs are found not only in adult insects but also
in larvae, pupae, and eggs. In fact, even unfertilised eggs within the female
body can give off light particularly in Diptera and Coleoptera.
[ I] Distribution of luminous organs
The luminous (light-producing) organs occurs in various parts of the insect
body as shown in the above table. However, they differ in their number
and location. The spring-tails emit light from the whole body, the
hypodermal fat layer functions as the luminous organ. In beetles, mostly
Lampyridae, there is a much diversity of the location of the luminous
organs amongst male, female and larvae. In male Photuris there is one pair
of lumninous organs in the ventral region of each of the sixth and seventh
abdominal segments while in female, only a single pair of organs are
located on the seventh segment and the larvae bear a single pair of organs
on the eighth segment. Among Diptera, the luminous organs of Bolitophila
are formed from the exceptionally enlarged tips of all four malpighian
tubules. In others, they are derived from the ventral hypodermal fat layer.
In Platyura, pair of luminous organs are found at the caudal end, whereas
Bioluminescence and Sound Production
Orders
Light-producing insects
Collembola
(lighting spring tails)
Homoptera
Diptera
(lantemfly)
(glow wonns)
Coleoptera
[ 217
(ftreflies)
Light-producing organs
whole body
Achrorutes muscorum
Onychiurus armatus
Fu/gora /anternaria
head
Arachnocampa luminosa.
Bo/ztophi/a lummosa
end of the malpighian tubules
Ceroplatus sessiodes
whole body
Platyura fultoni
caudal end
Cumpyloxenus
ventral sides of the abdomen
Diplocladon
each abdominal segments
Lampyris noctiluca,
Lucwla, Phengodes,
Photinus pyralis,
Photophorus
ventral sides of the abdomen
Photuris greeni
ventral regton of 6th and 7th
abdominal segments (in males)
and 7th segment (in females)
Phrixothrix
head, 2nd thoracic
abdominal segments
Pyrophorus noctilucus
either side of the thorax, at the
base of ventral surface of
abdomen
Ceroplatus
the whole body of larvae and pupae of
is luminous.
to
9th
Pyrophorus
emits light from a rounded area on either side of the thorax and at the
Diplocladon ,
base of the ventral surface of the abdomen . In
possess
three
luminous
organs,
two
lateral
and
one
median,
the larvae
on
each
abdominal segment.
[ II] Structure of luminous organs
The detail anatomy of the luminous organs of
Smith
Photuris
was studied by
( 1 963). The light organs consist of large sized cells, the photocytes,
lying just beneath the epidermis and backed by several layers of cells called
the dorsal layer cells (Fig.
1 ). The cuticle overlying the light organ is
transparent.
are
The
photocytes
so
arranged
that
they
form
cylinders
running at right angles to the cuticle and are richly supplied with tracheae
and nerves. Each trachea gives off branches at right angles which break up
into a number of tracheoles running between photocytes parallel with the
cuticle.
The
tracheae
are
placed
at
1 0- 1 5 µm apart from each other
providing a short path for oxygen diffusion. The inner membrane of the
tracheal end cell that binds the tracheoblast is highly folded.
The
nerves
entering
the
terminal processes between the
photocyte
plasma
cylinders,
membranes
end
as
spatulate
of the end cell
and
the tracheoblast within which the tracheole arise. The terminal processes
Bioluminescence and Sound Production
218 J
lumen of --.......
. -!­
..
cylinder
mitochondria
photocyte
granules
tracheoles
photocytes
epidermis
- - - - - - -- - ;;;:.--- transparent
cuticle
_
_
_ _
_
Fig. I . Diagrammatic section through part of the luminous organ of Photuris.
consist of two types of vesicles: large vesicles ( 1 000 A 0) and small
vesicles (200-400 A 0) . These vesicles resemble neurosecretary droplets
contatmng acetylcholine. The photocytes are packed with photocyte
granules. The granules each of which contains a cavity connecting with
the outside cytoplasm through a nee�. These granules contain luminous
substrate, luciferin. Small granules occur dorsally and ventrally.
Mitochondria are sparsely distributed in the photocytes. The dorsal layer
cells contain urate granules forming a reflecting layer. They suppose to
store the oxyluciferin irreversibly produced in light production. In
Photinus, two luminous organs together contain about 1 5 ,000 photocytes
forming some 6,000 cylinders each with 80- 1 00 end cells.
[ III] Mechanism of light production
Basically light is produced by the oxidation of luciferin, in the presence of
the enzyme, luciferase. Luciferin is first activated by ATP in the presence
of
magnesium
and
luciferase
to
produce
adenylluciferin.
The
adenylluciferin is then oxidised by an organic peroxide, again in the
presence of luciferase, to form so-called excited adenyloxyluciferin which
decays spontaneously to low energy adenyloxyluciferin with the production
of light. The energy for this process is obtained directly from the oxidation
process, not from the ATP and it is released in one large step. The
radiation is very efficient, about 98% of the energy involved being released
as light. The low energy adenyloxyluciferin produced inhibits further
reaction, probably by binding with luciferase. The pyrophosphate, however,
removes the inhibition. When the light organ is stimulated by a nerve, the
acetycholine released at nerve ending reacts with ATP and co-enzyme to
yield pyrophosphate. The pyrophosphate diffuses to the photocyte granules
Bioluminescence and Sound Production
nerve ---+
impulse
terminal
process
---+
[ 219
hydrolysis
acetylcholine
�
acetic acid + choline
ATP + coenzyme A
acetyl-coenzyme A + pyrophosphate + adenylic acid
'9moves inMbWon
free luciferase
luciferin
+
ATP
1
adenylluciferin
+
pyrophosphate
_:__ir
these reactions normally
prevented by inhi bition of
luciferase
1
..a...
inhibits luciferase
�
adenyloxy
luciferin
\
'
I
luciferin
I /
���
/�­
/I
? stored a s waste
I
L
l
••••
\'
+-- oxyluciferin
+
in dorsal layer cells
adenylic acid
Fig. 2. Scheme of the reactions involved in light production by insects.
and stimulates the production of light by removing the inhibition of
luciferase. During the reaction in the photocyte more pyrophosphate is
released and this may spread through the cell, extending the reaction. The
chemoluminescence reaction is given in Fig. 2.
[ IV] Colour of light produced by the luminous organs
The colour of the light produced by the insects varies with species.
Lampyris and Photinus produce yellow-green light, whereas Bolitophila
produces blue-green and Fulgora yields white light. The larvae and adult
females of Phrixothrix have green light producing organs on the thorax and
abdomen, and red ones on the head. The colour pattern varies with the
pH, temperature, concentration of urea and ions (Mg++ , zn++ , Cd++ ,
or Hg++ ).
[ V] Kinds of light produced by the luminous organs
The light may be produced as a continuous glow with uniform intensity and
without interruption (e.g., larviform females of Lampyris , Platyura,
Phenogodes); an intermittent glow that lasts only for a few seconds
(e.g., Photuris) ; a pulsation glow, the pulse ranging 60- 1 10/min in Lucio/a;
and a flash of glow that consists of a burst of light which lasts very shortly
(0. 1 to 0.2 second, e.g. Photinus, Photuris).
220 J
Bioluminescence and Sound Production
[ VI] Control of light production
The last two abdominal ganglia innervate the light producing organs of
Photuris. The axons, acting via the end cells, supply small parts of each
organ and these units can be stimulated to produce light independently of
the rest of the organ. The light is produced after some time of the nerve
stimulation suggesting that a chemical (possibly acetylcholine) diffuses a
certain distance that initiates the reaction in the photocytes before the
production of light.
[ VII] Significance of light production
In most insects, light production is important in the mutual attraction of
the sexes. Each species has a characteristic flashing rhythm; the length of
the flashes and the interval between flashes are of taxonomic importance
as these are species-specific. For example, the male fireflies (Photinus
pyralis) flying above 50 cm from the ground, display the light signal, and
the wingless larva-like female (glow-worms) signal back when they see the
appropriate flash sequence; size and brightness of the flash may be as
important as sequence. The males drop down to the females with
remarkable accuracy. Predatory glow-worms (Photuris) can mimic the
signal of Photinus females, and lure searching Photinus male to their death.
In certain moths the lights, which look like two eyes and shine forth most
brightly when their possessor is disturbed, are assumed to be a means of
escaping enemies away. The light produced by the cave dwelling
insectivorous Balitophilia larvae attracts the insects for food into networks
of the webs which they spin.
Sound Production
Several species of the · insects produce sounds by using vaneties of
mechanisms. Sounds are produced either by the vibration of the wings in
flight, by scraping a ridge over a series of striations on some other part of
the body (also known as stridulation), or by the direct action of a muscle.
The sounds produced by the insects are transmitted through all the media,
the air, water, and the substratum. These sounds are commonly correlated
with well developed organs of hearing and often play an important role in
various types of behaviour, e.g., inter- and intra-specific communication for
warnmg,
alarming,
courtship,
isolation,
aggregation
and
social
communication.
Insects
only
produce
sounds
under
particular
environmental conditions, the internal environment possibly being regulated
by hormones. Given suitable conditions sound production is regulated by
nervous system.
Bioluminescence and Sound Production
{ 221
[ I] Mechanisms of sound production
The insects produce sounds by several ways which can be classified as:
( 1 ) Sounds produced as a by-product of some usual activity of the insects;
(2) Sounds produce by the impact of some part of the body against the
substrate;
(3) Sounds produced by frictional methods, rubbing two parts of the body
together;
(4) Sounds produced by vibrating membrane; and
(5) Sounds produced by a pulsed air stream.
1. Sounds produced as a by-product of some usual activity of the
insects. In this category no specifically adapted structures are involved
in sound production. Such sounds are produced while insects are busy
in feeding, cleaning, courtship, flying and so on. The vibration of the
wings in flight produces sound. The wing beat of honey bee (Apis) is
about 250 cycle/s and that of mosquitoes from 280 to 350 cycle/s. The
frequency is almost constant for a species, but it may vary with
temperature, age and sex. The wings of certain insects also produce
sounds when they are not flying. The bumble bee (Bombus ) produces a
high frequency sound during the collection of pollen.
2. Sounds produced by the impact of some part of the body
against the substrate. Various insects produce sounds by striking the
substratum, mostly without any related structural modifications in the
body. A sexually mature death watch beetle (Xestobium ) produces
sounds by bending its head down and tapping it against the floor of its
burrow in the wood 7-8 times a second. The male grasshopper
( Oedipoda ) drums on the ground with its hindtibia at the rate of about
1 2 beats/s. Soldiers of the termite (Zootermopsis ) make vertical
oscillating movements using the middle legs as a fulcrum so that the
head moves up and down banging the tips of the mandibles on the
floor and, less frequently, the top of the head against the roof. Usually
two or three taps are produced successively followed by an interval of
about half-a-second before the taps are repeated.
3. Sounds produced by frictional mechanisms (stridulation).
Frictional mechanisms are found among several groups of insects.
Although these mechanisms are structurally diverse, they consists of
similar parts. Frictional mechanisms are located in the areas where two
surfaces (two wings, a leg and a wing, etc.) may be rubbed together.
Often it is possible to distinguish a long ridge or roughened file (strigil)
from a single scraper (plectrum). Movement of the scraper over the file
causes the membrane to which it is attached to vibrate so that a sound
is produced. Stridulation is partic;ularly associated with Orthoptera,
Heteroptera and Coleoptera. In Orthoptera two main methods of
222 1
Bioluminescence and Sound Production
articulation
functional file
left
right
Fig. 3. Elytra of a long-horn grasshopper from the ventral surface.
stridulation
are
wings)
crickets
in
employed:
(friction
stridulation
elytral stridulation
and
long-horn
between
(friction
grasshoppers
a
leg
and
between
and
two
femoro-elytral
wing)
in
short-horn
grasshoppers. In male crickets each elytron (forewing) has a cubital vein
near the base on its underside modified to form a dentate file while on
the edge of the opposite tegmen (forewing) is ridge forming the scraper.
The right tegmen overlaps the left so that only the right file and left
scraper are functional (Fig. 3). In producing the sound, the tegmina are
raised at an angle of 1 5-40% to the body and then opened and closed
so that the scraper rasps on the file causing the tegmen to vibrate and
produce
a
sound.
Sound
is produced on
closure
when they are opened, each impact between the
of the tegmina,
producing a single vibration of the tegminal membrane.
is thus driven by these impacts
so that the
�
not
scraper and a tooth
The membrane
frequency
of the sound
produced is the same as the frequency of impacts of the scraper on the
teeth. Each species of cricket has a number of different songs used in
different situations. These songs can be differentiated by fr.equency with
pulses of sound are produced. The
stridulatory
apparatus
of long-horn
grasshoppers is similar to that of crickets, but the left tegmen overlaps
the right and, in most fully winged forms, only the left file and the right
scraper are present.
Short-horn
grasshoppers
produce
sound
by
rubbing
the
hindfemora
against the tegmina (Fig. 4). Usually a row of pegs on the femur (file)
is rubbed against ridged veins of the tegmen (scraper). This causes the
tegmina to vibrate with their natural frequency and so produce a sound,
the
frequency
of which
frequency of the sounds
varies
from
2-50 kc/s. To some extent the
varies with the species, but even in a single
insect a wide frequency spectrum results from the different resonances
of different parts of the tegmina. Each movement of the femur produces
a single pulse of sound.
Bioluminescence and Sound Production
[ 223
femur
file
Fig. 4. Inside view of the left hindleg of a short-horn male grasshopper showing the
position of the stridulatory pegs.
4.
Sounds produced by a vibrating membranes (tymbal organs).
Sounds
produced
muscles
are
by
the
common
vibration
amongst
of a membrane
bugs
and
driven
some
tiger
directly
moths.
by
The
mechanism is most fully studied in cicada. In these insects the tymbal
organs are paired structures on the dorsolateral surface of the base of
the
abdomen.
(membrane)
In
on
cicada,
either
there
side
of
is
the
an
area
of
dorsolateral
very
surface
thin
of
cuticle
the
first
abdominal segment supported by a thick cuticular rim and a series of
dorsoventral
strengthening
ribs.
This area of cuticle
forms the tymbal
and it is protected by a forward extension of the abdomen forming the
tymbal cover (operculum). Internally a cuticular compression runs from
the ventral
surface to the posterior edge of the
fibrillar tymbal muscle,
running
parallel
with
supporting rim and a
the
compression
support,
arises ventrally and is inserted into an apodeme attached to the tymbal
(Fig.
5). The tymbal is associated by an . air-sac which surrounds the
muscle
and
opens
the
outside
presence
of
air-sac
minimum
of damping.
through
leaves
the
the
metathoracic
tymbal
free
to
spiracle.
vibrate
The
with
a
When the abdomen is raised the membrane is
stretched. When the tymbal muscle contracts, pulling on the tymbal so
that it buckles inwards producing a click. On relaxation of muscle the
tymbal returns to its original position by virtue of the elasticity of the
apodeme
airsacs
A
8
Fig. 5. Diagrammatic transverse section of the first abdominal segment of a cicada showing
the tymbal organ (A) and section of a tymbal (B ), the dash line and arrows indicate the
pattern of movement of the tymbal during sound production.
(Z-57)
224 J
Bioluminescence and Sound Production
inspiration
expiration
Fig. 6. Sagittal section of the head of the death's head hawk-moth showing the method of
sound production during inspiration and expiration.
surrounding cuticle and so produces a second click. Thus, a double
click of sound is produced by each muscle contraction.
In quite a different category is the piping of queen bees. This
sound is probably produced by vibration of the thoracic sclerites and so
may be regarded as a vibrating membrane mechanism but it does not
constitute the tymbal organ. The sound is only produced by virgin
queens. The piping of free virgin queens consists of a phase starting
with a long pulse of sound followed by a series of short pulses with a
fundamental frequency of 500 eyelets together with harmonics.
5.
Sound produced by a pulsed air stream. The only well
documented example of a sound produced by a pulsed air stream is by
a death's head hawk moth Acherontia (Lepidoptera). Air is sucked
through the proboscis by dilation of the pharynx causing the epipharynx
to vibrate and create a pulsed air stream. In this way a sound with a
frequency of about 280 els is produced. Contraction of the pharynx with
the epipharynx held erect expels the air producing a high pitched
whistle (Fig. 6).
[ II] Significance of the sounds produced
The sounds produced by insects function as a communicator between two
or other species (interspecific or extraspecific communication), or within
the members of the same species (intraspecific communication).
Sounds having interspecific significance are produced by both male
and female insects. Sounds of this type are presumed to be concerned
with defense and warning, perhaps alarming a potential predator or
warning other members of the species to the presence of a predator. In
tiger moth the sound production is associated with a display of warning
colours. These are distasteful species amongst them, these being the
forms which most need to reinforce their display if predators are to
learn to avoid them. In addition, it the sounds produced by these moths
(Z-57)
Bioluminescence and Sound Production
[ 225
disrupts the echolocation system of the bats while they are hunting
them.
Sounds having intraspecific significance are concerned with courtship
behaviour and thus help in sexual isolation. Usually such sounds are
produced by the male insects. The role of song in insect courtship has
been most fully studied in Orthoptera. The Orthoptera have five main
classes of song concerned with calling, courtship, copulation, aggression
and alarm, and differing from each other in the pulse repetition
frequency and the form of the pulse. In the grasshoppers, the female
only stridulates when she is in the responsive state. The different songs
of different species of grasshoppers, crickets and cicadas have the effect
of enhancing the isolation of species due to other factors. Usually, it is
believed, the difference in song has arisen after species have become
morphologically isolated. Certain sibling species of crickets can only be
identified by the sounds they produce.
In some insects sounds lead to aggregation. This occurs in cicadas
which have song leading to aggregation of males and females, and
resulting in a clumping of the species within a habitat so that particular
trees may be occupied by a particular species.
Aggressive stridulation is well illustrated by crickets. Each male has
a territory of some 50 sq cm in which he sings his normal song. If
another cricket intrudes, the male sings an aggressive song quite distinct
from other songs and the intruding male replies. Fighting may occur,
the maks lashing each other with their antennae, sparring and hiting
until one male retires.
In honey bees the sounds play an essential role in the transmission
of information within the colony which is very vital for them. The bees
do not survive in sound-proof hives. The piping of queen bees may be
important in informing the colony of the presence of a virgin queen in
the colony and indicating whether she is free or still enclosed within the
cell.
Important Questions
I.
2.
3.
Write an essay on bioluminescence in msects.
Describe the various ways by which insects produce sound.
Write short notes o n · (i) Stridulation; (ii) Tymbal organ; (iii) Mechanism ot " " ...
production in insects; (iv) Significance of light production and (v) Significance of sound
production.
16
Insects and
The Abiotic Environment
Abiotic factors that directly or indirectly affect the population of insects
include temperature,
moisture,
light,
and several other physical/chemical
parameters. Weather is a composite condition of influence of temperature,
light, humidity, rainfall and wind at any given moment in time. It varies
continually
throughout
influence on
insect
days,
weeks,
abundance,
months,
distribution,
and
years
longevity,
and
exerts
an
development rate,
and so on, from one year or season to the next. Climate, on the other
hand, is the annual average condition of the weather in a locality over a
period of several years. Weather changes rapidly, often violently, whereas
climate tends to remain much the same or change very slowly over a
period
of
many
temperature,
years.
moisture,
The
and
main
light,
components
although
several
of
the
other
weather· are
physical
and
environmental factors exert a degree of influence on insects.
1. Temperature
Insects are basically poikilothermic animals. It implies that they have little
physiological
regulation
of
body
temperature.
However,
they
possess
behavioural adaptations that maintain the body temperature as nearer to
an
overall
optimum
of the
species
as
environmental
conditions
allow.
Because of this, the body temperature is not always the same as that of the
environment. The range of temperature within which the insects are able
to survive is species specific and beyond this range they die. Not only this,
the range of tolerable temperatures varies even within ·a species, and with
the physiological state of an individual. Thus, an insect may survive high or
(Z-57)
Insects and The Abiotic Environment
[ 227
low temperatures during certain stage of life cycle, e.g., many insects are
able to survive much lower temperatures in winter than in the summer.
Tropical species are generally less tolerant of cold than those in temperate
species. Terrestrial insects usually tolerate wider range of temperature than
aquatic insects as the range of temperature variation in terrestrial habitats
is usually greater than that in aquatic habitats .
( I] The range of optimum temperature
Most of the insects survive somewhere between O"C and 50"C, although it
is likely that no one species can thrive throughout this entire range. The
optimal temperature range for most species is 22"C to 38"C. However,
some species are able to survive at temperatures beyond these ranges, e.g.,
some dipteran larvae may survive at 55"C or even higher, while certain
beetles may develop around O"C. The optimal temperature range for one
species of Grylloblatta living at high altitude is 3"C- l 2"C whereas, the
firebrat, Thermobia domestica that inhabits hot environments may live at
42"C and higher. If the insects of a given species are exposed to a range
of temperature, they move until they reach the preferred temperature, at
which point they will tend to congregate. Near the upper and lower
tolerable limits of temperature, insects become dormant.
1. Lower lethal temperature. Lower lethal temperature is the
temperature below the optimum range of temperature at which the
insects become less active. If the exposure period is prolonged they die
due to starvation, e.g., Locusta stops feeding below 20"C. At
temperature below freezing point, the majority of insects die as a result
of formation of ice-crystals or the metabolic balance may be disturbed.
However, some insects are able to avoid freezing because they can
withstand supercooling (i .e., being cooled below the point of freezing,
but without freezing actually taking place) as they possess certain
cryoprotective compounds like glycerol, sorbitol, and erythritol in their
tissues. Glycerol decreases the haemolymph freezing point of Bracon (a
parasitic wasp) larvae as much as l 7.5"C and that this compound plays
a key role in cold-hardiness. Most of the caterpillars are able to survive
the formation of ice within their bodies as they have higher
concentrations of cryoprotective compounds in their haemolymph that
depress the haemolymph freezing point.
2. Upper lethal temperature. Upper lethal temperature is the
temperature above the preferred or optimum range of temperature at
which the activity of the insects sharply increase. At extreme high
temperature at which an insect survives, the insect becomes enable to
move, a phase known as heat stupor following death. The temperature
at which death occurs is species-specific and exposure time-dependent.
It also depends upon the interaction with the humidity, e.g., Periplaneta
Insects and The Abiotic Environment
228 J
dies at 38"C at high humidity but can survive up to 48°C at the low
humidity. However, longer exposure to low humidity tends to dehydrate
the insects causing death. For many insects the lethal temperature for
short-term exposure
is
within
40-5<Y'C.
Previous
experience
of
temperature influence the lethal temperature for an insect species, e.g.,
Drosophila reared upto adulthood at 1 5"C, the adults survive for about
50 minutes in dry air at 33.5"C, but if they are reared at 25"C, they
survive for about 1 80 min at that temperature. Death at high
temperature is due to protein denaturation, metabolic imbalance,
disruption of ordered molecules (e.g., in the wax layer of the epicuticle)
and desiccation.
3. Acclimatisation of low and high lethal temperatures. The
responses to temperature are continuous and vary according to the
experience of the insects in past. Such modification is called
acclimatisation or acclimation, e.g., the larvae of mosquito Aedes reared
at 30"C die at
0.5"C, but survive if exposed for 24 hours at 1 8"C or
20"C. In field situations, the importance of the acclimation lies in
tending to fit the insect to the prevailing conditions.
4. Control of body temperature. There are certain insects that
possess some measure of control over their temperature. It comprises
mainly by behavioural adaptations rather than physiological control.
There are very few insect that have ability to regulate the body
temperature by physiological means. However by gaining and losing
heat, the insects maintain a narrow range of body temperature that
make them fit in that situation. The major sources of heat gain are
solar radiation and metabolic heat and heat loss include evaporation,
convection, conduction, and long-wave radiation.
(a) Heat gain. Solar radiation may cause the temperature of an
exposed insect to be significantly different from the surrounding
temperature. It is influenced by the size, larger insects being more
affected than smaller ones; colour, darker colours absorbing more
radiation than lighter ones; shape, the more surface directly exposed to
radiation, the greater the absorption; and orientation of insects with
respect to the sun, some insects orienting in such a way that a large or
small amount of body surface is exposed. Part of the metabolic heat
results
from
the
breakdown
of
complex
organic
molecules
(carbohydrates, fat and proteins) is stored in the high-energy bonds of
ATP while the remainder is released as heat. In the absence of solar
radiation, this is the sole source of heat.
(b) Heat loss. Evaporation of water from an insect has a cooling
effect and is the major cause of heat loss in the absence of solar
radiation. The rate of evaporation of water from an insect is dependent
partly on the size of the insect, smaller insects having a larger ratio of
-
Insects and The Abiotic Environment
[ 229
surface area to volume, have greater tendencies to lose water through
evaporation than larger ones. The other causes of heat loss are
convection, conduction, and long-wave radiation. These are significant in
the absence of solar radiation at which time the temperature differential
between an insect and its surroundings is usually greater than in the
presence of solar radiation. Tracheal system helps in removing heat
from the body. Dense coverings of hairs and scales, on the other hand,
may serve as an insulat�ng layer and retard heat loss.
(c) Behavioural adaptations. Temperature regulation is most highly
developed in social insects. The ants carry their young stages to the
most favourable situations. On hot days they block the entrance of their
nests so as to prevent the entry of warm air. On cooler days, moth
used to fan their wings generating metabolic heat to warm their body.
Most of the insects have a preference range of temperature and tend
to remain their for a longer period, e.g., the desert locust, Schistocerca.
Overwintering honey bees form clusters within their hives to prevent
heat loss.
[ II] Influence of temperature on the life-system of the insects
Temperature has direct or indirect effects on various life-systems of the
insects like the metabolic processes, longevity, fecundity, progeny sex ratio,
development period, rate of utilisation of food reserves when food is
present in limited quantities, etc.
1. Effect of temperature on longevity. Extreme low and high
temperatures are always lethal to the insects. The maximum survival of
the insects is at the lower tolerable range of temperatures which
decrease with rise of temperature. The cause of such decrease in
longevity at high temperature is an increase in metabolic rate. Certain
insects such as the blood-sucking tsetse fly (Glossina), that depends on
stored food reserves between meals, increase in temperature shorten the
survival period between meals. Evidently, if a fly uses up its reserves
from one meal before it is able to obtain another, it will die.
2. Effect of temperature on fecundity. In general, increase in
temperature within tolerable range increases the fecundity being
maximum at optimum temperature, e.g., Binodoxys indicus (an aphid
parasitoid) paras1t1se on average 234 aphids at 22"C (optimum
temperature) but only 98 at 12"C and 85 at 32"C. Similarly, corn aphid
Rhopalosipum maidis larviposit 32 nymphs/female at 20"C but only 1 4
nymphs/female at IO"C and 10 nymphs/female at 27.5"C o n sorghum.
The human louse, Pediculus do not lay eggs below 25"C.
3. Effect of temperature on progeny sex ratio. In most of the
insects the progeny sex ratio (proportion of males in the population) in
the population tends to stabilise at 0.5. However, extreme low and high
230 J
Insects and The Abiotic Environment
temperature increase the sex ratio particularly in insects reproducing
arrhenotokously where males develop parthenogenetically by haploid
eggs and female by diploid eggs, e.g., most of the parasitic wasps.
4. Effect of temperature on development rate. Within the tolerable
ranges, egg development, and the rate of larval and pupal development
usually increase with increasing temperature. For example, the duration
of pupal life of the mealworm beetle, Tenebrio monitor is decreased by
1 80 hours (from 320 to 1 40) as the temperature is increased by 1 2"C
(from 2 1 °C to 33"C). Similarly, total development period of Binodoxys
indicus is decreased by 14. l days (from 22.3 to 8.2 days) as the
temperature is increased by 25"C (from 1 2 to 37"C).
Because
temperature exerts such a strong influence on the rate of development,
it influences the timing of the various life stages of an insect.
Knowledge of the t1mmg and duration of these life stages is of great
importance in applied entomology as it is easy to predict the destructive
stage to be reached, or perhaps the stage most vulnerable to a
particular control measure to be reached. It helps applied entomologists
in applying timely control measures.
A useful method for the prediction of the timing of developmental
events (eclosion, larval moults, pupation, adult emergence, etc.) is the
use of physiological time scale, i .e., degree-days (0days ). It integrates
calendar time and temperature to yield a more accurate time schedule
and represent the accumulation of heat units above some minimum
temperature (lower developmental threshold or tL, the temperature
below which no measurable development occurs) for a 24-hour period.
For 0n accumulation, if the tL of an insect is 1 5°C and average
temperature for the day is 27"C, then 1 2°n, would have accumulated on
that day. For example, the thermal constant for egg to adult
development of an aphid parasitoid, Binodoxys indicus, is 330 °n, but
for the painted lady butterfly, Vanessa cardui, is 440 °n. However, tL
for these insects are 3"C and 1 2"C, respectively. Because of significant
differences, tL values must be determined for each species of interest.
The 0n programmes are a basic component of many insect pest
management programmes where 0n accumulations are made from the
beginning of a growing season and are continuously compared with
thermal constants that indicate time of potential crop injury. Thereafter,
samples are taken, density estimates are compared with economic
threshold levels. and decisions are made as to whether control measure
is necessary or not.
5. Effect of temperature on the distribution. The temperature
greatly affect the distribution of an insect species both horizontally
(latitudinal) and vertically (altitudinal). In temperate zones the northern
extreme of a given insect's distribution is commonly determined by low
Insects and The Abiotic Environment
[ 23 1
temperature extremes. For example, the com earworm, Heliothis zea, in
eastern North America must become completely re-established during
the warm season each year in Canada and to a progressively lesser
extent in the United States proceeding Southward.
6. Effect of temperature on dispersion and migration.
The
dispersion and hence migration is accomplished mainly by flight and the
body temperature of an insect is of overriding importance in limiting
flight. All insects have a minimum body temperature below which flight
is quite impossible, e.g., the minimum air temperature for flight for
Culex is 15°C but the arctic Aedes may fly at 2.5°C.
7. Effect of temperature on diapause. Temperature alone or with
photoperiod and other factors like water and fat content, induces
diapause among insects.
In general, m temperate regions high
temperature suppresses and low temperature enhances any tendency to
enter diapause.
2. Humidity
Humidity or moisture or water vapour content in the environment directly
or indirectly affects the reproduction and survival of the insects by various
ways as it may affect their metabolism.
[ I] Water content in insect body
The water content of insects varies from less than 50% to 90% of total
body weight depending upon the species as well as developing stages.
Soft-bodied insects (e.g., caterpillars) have larger amount of water in their
tissues than insects with hard-bodies. Active stages commonly have higher
water content than dormant stages. Some insects are able to maintain the
water content of the body within certain limits, which are influenced
by several other environmental factors (e.g., temperature, pressure,
air movement, available surface water) . Insects such as some Thysanura
(silverfish and relatives) are able to absorb moisture directly from
the atmosphere.
[ II] Environmental moisture
The precipitation (e.g., Rainfall, snow, hail and sleet), condensation
(e.g., dew, fog, and white frost) and available surface water are common
forms of water available in the environment and its annual and seasonal
amounts are primarily determined by the movements of large masses of air
and by topographical and soil characteristics. Humidity in the air depends
on temperature and atmospheric pressure. The relative humidity varies
with location, time of day or year, topography, vegetation, and so on,
and commonly tends to be comparatively high during the night and
-
.
.
232 1
Insects and The Abiotic Environment
lower during the day. It may also be different at different heights above
the ground.
1. Optimum moisture range for insects. Most of the insects survive
at an optimal moisture range which varies with insect species and
developing stages. Some insects may develop and survive as low as 5%
RH (e.g., Tenebrio) while some insects prefer 60 to 70% RH (e.g.,
Schistocerca). Extremely low and high humidity causes greater mortality
of the insects particularly active stages. Death under very dry conditions
is generally due to dehydration (decreased amount of body water). In
addition low moisture content of food may interfere the feeding
behaviour of the insects particularly bugs having piercing-sucking
mouthparts.
2.
Effect of humidity on the longevity, development and
reproduction - of insects. The longevity of insects at different relative
humidities depends on its ability to maintain its water content. If this
falls too low the insect dies. The developing stages (egg and pupa)
which are unable to replenish the water loss dies quicker at low
humidity whereas the larvae and adults which can replace the loss of
water are able to tolerate extremes of humidity. High humidity is
detrimental for the insects indirectly by favouring the spread of viral,
fungal, and bacterial diseases. The gypsy moth (Lymantria dispar) is
readily infected with viruses at high humidity and warm environment.
Many viruses only develop at temperatures of 2 1 to 29"C and relative
humidities of 50 to 60% .
The rate of development is also affected by humidity. The
incubation period of Ptinus eggs is 15 days at 20"C and 30% RH but is
10 days at 90% RH at the same temperature. However, the incubation
period of insects living in dry habitats, e.g., bedbug (Cimex) is not
influenced by the humidity while the rate of development of Locusta is
fastest at 70% RH, being slower at lower and higher humidities. The
preferred range of humidity of any insect may vary due to differences in
its water content. Thus, Tribolium normally has a preference for dry
conditions, but after three or four days without food or water a
preference for higher humidities develops.
Certain insects, e.g., migratory locusts do not produce eggs below
40% RH. Generally dry conditions adversely affect the rates of
oviposition, which increase as humidity increases. Heavy rainfall cause
mechanical damage to certain insects (e.g., aphids).
3. Light
Unlike other physical environmental factors, the parameters of light
(photoperiod, intensity, wavelength, etc.) are more or less constant. It is
seldom, if ever, directly lethal to the insects under natural conditions.
Insects and The Abiotic Environment
[ 233
However, it influences the survival, movement and reproduction of many
insects.
However,
insects
to
as
with
photoperiod
temperature
and
other
and
light
humidity,
parameters
the
vary
response
both
of
among
different species and among different life stages of the same species.
[ I] Photoperiod
Outside the tropics, long day (photoperiod more than 1 2 hours) qccur in
summer and short day (photoperiod less than
12 hours) in winter with
increasing or decreasing daylength in spring or autumn. The photoperiod
is one of the major stimuli that induces diapause in insects
Regular change
in photoperiod serves as an annual clock for insects and is used by many
to maintain synchrony with the seasons and their host plants. Photoperiod
less than 16 hours induces diapause in the pupae of
photoperiod
However,
exposure
more
in
to
few
than
insects,
extremely
hours
14
Euproctis
e.g.
short
initiates
(less
than
diapause
larval
15
Acronycta where as
in Bombyx mori.
diapause
hour)
or
results
from
extremely
large
photoperiods (more than 20 hours).
Photoperiodism influences the motor activity rhythms of insects such
as
locomotion,
also moulting
most
of
during
feeding,
temperate-zone
favourable
diapause
adult
and growth in
emergence,
some
insects
periods
is
and
mating
species.
so
the
tuned
and
oviposition,
The reproductive
that they
remaining
period
reproduce
is
and
cycle
passed
in
only
in
state.
[ II] Light intensity
The light intensity is the major factor that control the flight behaviour in
many insects. Diurnal insects (e.g., butterflies, wasps and bees) are not
active in the dark and aphids do not fly at low light intensity. Nocturnal
insects, e.g.,
moths, do not fly in light. In some insects, a cha11ge in light
intensity provides an important stimulus for take-off, e.g., locusts. Cloud
cover reduces light intensity and thereby reduces the flight and oviposition
of the cabbage butterfly reducing population growth.
The
effect
of
light
on
both
aquatic
and
terrestrial
plants
may
indirectly influence the activities of insects. For example, the amount of
light reaching submerged aquatic vegetation will affect oxygen-generating
photosynthesis and in tum affect the oxygen concentration in the water,
which then influences aquatic insects.
[ III] Different wavelengths
The
phytophagous
insects
locate
wavelengths of reflected light.
their
food
plants
The position of the
sun
using
different
and degree of
polarisation of light in different parts of the sky are important to many
insects in orientation and navigation. The honey bees and ants are able to
234 J
Insects and The Abiotic Environment
detect the polarisation pattern of the sky, which varies with the position of
the sun and enables them to determine direction. This directional ability is
important in finding the hive or nest after a foraging or hunting trip.
Insects other than social insects, e.g., diurnal crickets also possess the
ability to use polarised light in returning home.
4. Other Factors
Other environmental factors that under some circumstances also influence
insects include currents in air and water, gases dissolved in water, air
composition, electricity, ionising radiation, and soil composition.
Important Questions
I.
2
3.
Describe the abiotic factors that influence insect population.
Give an account on the ways by which insects maintain their body temperature.
Write short notes on : (i) Temperature threshold, (ii) Effect of temperature on
diapause.
17
Insect Population and Pest Outbreak
One of the major problems in agriculture is the control of insect pests that
cause about 34% loss of the crop, in spite of annual use of more than 500
million tonnes of pesticides plus various biological and other non-chemical
control methods. It accounts total loss from only insects worth more than
Rs. 10 trillion per year. H owever, all insects are not pest. W.henever the
population of an insect species in a habitat increased causing considerable
economic loss, the insect species will be said to be a pest. It implies,
therefore, that it is the population of insects that makes them pest and not
the species.
Insect Population
Insect populations are groups of individuals set in a frame that is limited
in time and space. Often, the boundaries of time and space for a
population are somewhat vague and are fixed, for convenience, by the
ecologists. The aspects of time and space are of significant importance . in
studying the population and necessarily need to be defined for any
consideration of population dynamics.
Every insect population has unique attributes that include density,
dispersion, natality, mortality, age distribution, and growth form of the
population. Density and dispersion are complementary attributes. Density
is the number of individuals per unit of measure (e.g., number of house
flies per square metre), whereas dispersion is the spatial arrangement of
those numbers. The dispersion may be clumped or aggregated (e.g., aphid
population) or it may be random. Natality is birth rate, often measured as
the total number of eggs or eggs/female/unit time whereas the mortality is
23 6 1
Insect Population and Pest Outbreak
the death rate, or numbers dying per unit of time. The natality adds
numbers and mortality subtracts them, and both determine the population
density as well as dispersion. Age distribution is the particular proportions
of individuals in different age groups at a given time, e.g., in early season
the age distribution of an insect population may be 75% adults, 20% eggs,
5% first-stage larvae. This distribution would change to mostly eggs and
larvae as time progressed, and so on.
[ I] Population change
Change in numbers from one time to the next and from one place to the
next is one of the most common properties of any population. A widely
used expression with the primary factors of this change is as follows :
N
N0e ( b- d)t
E t + It
t
=
-
where Nt =number at the end of a short time period, N0 =number at the
beginning of the time period, e= base of natural logarithms = 2.71 83, b
= birth rate, d= death rate, t= time period, E= emigration ( = movement
out of an area), !=immigration (=movement into an area).
The above expression is a general model of change m any
population and shows the mathematical relationship among the primary
factors of this change: births, deaths, and movements.
To explain population numbers, it is necessary to account
quantitatively for these primary factors. However, the prediction of
change involves the understanding of the environmental factors (e.g.,
weather, natural enemies, breeding habitat, overwintering space) that
could modify these primary factors.
1. Birth rate. Birth rate is a major process which adds new
individuals to a population. It is often expressed as numbers born per
female or per l ,000 females in the population during a specific time
period. The major factors that determine birth rate are fecundity,
fertility, and sex ratio. Fecundity is the rate at which females produce
ova, whereas fertility is the rate at which they produce new individuals,
e.g., fertilised eggs. Sex ratios in sexually reproducing populations are
0.5 (proportion of males). However, it varies in parasitic Hymenoptera
and, of course, parthenogenetic species. Overall, changes in birth rate
may be caused by changes in the average production of eggs per
female. changes in mating success, changes in the proportion of female
individuals in the population, or a combination of these.
The environmental factors like temperature, moisture, and food
strongly influence the fecundity and fertility of a female. For example,
European corn borer, Ostrinia nubilalis, produce 708 fertile eggs at a
temperature of 2 l"C, but only 533 were produced at 32"C. Likewise,
female egg production can be greatly influenced by nutrition during the
Insect Population and Pest Outbreak
[ 2 37
immature stage. Crowding affects birth rate partly through affecting the
quality of the food but also by more direct influences such as
stimulating restlessness of individuals. Variability in birth rate of a
population is often overlooked as a cause of population change. The
attempts to explain this variability are important as the factors
responsible may be the essence of a population outbreak.
2. Death rate. Numbers of insects dying over a period of time are
often represented in survivorship curves or in life tables, a tabular form
for accounting for deaths. In many insect populations, high death rates
(mortalities) are the rule, with sometimes less than 1 % of the
individuals of a generation reaching adulthood. Mortality factors may be
biotic or abiotic and may operate in a single life stage or over many
stages. For example, intense rain causes mortality of all stages of aphids,
while their parasitoids (e.g., Binodoxys indicus, Diaeretiella rapae)
parasitise and kill usually second and third instar nymphs. Crowding
may lead to cannibalism or starvation.
Like birth rates, death rates can vary greatly from one time and
place to the next. Indeed, relaxation of an important mortality factor
(e.g., natural enemies) may be the primary cause of a population
outbreak.
Following are the causes of mortality of insects
aging
(physiological death), low vitality (ability to survive), accident (abnormal
events in the insect life cycle resulting in death), physicochemical
conditions (physical and chemical conditions of air, water, and substrates
in or on which the insect population lives), natural enemies (predators,
parasite, parasitoids, and pathogens), food shortage and lack of shelter.
3.
Movements.
Knowledge
of movements
into
an
area
(immigration) and out of an area (emigration) is vital for understanding
population dynamics. Indeed, movements, rather than births or deaths,
may be the main cause of rapid population change within a season; that
is, predictable movement can be a major population characteristic.
Information obtained from studying insect movements may allow such
activities as pinpointing sources of plant-disease vectors or predicting the
seasonal arrival of an important pest that does not overwinter locally.
Movement of all kinds is a rule during the life cycle of most
insects. It may involve persistent crawling, walking, or hopping along
terrestrial surfaces; swimming on or in the water; or flying or being
carried passively (as with wingless insects) in the air. Most movements
can be categorised as nonmigratory or migratory.
(a) Nonmigratory movements. It involves displacements of insects
within or close to the breeding habitat that are frequently interrupted
when the insect encounters food, a mate, or an oviposition site. Such
movements usually take place throughout the life of individuals while
238 I
Insect Population and Pest Outbreak
carrying out their life functions, except during egg and pupal stages and
during dormancy, e.g., a butterfly flies from flower to flower, sometimes
moving among several habitats, feeding on nectar.
(b) Migratory movements. It involves great distances, perhaps
hundreds of kilometres, during which time locomotion is not inhibited
by food, mates, or oviposition sites. Migration is usually accomplished by
a peculiar type of flight which is an adaptation to periodically transport
insects beyond the boundaries of their old reproductive sites and into
new ones. During migratory flights, great mortality of insects may occur
as a number of individuals are deposited in areas where they cannot
survive, e.g., open seas, lakes, glaciers, and snow fields. However, at
least a small portion of the migrants succeeds to locate suitable habitats
where they can reproduce.
[ II] Factors affecting insect population
The factors that could cause variation in birth and death rates as well as
movements fall under two categories: climate and competition (Fig. 1 ).
However, for the long-term regulation of insect numbers at a particular
level, the factor that varies in its effect with density of the insect population
is more important than the factor which is related to climate or
competition. This is explained in Figure 2. The insect populations like
others tend to increase geometrically, a typical population increase can be
represented as a straight line plot of the logarithm of density against time
(Fig. 2a). If there is a restriction on birth rate, the line will have a
shallower slope (Fig. 2b). If the restriction is very strong, the population
may actually decline (Fig. 2c). Any factor which causes a simple change in
climate
<
directly
indirectly
<
mortality e.g, frost, storms, rainfall
<
natality
<
growth and condition of food plant
differential effect on organism
and its natural enemies
mortality
natality
with natural enemies (for own survival)
competition
� with other species e.g, for food, oviposition site
� within the species e.g., overcrowding, starvation
Fig. I . Factors causmg variation m birth and death rates
in msects.
as
well
as
movements
Insect Population and Pest Outbreak
�
Ul
c
CD
:g,
Ul
a
[ 239
I
I
b
a;
e
.0
E
:::i
c
OJ
.Q
c
time
Fig. 2. Pattern of relationship between insect density and time.
the rate of increase of this kind will, if extrapolated, lead either to
enormous populations or extinction.
There is clearly a limit to the size of any population if the growth
rate is positive (Fig. 2a or b). At some stage, density will reach a point
where competition for space or food becomes intense and the growth
rate will then rapidly slow down (Fig. 2d). Such slowing down is an
example of the action of a density-dependent factor; it has little or no
influence on the population growth rate until a certain density is
reached, when its effect suddenly becomes rapid and dramatic. Until
then (Fig. 2a, b or c) the population increase rate has largely been
determined by density-independent factors.
If population growth is illustrated by a straight line of logarithm of
density against time, a density-dependent relationship can be shown by a
change of a straight line into a curve. Figure 2e shows a
density-dependent interaction operating long before overcrowding occurs,
and the effect of this constraint becomes greater as the population
increases (illustrated in Figure 2e by a series of increasing changes in
angle forming a curve). Natural enemies usually show such a
density-dependent relationship with the density of their prey or host
(Fig. 2f). The rate of parasitism usually increase with rise of host
density which is partly due to increasing numbers of parasitoids locating
the host resource and partly due to their greater retention period at the
host patch. At higher host densities the host searching time is also less.
Similarly, many predators spend less time feeding per prey individual
when prey is abundant, moving on to new prey without totally
consuming earlier one.
Even crowding may exert a density-dependent relationship at a
much lower density than would be expected. Encounters between
individuals of the same species will lead to some cannibalism at quite
low prey densities or stimulate the production of emigrant individuals
(Z-57)
240 J
Insect Population and Pest Outbreak
long before overcrowding is apparent. Clearly natural selection will
operate against the dangers to subsequent generations when individuals
in an 0•1erpopulation situation have to face intense competition for
resources. Competition is a widespread phenomenon which very often
takes the form of a territorial behaviour; additionally , increasing contacts
between individuals as density increases may reduce fecundity, promote
cannibalism and induce emigration or an arrest in reproduction.
1. Density-independent relationships. In the agro-ecosystem, plants
are specially selected and properly managed by thinning, using fertilisers,
insecticides and other agro-practices. Whenever, a pest insect arrives at
a particular time, it finds a very large proportion of the plants at a high
level of suitability. Fertilisers and thinning are among the management
practices which maintain a high plant quality suitable for the rapid
development of a pest infestation.
2. Density-dependent relationships. As mentioned earlier, the
relationship
between
insect pests
and their natural enemies is
density-dependent. Following facts are responsible which dissociate the
interaction of insect pests and their natural enemies.
(a) Exotic pests. Many pests have been introduced from abroad, and
have been separated from their natural enemies which may not survive
the new climate. The woolly aphid (Eriosoma lanigerum) entered into
Britain from the USA but its effective wasp parasitoid (Aphelinus mali)
does not survive the British winter.
(b) Crop harvesting. Most crops are harvested to leave fallow land
which dissociate the interaction between insect pests and their natural
enemies. Each new crop must, therefore, largely be colonised by natural
enemies from outside.
(c) Use of insecticides. Insecticides depletes the natural enemy fauna
in crops.
Pest Outbreak
The word 'pest' is not a natural (ecological) word but a man made word.
From global perspective we were to designate as pests because pests
should be defined as those biological species which impair ecosystem,
productivity, diversity, and stability. According to this definition the world' s
major pest is man himself (earth pest) whose number and activities are
threatening the stability of Lhe biosphere. However, here for our own
interests we define pests as an organism, if it becomes abundant enough to
harm man directly or indirectly. Thus, still we are concerned with numbers
or populations of a species rather than its mere presence. This is basic to
the concept of economic levels of the pest to survive and reproduce in the
habitat and the way in which any factor inherent or extrinsic to the pests
physiology and behaviour violate its success or failure.
(Z-571
Insect Population and Pest Outbreak
[ 24 1
[ I] Kinds of pests
There are six kinds of pests: regular, sporadic, occasional, seasonal,
persistent and potential pests.
1. Regular pests. The regular pests are those insects that occur
most frequently on a crop and such insects have close association with
a particular crop, e.g., jassids (Nephotettix bipunctatus) on paddy, aphids
(Lipaphis erysimi) on mustard, fruit borer (Leucinodes orbonalis) on
brinjal, etc. These insect pests are expected to arrive on the crop
sometime before harvest.
2. Spo. adic pests. The sporadic pests assume pest status
occasionally in certain years in a few isolated localities. It includes rice
stink bug (Leptocorisa acuta), locusts, grasshoppers, hairy caterpillars,
crickets, cutworms and cotton semi-looper.
3. Occasional pests. Many insects occur rather infrequently and a
close association with a particular crop is absent, e.g., caseworm of rice,
the castor slug caterpillar, the mango stem borer, etc.
4. Seasonal pests. Seasonal pests are those insects which ocrur
mostly during a particular part of the year. The incidence of these pests
are largley governed by the environmental factors in a locality, e.g., red
hairy caterpillar (Amsacta albistriga) on groundnut during April-May in
certain localities of south India and the ric1<_grasshopper (Heiroglyphus
banian) during August-September in northeastern U.P.
5. Persistent pests. Insects which occur on a crop almost throughout
the year are known as persistent pests. Scale insects and mealy bugs on
sugarcane are examples.
6. Potential pests. The potential pests normally cause little loss but
may become highly destructive resulting from some disturbance in the
environment and the consequent increase in their number, e.g., brown
plant hopper infesting paddy crops in eastern U.P.
[ II] Causes that make the insect as pest
There are three main causes by which an insect population attains a pest
status-(i) invasion (ii) ecological changes and (iii) socio-economic
changes :
1. Invasion. Advances in transportation technology over the past
century have increased global interaction among the nations and
encouraged trade. It promoted the exchange of perishable as well as
other agricultural commodities, however, the increased trade results an
increased risk of new pest invasion. In spite of advanced technology
used to limit the invasion, many insects used to evade inspection.
Indeed,
large
proportions
of the
major
pests
in
a
riiodern
(Z-57)
242 J
msect Population and Pest Outbreak
agro-ecosystem are exotic in ongm. Introduction of San Jose scale and
cottony cushion scale of citrus into India, gypsy moth and European
com-borer into North America, and the insect pests of Eucalyptus into
New Zealand and South Africa are examples.
2. Ecological changes. Ecological changes are another major cause
of pest out breaks. The history of agriculture has been the history of
constant ecological changes. Through various agrotechnical practices,
e.g., monoculture, selection of high yielding plant cultivars or,
elimination of natural enemies, etc., that created conditions favourable to
certain insect species and has thus induced many folds increases in their
popula•ion. Actually, these factors disrupt the interaction between
.
phytophagous insects and their natural enemies, the essential ecological
processes that contribute to the regulation of insect population.
Whenever, this interaction between phytophagous and entomophages is
disrupted, the population of the phytophagous insects increased
tremendously and they attain pest status because they become free from
the constraints imposed by them.
3. Socio-economic changes. Increased urbanisation has changed the
life-style of human beings.
For example, the human activities have
provided conditions conducive to breeding of mosquitoes and use of
insecticides by man has created a number of secondary insect pests.
Similarly, the cotton jassid was not so serious a problem on the
indigenous cotton in India, but with the introduction of American cotton
in the beginning of the century this insect became the dominant pest of
this crop in central and south India. Introduction of susceptible and
nutritious host plants into the environment of a pest species usually lead
to its fast development and abundance.
There is a considerable difference between the insect population
harbouring crops and non-crop plants. The insects feeding on wild
plants they do not occur in vast numbers, and also the plants do not
appear to suffer extensively by their activities. However, contrast to this
endemic situation, the insect population is extensively enormous in the
cultivated fields (epidemic situation). It is, therefore, worth comparing
how numbers of insects are affected in contrasting situations, such as
uncultivated land and crops, or comparing the same species in countries
where it is a pest and in others where it does not attain pest status.
[ III] Factors causing pest outbreaks
As mentioned earlier, there are several factors that influence the
population growth of insects. The factors influencing birth and death rates
and movements interact in a manner that in natural system there is a
balance between the phytophagous and entomophagous insects and other
(Z-57)
Insect Population and Pest Outbreak
[ 243
flora and fauna. The abiotic factors that act density-independent has little
role on the pest outbreaks while the biotic factors particularly the natural
enemies of the insects exert density-dependent action and put the
population of phytophagous insects below the economic threshold level.
Therefore, any factor that could cause the detrimental effect on the natural
enemies of insect pests and create a suitable environment for the latter
cause pest outbreak and a knowledge of these factors is essential.
1. Deforestation for crop cultivation. The destruction of forest for
agriculture directly or indirectly influence the climate, viz., temperature,
humidity, rainfall, wind velocity, etc. in that locality that create situations
favouring some insects to reproduce more profoundly and assuming pest
status. The insects feeding on the forest vegetations are forced to infest
the cultivated crops and become new pests.
2. Destruction of natural enemies. The natural enemies (parasites,
parasitoids and predators) of the insects prevent the increase of their
population. Their destf\lction either by man or other agencies (by using
synthetic insecticides, burning crop wastes having harborage of dormant
stages of parasitoids, e.g., burning of sugarcane leaves destroys the
developmental stages of parasitoids of pyrilla) tends to increase the
population of insects in an area. Sometimes the weather conditions may
be favourable for the tremendous multiplication of a pest and
unfavourable to its natural enemies. The phytophagous insects are more
resistant to insecticides than the parasitoids and predators.
3. Modern agronomic practices. Modern agricultural practices, viz ..
mechanical tillage, timely irrigation and application of synthetic fertilisers
improve the growth of the crop which reduce competition for food for
insects due to which they reproduce very fast. Particularly, uses of
nitrogen fertilisers increase the palatability of the crop for insect, e.g ..
incidence of stem borers in rice and aphids in cotton increases when
high amount of urea is applied. Sometimes use of insecticides as a
prophylactic measure results in reducing one of the competing species
of pests while allowing the other to multiply unhindered.
4. Intensive and extensive cultivation of crops. When one or more
crops are cultivated in extensive areas, due to high food availability, no
problem exists for the insect for food and shelter. This affords
conditions that favour the proper development of the insects.
The
effect is more pronounced if cropping is done in more than one season
for the year. If the different crops in rotation are closely related to
each other or when there are alternative food plants for the insect
concerned, again the population of insect is likely to increase. For
example, incidence of · borers and leaf hoppers when sugarcane crop is
raised over extensive areas.
244 J
Insect Population and Pest Outbreak
5. Introduction of new crops and improved strains. Plants or trees
introduced into an area where it was not previously present, may serve
as new hosts for some of the insect species. Most often improved
cultivars of crops are susceptible to insects.
Sometimes the insects
which are considered of minor importance become of major importance
with the introduction of such new varieties. The improved Cambodea
cotton strains are highly susceptible to the spotted bollworms, Earias
spp. and the stem weevil, Pempherulus affinis than the desi cotton
strains. The growing of cabbage crop in the plains of Tamil Nadu as a
new venture resulted in the widespread incidence of the green
semilooper Trichoplusia ni.
6. Introduction of a new pest in a new area. When an insect
somehow introduced into a new area it becomes more abundant because
of want of their natural enemies. For example, the woolly aphid
(Eriosoma lanigerum) became a major pest of apple on the Nilgiris as
there was no natural enemy of the pest to exercise natural control over
its multiplication. Nowadays it is under control by introduction of its
specific parasitoid Aphelinus mali collected from Punjab and Himachal
Pradesh.
7. Accidental introduction of foreign pests. Modem fast transport
and global trades (import and export of commodities) has increased the
chances of introduction of foreign pests into areas where they were not
present before. Stored grain pests, adult insects which. adhere closely to
the plant (e.g., scales and whiteflies), and fruit and stem borers, leaf
miners, gall insects, etc., are more liable to be introduced into other
countries. Some of the accidentally introduced insect pests into India
from foreign countries are the diamond back moth (Plutella xylostella)
on brassica vegetables, the San Jose Scale (Quadraspidiotus pemiciosus)
on fruit trees on the hills, the green bug (Coccus viridis) on coffee and
the potato tuber moth (Phthorimaea operculella).
8. Resurgence of sucking pests. Sometimes plants treated with
systemic and contact insecticides though afford initial protection against
sucking insects tend to offer physiological conditions favourable for their
rapid reproduction and ultimately heavy build up of their population.
For example, in recent years in northeastern U.P. brown plant hopper
(Nilaparvata lugens) resurge on paddy coop due to heavy use of contact
poisons against stink bug (Leptaconsa acuta).
9. Large scale storage of food grains.
Storage of grains m
warehouses leads to increase the pest population if poorly managed.
Insect Population and Pest Outbreak
[ 245
10. Biotypes of pest species. Insects have great adaptations for their
survival. There are several insect species that develop biotypes that are
able to develop and multiply even on resistant crop cultivars. Several
biotypes of aphids (Lipaphis erysimi) on mustard crop, gall midge and
brown plant hopper (Nilaparvata lugens) on paddy and whitefly (Bemis ia
tabac1) on cotton, etc. have been observed in India.
Important Questions
I.
2.
3.
4.
What are the parameters for measurement of population changes of insects ?
Describe the causes of pest outbreaks.
Define the word 'pest'. Describe the main causes by which an insect population attains
a pest status.
Write short notes on : (i) Birth rate, (it) Kinds of pest, (iii) Emmigration and migration.
18
Insect-Plant Interaction
Insect-plant association begins with the evolution of insects. Almost half of
the insect sp�cies are phytophagous and thus depend on plants for their
food requirements. Others feed upon phytophagous insects and indirectly
related with the plants as it is the plants that capture the sun's energy and
use it to make organic molecules from inorganic ones. On the other hand,
plants have several mechanisms to defend the insect attacks as well as
evolve morphological and physiological features to attract the insects for
the pollination. The insects have developed to break down the defense
posed by the plants and simultaneously evolved sensory mechanisms to
respond each and every stimulus received from the plants.
Phytophagous Insects
Half of the insect species depend upon plants directly for food. Plant
tissues are major source of sugars, proteins, fats, salts, water and vitamins.
Most plant tissues provide adequate nourishment for insects, although
different insects do have different nutritional requirements . Insects feed on
all parts of the plants (e.g., leaves, buds, stems, roots, fruits, seeds, flowers,
nectars, pollens etc.) as well as on plant tissue in various stages of decay.
They feed externally or internally, as borers or leaf miners; they produce
galls and other distortions; sucking insects imbibe plant juices and may
cause weakening and yellowing. Some insects that do not feed plant tissues,
also use plants as shelter, and some make still other uses of plants (e.g.,
leaf-cutter ants harvest bits of leaves that they use as a substrate for
growing fungi in their nests).
Insect-Plant Interaction
[ 247
It is hard to find any plant species immune to insect attack. Most
o f the plan ts are �ttacked by hundred:; of the insect species. Similarly, a
.
_
smgle species of msect may attack more than one plant species. On the
basis
of
feeding
habits
insects
may
be
monophagous
(feeding
single
plant species, e.g., silkmoth), oligophagous (feeding on a single group of
plants, e.g., mustard sawfly, Colorado potato beetle, brinjal fruit borer)
and polyphagous (feeding on distantly related plant species, e.g., locusts,
gypsy moth, cotton aphid). Few insects which feed both upon plant as
well as animal tissues are called omnivorous (e.g., crickets, cockroaches,
ants).
[ I] Host plant selection
The phytophagous insects may reach its host in one of three ways: the
hosts may be selected by trial and error, e.g., grasshoppers; the host may
be selected by the mothers, who lay eggs in response to some cues, often
chemical, e.g., most moths and butterflies; and the insects may live in an
aggregation
that extends
through
several
generations,
so
that emerging
young find themselves already settled on their host plants, e.g., aphids and
social insects.
The insects have several ways to find their host plants. They have to
pass at least following stages in host finding: host habitat location; host
location; host recognition and acceptance; and host suitability.
1. Host habitat location. The location of habitat and orientation
towards the prospective food plant appears to be the first step in the
process of host plant selection by phytophagous insects. Visual cues are
usually involved for finding the hosts from a distance but detection of
host plants from within close range is mostly mediated through olfactory
cues.
Desert
locusts
are
responsive
towards
the
pattern
of
vertical
stripes. In addition to light, other factors like wind, gravity, and perhaps
temperature
and
humidity
also
help
orient
dispersing
insects
to
the
overall location of the host.
2. Host location. Once in a host habitat, the next step is to find a
proper host plant.
Most insects rely on
vision
and/or smell to find a
host plant. Remote factors in locating the plant include colour, size, and
shape.
Aphids, whiteflies and apple
yellow-green
surface.
Red
cultivars
from the insect attack than
maggot flies are attracted towards
of cotton
other cultivars.
and
cabbage
suffer
less
Some cotton cultivars that
blossom yellow flowers, attract insects for pollination but once pollinated
they turn pink or red. It prevents revisit of insects and thus promotes
the pollination of other flowers.
In addition to colour, some insects,
like fruit flies are known to associate shape and size of trees in locating
hosts.
Once
insects
are
in
close
contact
with
the
plants,
short-range
stimuli arrest further movement. These stimuli are both physical, exciting
Insect-Plant Interaction
248 1
tactile
receptors
antennae,
and
and
chemical,
exciting
chemoreceptors
on
tarsi,
mouthparts.
3. Host recognition and acceptance. Subsequent to host finding,
insects use to take test bites to taste the plant tissue to confirm host
recognition. Continuous feeding seemingly is regulated by the stimulation
from various chemicals present in the plant tissues. Such chemicals are
known
as
phagostimulants
and
they
may
either
be
secondary
plant
metabolites or the nutrients including minerals, amino acids and sugars.
For
example,
sinigrin
(a
glucoside
found
in
plants
of
Brassicaceae
family) serves both as phagostimulant and attractants for oviposition for
the
insects
cabbage
infesting
aphid,
Similarly,
in
a
the
plants (e.g., mustard
Brevicoryne brassicae;
monophagous
insect,
aphid,
Lipaphis erysimi;
Pieris rapae).
(Bombyx mon), a
cabbageworm,
the
silkworm
series of substances (morin, inositol, sucrose) are perceived in mulberry
leaves
that
elicit
biting,
European corn borers,
swallowing,
and
continued
feeding.
For
sucrose concentration in the plant tissue triggers
feeding response. Feeding to satiation then
follows
in the presence
of
appropriate chemicals. Physical factors such as leaf and stem toughness,
leaf surface
waxes,
and
pubescence
may
be
important
in
relation
to
feeding and/or oviposition. For example, female of diamond back moth
lay eggs more readily on rough than on smooth surface.
4. Host suitability. The plants that provide adequate nutrients and
no toxic chemicals and
insect completes development within a normal
time period and becomes an adult are said to be
suitable host plants.
The plant
adult longevity
suitability
is
also
reflected
in
normal
and
fecundity.
[ II] Feeding habits
Plants are complex and variable structures. The nutnt1ous parts of plants,
cytoplasm and fluids, are surrounded by wooden walls that insects cannot
directly digest, so they must mechanically disrupt or otherwise avoid them.
Insects have evolved specific morphological, physiological, behavioural and
other adaptations to cope with and use plants. On the basis of feeding
habits the insects can be classified into several groups as follows.
.
1. Chewers. Chewing is the most common way by which insects feed
plant
materials
Phytophagous
(leaves,
insects
stems,
belonging
flowers,
to
the
orders
pollen,
seeds,
Orthoptera,
roots).
Phasmida,
Coleoptera and Hymenoptera both larval and adult stages have chewing
mouthparts.
In
the
Diptera
and
most
Lepidoptera,
only
larvae
have
chewing mothparts.
2. Miners and borers. Larvae of many insects feed within plant
tissues. Leaf miners are chewing insects that eat one or more of the
tissue layers between the intact upper and lower epidermis of leaves. As
Insect-Plant Interaction
[ 249
the insect feeds, a tunnel is made whose pattern is often characteristic
for the species. Leaf-mining insects are found in Lepidoptera, Diptera,
Hymenoptera and Coleoptera. Similarly, boring insects live in the woody
tissue of plants or fruits. The adult cerambycid beetles bore into the
hard wood and feed it. Larvae of sugarcane top borer and stem borer
feed the content of the stem. Most of the fruit borers (Earias spp. on
okra, cotton; Leucinodes orbonalis on brinjal; Bactrocera (=Dacus
cucurbitae on cucurbits) and stem borers (Chilo spp. on corn, s orghum;
Tryporyza spp. on sugarcane, rice) are injurious to crops.
3. Gall-makers. Many phytophagous insects induce the production of
abnormal growth reactions, galls, in the tissues of their host plants,
inside which they live and feed. Galls are found in buds, leaves, stems,
flowers, or roots and their shape and location are often characteristic of
the plant and insect species concerned. The gall is entirely a product
of the plant, developing in response to a chemical stimulus from the
secretions of the insect. The gall-maker species are found primarily in
Diptera (gall midges, Cecidomyiidae), Hymenoptera (gall
wasps,
Cynipidae, Agcanidae), Homoptera (aphids, Aphididae) are found on
several plant species like oak, rose, sunflower, eucalyptus, etc. The
simplest galls are mere swellings that involve no major distortion or
discoloration; these are called indeterminate galls. In some cases these
are expanded to form pouch galls, which are often open to the outside
and are especially characteristic of aphids. The majority of gall midges
and wasps make determinate galls, which have a form and colour quite
different from that of the host plant, e.g., oak galls.
4. Sap suckers. Some insects do not physically harm the plant and
by using highly modified mouthparts pierce the plant epidermis and
suck plant sap. In this way, the true bugs, aphids, whiteflies, thrips and
males of mosquitoes avoid consuming indigestible part of the plant as
·
well as toxic materials produced by the plant. Sap feeders either take
xylem (e.g., cicada) or phloem (e.g., aphids) contents of the plant or
pierce and macerate the contents of individual cells with their styles and
then suck the liquefied material through their proboscis (e.g., thrips).
Phloem feeders usually do not physically harm the plant tissues.
However, they have to excrete extra sugars as honeydew, e.g. , aphids
and leafhoppers. This honeydew causes the growth of black moulds that
inhibits photosynthesis reducing the yield. The honeydew serves as food
source for many insects like bees, wasps, ants etc.
5. Seed feeders. Seed-feeders and seedling-feeders are the only true
plant predators among insects because they kill plants by consuming
them, e.g., seed beetles (Bruchidae).
250 j
Insect-Plant Interaction
[ Ill] Insects as vectors of plant diseases
Insects transmit almost all kind of plant diseases caused by bacteria,
viruses, fungi and mycoplasma by several ways.
1. Mode of transmission. Insect transmit the plant diseases by two
means : mechanical and circulatory.
(a) Mechanical transmission. When the causative pathogens are
borne on the surface of the insect, usually the mouthparts, and in this
way carried from plant to plant, the mode of transmission is said to be
mechanical. In case of sucking insects, e.g., aphids, it is called as stylet
borne transmission.
Such a transmission is usually nonpersistent since
the pathogens survive for only a short period.
(b) Circulatory transmission. When an insect from the diseased plant
ingests the causative pathogens, the pathogens circulate in the body, and
are later discharged into salivary fluids. During feeding the insects inject
the pathogens into the plant tissue. Such a mode of transmission is said
to be circulatory. It is persistent , also as the pathogens survive for a
longer duration.
The transmission may be by inoculative, in case of sucking insects
(e.g., aphids) or by surface deposit of pathogens, which must then
invade the plant tissues often through the wounds made by the insects.
2. Types of diseases transmitted by insects. Several thousand insect
species are known that transmit several hundred plant diseases
throughout the world.
(a) Bacterial diseases. Plant pathogenic bacteria are bacilli, usually
non-spore formers, which are able to enter plant tissue only through
wounds or by inoculation. The transmission is usually mechanical and
nonpersistent. For example, cucurbit wilt is caused by Erwinia
tracheiphila that invade and blocks the vascular bundles of cucumbers.
(b) Viral diseases. Viruses produce a great number of plant diseases.
A single species of aphid, Myzus persicae are known to transmit more
than 1 00 plant viruses. Symptoms of virus disease are diverse; they
include blotching and mottling of leaves (termed mosaic), leaf curl,
tumors, rosettes, distortions of flowers and fruits, yellowing and necrosis.
(c) Fungal diseases. A great variety of rots, wilts, cankers and root
infections are produced by fungi and the spores of such fungi are
transmitted from plant to plant by insects. Unlike bacteria, fungi
penetrate the plant tissue without requiring a wound. Transmission is
usually mechanical. For example, blue stain of conifers is caused by the
fungus Ceratostomella and is mechanically transmitted by bark beetles.
(d) Mycoplasmal diseases. The mycoplasmas are pleomorphic, i.e., the
cells undergoing changes in form throughout their life cycle. More than a
dozen plant diseases are caused by mycoplasmas and all of them are
Insect-Plant Interaction
[ 251
transmitted by leaf hoppers. For example, aster yellow that infects at least
40 plant families including potatoes, carrots, and spinach. In this disease,
plants become yellow with various malformations.
3. Insects causing phytotoxemia. An insect whose feeding produces
symptoms of disease is said to be toxicogenic, and the condition is
called as a phytotoxemia. Several kinds of phytotoxemia are recognised
as given below.
(a) Localised lesions. Leafhoppers and mealybugs while feeding inject
saliva into the plant tissue due to which the feeding spot become paler
or darker in colour than the surroundings. The damage caused is
localised.
(b) Malformations. Leaf curling, rolling, production of witches
brooms, shortening of intemodes, and other distortions of plants are
caused by insects. For example, feeding of leaf hoppers (Cicadellidae)
often called hopperbum produces browning and curling of leaf edges.
Several plants are subject to hopperbum, such as potatoes and melons.
(c) Systemic toxemias. It includes yellowing, wilting, redµction in
growth, or killing of part or all of the plant. These conditions result
from translocation throughout the plant of toxins produced by sucking
insects like aphids, psyllids and leaf hoppers. Psyllid yellows of potatoes
is one of the best known of systemic toxemias. The saliva of most of
the insects contains phytotoxic chemicals or growth inhibitors.
Defense Mechanisms of Plants Against Insects
Plants have evolved at least two kind of defense mechanism against
phytophagous insects: physical defense and chemical defense.
[ I] Physical defense
Physical defense or resistance against insects involves plant structures that
interfere physically with the insect's locomotory behaviour, feeding, or
reproductive functions. These functions may include host selection, feeding,
ingestion, digestion, mating, or oviposition. Colour, shape, tough cell walls,
trichomes etc. are certain morphological characteristics that help plants to
defend attack of insects.
1. Colour. Certain insects seem to be most attracted to leaves that
are yellow-green, i.e., those that reflect light with wavelengths within
500-600 nm range. For example, aphids are attracted to yellow leaves or
flowers irrespective of the plant species. Dark green plants are less
attractive to these insects. Red cotton varieties suffer less from boll
weevil than green varieties. Similarly, red cabbage varieties are less
susceptible to oviposition of the cabbage butterfly, Pieris mpae than any
green varieties.
252 J
Insect-Plant Interaction
2. Shape. Tropical Heliconius butterflies locate their passion vine
host (Passiflora sp.) by visual cues relating to the leaf shape. The
passion vines have a range of defensive chemicals and also have
extrafloral nectaries that attract ants as well as parasitoids, both add to
the defense of the plant. The vines have developed leaves of various
shape that make it difficult for Heliconius to locate. The leaves of
certain vine plants resemble those of other tropical plants that
Heliconius caterpillars find inedible (an example of plant mimicry).
3. Thickened cell wall. Cell walls of the plants that are thicker than
normal, usually owing to deposition of additional cellulose and lignin,
are more resistant to the tearing action of insect mandibles or
penetration of the stylets or ovipositor. If eggs are laid onto/into the
plant having tough cell walls, it is difficult for the larvae to feed
properly and their rate of mortality increased. In some cases, thick cell
walls also inhibit digestion. For example, thick hypodermal layers have
been considered a .factor in resistance in rice to the rice stem borer.
4. Stem characteristics. Insects living inside stem are sometimes
seriously affected by differences in stem characteristics, and in many
cases resistance to stem borers is related to the nature of the stem
tissues. For example, solid stems are much more resistant to mustard
sawfly than hollow stem varieties. Similarly, thick cortex in the stem of
tomato prevents the aphid Myzus persicae, from reaching the vascular
tissues.
5. Trichomes and glandular secretions. Trichomes are cellular,
hairlike outgrowths of the plant epidermis, which may occur on leaves,
shoots, or roots. Trichomes are important for various physiological
reasons but are particular value in water conservation and are probably
the plant's most important morphological defense against insect attack.
These structures interfere with insect oviposition, attachment of the
insect to the plant, feeding, or ingestion. The mechanical effects of
trichomes depend on following attributes: density, erectness, length, and
shape. Some trichomes posses glands that exude secondary plant
metabolites and if these chemicals are defensive, it may combine with
physical defense mechanisms. However, certain secretions are sticky and
physically glue the insects on the plant surface reducing locomotion.
Pubescent surface (those with high densities of trichomes) prevents
sucking insects to feed as the stylets do not reach to the conductive
tissues of the plants. Even insects having mandibulate mouthparts find
difficult to feed on the plant and having dense and erect trichomes. If
eggs are laid or the leaf surface having dense hairs the younger larvae
have to feed trichomes first to reach the epidermis. In doing so, they
ingest large amount of cellulose and lignins, the basic constituents of the
trichomes, and death resulted from this inadequate diet. Thus trichomes
Insect-Plant Interaction
may
act as
defense
[ 253
mechanisms
in
several
ways:
they
discourage the
females from oviposition on the leaf surface; if eggs are laid, tend to
dehydrate;
if the
eggs
hatch,
the
surviving
larvae
starve
because
of
nutritional deficiency; and some still survive, their gut walls are damaged
due to spike-like trichomes.
6. Silica. Silica is incorporated in the epidermal wall of several
plant
families
against
and
attack
this
seems
by some
to
be
an
effective
defense
insects. For example, rice
mechanism
plants incorporate
silica from the soil into the epidermal walls and physically damage the
mandibles of the stem borers.
7. Surface waxes. The cuticle of most vascular plants is covered by
a
thin
layer
of hydrophobic
waxy
material
and
may
prevent
Brassica oleracea
attack. For example, normal waxy leaves of
insect
are more
resistant to attack by cabbage flea beetle than non-waxy strains.
······
[ II] Chemical defense
All plants require inorganic ions and must produce enzymes, hormones,
carbohydrates,
metabolic
lipids,
products)
compounds
(primary
for their proper growth and reproduction.
proteins,
and
phosphorus
However,
the plants contain thousands of other chemicals which are by-products in
the synthesis of primary metabolic products. Plants instead of excreting
these secondary metabolites store them in any convenient place.
These
secondary metabolites are used by plants as chemical defense against the
phytophages,
however,
several insects
not only
overcome
such
a plant
defense but also use these chemicals as cues for host plant selection and
acceptance. For example, the plants of Brassicaceae family contain mustard
oil which causes serious damage to animal tissue and prevents attack by
most
of the
insects
cabbage aphid,
but
the
mustard
Lipaphis erysimi
aphid,
Brevicoryne brassicae are
and
the
well adapted ag�inst this and also
use it for host selection and acceptance.
The
secondary
plant metabolites used
by
the
plants
against
insect
attack are commonly known as allomones which may act as repellents,
feeding deterrents,
In
the
tritrophic
phytophages,
These
chemicals
compounds,
toxins,
interactions,
growth regulators,
thus
defending
the
are
grouped
into
terpenoids,
and may impair digestion.
the allomones · attract
phenolics,
plants
five
the
from
major
proteinase
natural
the
enemies
attack
categories :
inhibitors,
of
of latter.
nitrogen
and
growth
regulators related to insect hormones.
1. Nitrogen compounds (primary alkaloids).
Certain
nonprotein
amino acids act as antimetabolites and prevent insect feeding. Alkaloids
are complex nitrogenous bases of diverse molecular structure occurring
in many plants and are best known toxins serving as defenses against
insects. For example, nicotine, which has a long history of use as an
254 J
Insect-Plant Interaction
insecticide. The family Solanaceae is well known for producing the
alkaloids, e.g., green parts of the potato, Solanum tuberosum, contain
solanine; tobacco, Nicotiana spp., contains nicotine; and the' deadly
nightshade, Atropa belladonna, produces atropine. Only a few insects are
able to overcome these chemicals and attack the members of this family,
e.g., Colorado potato beetle, Leptinotarsa decemlineata and certain flea
beetles, and tobacco and tomato homworms, Manduca spp. However,
the major alkaloid in tomato, the tomatine, discourage Leptinotarsa
decemlineata for feeding, but if feeding ensues, beetle mortality may
result.
The
members
of
two
closely
related
plant
families,
Asclepiadaceae and Apocynaceae, have a milky sap and cardiac
glycosides and only few species have evolved adaptation to feed them.
2. Terpenoids. Terpenoids are biologically most important class of
natural plant products, acting as attractants for pollinators but as
feeding deterrents and as toxins for others. For example, pyrethroids are
toxic monoterpenes from Chrysanthemum . The dried flowers and various
extracts of Chrysanthemum have been used for centuries as insecticide.
Synthetic formulations are widely used in agriculture against insects.
Similarly, sesquiterpenoid gossypol makes the plant resistant against
several insects like cotton bollworm. For this, cotton cultivars having
high gossypol are recommended for growers. Cucurbitacins which are
triterpenoids and found in the family Cucurbitaceae impart a bitter taste
to plant materials and are potent feeding deterrents for a wide variety
of phytophages, however, they serve as attractants for red pumpkin
beetle.
Azadirachtin, a triterpenoid isolated from the neem tree (Azadirachta
indica) is one of the most promising natural feeding deterrents to insects.
Azadiractin is very effective against a wide variety of insect species and, at
present, is one of the most saleable plant based insecticides.
3. Phenolics. Phenolics are nonnitrogenous compounds that contain
one or more hydroxyl groups attached to benzene rings. Flavonoids are
the most important phenolic compound. Rotenone, a isoflavonoid, having
a bitter taste is used commercially as an insecticide. Tannins are
polymeric phenolic compounds with strong protein adsorbing properties
and reduce the fecundity of insects.
Proanthocyanidins or condensed
tannins not only discourage feeding but also reduce protein digestion.
However, anthocyanin (flower colouring agent) attract pollinators.
4. Proteinase inhibitors. In plants, proteinase inhibitors which are
proteins or polypeptides, are found in large amounts in seeds, tubers
and foliage. They inhibit the activity of digestive proteinases and thus
reduce protein digestion and thus provide protection from insects. The
level of such chemicals increases in plants when they are attacked by
insects.
Insect-Plant Interaction
{ 255
5. Insect growth regulators. There are so many plants that produce
chemicals having insect hormone act1v1ty. This situation indicates
convergence of defense strategies in the coevolutionary warfare. The
hormone mimics may be of the juvenile hormone type (juvabione) that
maintain the immature condition, or of the moulting hormone type
(phytoecdysone) that synchronise moulting activity, both vital processes
in insect development. When European bug, Pyrrhocoris apterus was
reared in contact with the papers manufactured by pulp tree, balsam fir
(Abies balsamea), did not mature but developed into giant nymphs as
the fir contains juvabione. Similarly, Bracken fem yields high
concentration
of
phytoecdysone
and
detrimentally
hamper
the
metamorphosis. Some are certainly insecticidal when applied topically
and others do inhibit normal development.
Response of Insects to Chemical Defense
Many phytophagous insects have developed several ways to withstand the
chemical defenses of plants either by detoxifying the defense chemicals or
avoiding feeding.
[ I] Detoxification of defense chemicals
The caterpillars synthesise enzymes in midgut that detoxify the plant
defense chemicals and such enzymes are instrumental in the development
of insecticide resistance. For example, adaptive biochemical traits are
acquired by the bruchid beetle, Caryedes brasiliensis, which feed on the
large seeds of neotropical legume, Dioclea megacarpa that contains the
nonprotein amino acid canavanine in very high concentrations. Canavanine
is highly toxic to the vast majority of organisms because, as an analogue of
arginine, it becomes incorporated into polypeptide chains and adversely
affects the normal function of all proteins formed of canavanine containing
polypeptide chains. The chemical is therefore a very effective defense
against all phytophages because the nitrogen store in seeds is locked into
a toxic chemical. However, Caryedes brasiliensis breaks through this
chemical barrier and is the only species known to feed on the seeds of
Dioclea megacarpa. The t-RNAs in the beetle are able to discriminate
between arginine and canavanine, so that even in the presence of abundant
canavanine, it is not incorporated into polypeptide chains. Also, in the
beetle the enzyme arginase converts canavanine to canaline and urea, and
urease converts the urea to carbon dioxide and ammonia, thus making
canavanine a rich nitrogen source for the synthesis of protein amino acids.
Canaline on the other hand, is also highly toxic to most insects, but the
beetle breaks down the canaline to homoserine and ammonia providing
more nitrogen for protein biosynthesis. Unlike most terrestrial insects,
Caryedes brasiliensis excrete nitrogenous wastes mainly as ammonia and
(Z-57)
256 J
Insect-Plant Interaction
urea necessitating further adaptations for dealing with the highly toxic
ammonia.
[ II] Avoidance of feeding toxic chemicals
Avoidance of toxins is also possible for some insects. Numerous insects
such as some aphids feed on dying foliage and so avoid toxic compounds
or other defensive strategies on the part of the plant. The very fine
piercing mouthparts of the true bugs enable them to feed between pockets
or ducts of toxin in the host plant and thus they achieve a spatial
avoidance.
Once the insect species has evolved a mechanism for tolerating or
detoxifying the plant' s chemical defense, the insects utilise those
chemicals as cues to identify the plant for feeding and breeding
purposes. By evolving such a physiological adaptation, the advantages for
the insects are at least fourfold
(i) Minimisation of the food
competition, the insect gains a source of food that cannot be usually
utilised by other phytophages; (ii) E asy recognition - this food is very
easily recognised by its secondary metabolites; (iii) Reduction of
predation/parasitism pressure, feeding such food may also impart a toxic
or
unpalatable characteristic
to
the phytophagous so that the
predator/paras1toids
may
not
prefer
them;
(iv) Protection from
pathogens, the antibiotic properties of many toxic chemicals may protect
the phytophage insects against pathogens.
[ III] Regurgitation of defense chemicals
Some insects, which feed plant defense chemicals, store and regurgitate in
defense against predators. These regurgitated chemcials are not secretory
but are sequestered substances. For example, a diprionid sawfly that feeds
pine needles having high resin content, stores it in the diverticula of the
gut, and regurgitates it as a sticky blob. This is not only distasteful to
predators but also gums up their mouthparts.
Tritrophic Interactions
An agro-ecosystem consists of at least three trophic levels: plants,
phytophagous species and the enemies of the phytophages. The
characteristic feature in this trophic system is that members of alternate
trophic levels usually acl in mutualistic manner. Natural enemies of
phytophages benefit the plants by reducing the abundance of phytophages.
Plants may benefit the enemies of the phytophages by making them more
accessible to the trophic level above. Therefore, plants defend themselves
either by producing chemicals, such as toxin, or digestibility reducers, or
through physical defense by trichomes or toughness, or by a combination
of the two, as with glandular trichomes or resins (intrinsic defense of the
(Z-57)
Insect-Plant Interaction
[ 257
plants) as mentioned earlier and by benefiting natural enemies of the
phytophages (extrinsic defense of the plants). It is now recognised that
almost every mechanism of the intrinsic defense of a plant has an effect up
the trophic system and that intrinsic defense may impact positively or
negatively upon the third trophic level as well as on those factors involved
with extrinsic defense. The intrinsic and extrinsic defenses of plants reduce
the colonisation rate of the phytophages. The conflict between intrinsic and
extrinsic defenses affected the evolution of plant allelochemistry. The
plants have three options either (i) they become highly attractive to
beneficial insects, thus reducing the phytophage population or (ii) they
become poisonous to phytophages, the second option may harm third
trophic level (extrinsic defense), or (iii) they achieve some compromise
which exploits both protective mechanisms. The toxic substances of plant
tissues which repel, retard growth, reduce vigour, or kill susceptible
phytophages may poison bioagents or cause physiological/metabolic
changes in phytophages which reduce its value as a food source for the
entomophages. The study of intrinsic defense of plants by manipulating the
plant characteristics is the subject of plant breeders while the study of
extrinsic defense of plants is the subject of biological control workers.
Combination of these two approaches has been suggested because
the understanding of the multitrophic interactions is essential in
evaluating the roles of natural enemies in population dynamics of
phytophages. In past, the synergism has been shown between intrinsic
defenses and extrinsic defenses of plant. For example, resistant varieties
of sorghum and oats enhance the efficiency of the greenbug parasitoid
Lysiphlebus testaceipes in reducing the population of cereal aphid
Schizaphis graminum. In recent years, the compatibility of host plant
resistance and biological control in integrated pest management has
been argued.
As stated earlier, several attributes of the plants defend them from
insect' attacks or in other words, make them resistant, such as its
community, its phenological characteristics, its physiological state and its
physical and chemical properties. All these properties of the plants are
known to have influence upon the phytophagous insects and host
seeking ability of their natural enemies. There are three kinds of
interactions among three trophic levels : semiochemically, chemically
and physically mediated interactions.
[ I] Semiochemically mediated interactions
All the organisms have body odours which may be used by their enemies
to aid in their detection. These chemicals may be synthesised by the
organism themselves, derived from food unchanged or precursors only
slightly modified. In the world of insect phytophages and their enemies,
(Z-57)
258 J
Insect-Plant Interaction
chemical signals are exceedingly important in influencing behaviour. The
chemical message provided by plants influences the third trophic level in
at least four general ways : (i) plants provide chemical cues for searching
hosts by their natural enemies, (ii) plant chemicals become kairomones in
the phytophage-enemy interaction, (iii) associated plants provide chemical
cues for searching enemies, and (iv) associated plants produce chemicals
that mask attractants to enemies.
Plant chemicals are used as cues for phytophagous insects as well as
for their natural enemies (parasitoids and predators). The cabbage
aphid, Brevicoryne brassicae uses chemical sinigrin (present in brassica
plants) as a cue to find host plants while its parasitoid Diaeretiella rapae
uses a related compound allyl-isothiocyanate (present in mustard oil) to
find the plant and then the aphid. Similarly, tricosane, the kairomone
for Trichogramma evanescens, first isolated from the corn earwonn,
Heliothis zea was isolated from its food plant, Zea mays. The tricosane
synthesised by the food plant becomes incorporated into the eggs of
Heliothis zea and is used as a searching cue by Tricogramma evanescens.
Fewer Myzus persicae are parasitised by Diaeretiella rapae in
weedless than in weedy plants, especially if the weeds in adjacent plants
were kept trimmed. In contrast, parasitism by Aphidius species was
higher in plot containing weeds that served as alternate hosts. The
population dynamics of Aphidius ervi, which is an important pea aphid
parasitoid in Europe, was influenced by the availability of legumes
serving as overwintering host plants of the pea aphid and of reservoir
such as Microlophium camosum on stinging nettle. Therefore, the
intrinsic defense of the plant has direct and indirect effects on natural
enemies that may be important in biological control and extrinsic plant
defense.
[ II] Chemically mediated interactions
The direct and indirect effects of plants on phytophages and their natural
enemies at the chemical level have received considerable attention in the
literature. The parasitoids feed on pollen and nectar in nature. The
honeydew excreted by the aphids also serve as food for the parasitoids,
thus plants provide food to parasitoids by sustaining the phytophages that
produce honeydew. Honeydew also contains kairomones and increase
chances of host findings by their parasitoids. Colour of the host aphids is
known to vary with food plants which in turn influence their acceptability
by their parasitoids.
The products in foliage may affect the success of parasitoids. The
reduced percentage of parasitisation of Mandua on tobacco plants by a
braconid wasp Apanteles resulted by the ingestion of nicotine by the
host and the subsequent toxification of the parasitoid. It has been
(Z-57)
Insect-Plant Interaction
[ 259
observed that parasitoid Binodoxys indicus parasitises Aphis gossypii
more easily on Lagenaria vulgaris and Luffa cylindrica than Cucurbita
maxima that have different chemical composition.
Thus, chemical effects of plants at the third trophic level may be
direct or indirect. They cause strong linkage and interaction between the
intrinsic defensive system of the plants, or resistance to phytophages and
extrinsic defense caused by natural enemies of the phytophages or
agents used in biological control.
[ III] Physically mediated interactions
The physical effects on the third trophic level interactions are as important
and diverse as chemical effects, however, they have received much less
attention. Sometimes, differences between physical and chemical influences
are not clear, e.g., a plant resin may be used by phytophage in defense
against its enemies both in terms of its physical stickness and chemical
deterrence. For example, Lysiphlebus testaceipes usually entrapped in the
glandular hairs of petunia. Similalry, a decrease in adult survival of
Aphidius matricariae with increasing glandular trichome densities was
observed. Plants sometime provide the phytophages physical protection
from their natural enemies.
The physical nature of plants such as leaf toughness, trichomes and
cuticle thickness has direct effects on the efficiency of parasitoids. For
example, trichomes on leaves may reduce the searching rate of
predators and parasitoids to the point where enemies become
ineffective. Galls that grow relatively large provide more protection
against the herbivores' enemies than smaller galls. The galls with
extrafloral nectaries attract foraging ants which interfere with attack by
parasitic wasps. The aphid parasitoid, Aphidius nigripes has the ability to
modify host behaviour in order to select a suitable micro-habitat for
population with in host remains. The plant architecture also influences
interactions
over
several
trophic
levels.
Spatial
distribution
of
phytophages on the plants is dependent on the architect of the plant
which indirectly influences the searching efficiency of the natural
enemies. Even plant dispersion also influences the effectiveness of the
natural enemies. Not only this, the density of planting in agricultural
crop also affects micro-climate of the phytophage density and thus the
abundance of the natural enemies.
Phytophagous Insects Beneficial to Plants
All insects that feed on phytophagous insects provide extrinsic defense to
the plants. However, a number of insects species which are phytophagous
are beneficial to the plants, e.g., by conserving their nutrition, pollinating
their flowers etc.
260 J
Insect-Plant Interaction
[ I] Conservation of nutrients
Much less attention has been paid to phytophagous insects that benefit
plants than to the antagonistic relationship. The activity of phytophagous
insects is known to increase conservation of nutrients by c;ausing leaf fall
over a prolonged season. The honeydew producers may increase nitrogen
fixation beneath the plant. Also, the phytophagous insects contain plant
growth regulators in relatively high concentrations, however, the growth
regulation is variable depending on the plant and phytophage involved.
[ II) Insect pollinators
A number of insects have developed total dependence on floral products
for food. Therefore, the early insect pollination was surely be accidental,
with plant-feeding insects, becoming contaminated with pollen, and
transporting a few grains to the next plant visited. For this, the plants
develop sticky pollen grains to facilitate adhering onto the insect bodies.
Later on they began to rewarding the insects by secreting small amount of
sweet fluid (nectar, floral as well as extra-floral) followed by secretion of
attractive odours that increased the frequency of insect visits. Nectar
contains various sugars, proteins, amino acids, salts etc. With time the
flowers also acquire colours other than green and allowed them to be more
easily seen by the insects. Along with this, insects develop sensory
mechanisms especially the perception of colours and odours and the ability
to link these characteristics with food. Most of the modem pollinator
insects are able to see in ultraviolet light. Ultraviolet colour is one of the
important characteristics of insect-pollinated plants as they attract the
insects no need to reset the sentences; The insects in locating nectar
source use the specialised floral patterns (nectar guides) that appear in
ultraviolet light.
There exists distinct correlation between the anatomical and
physiological characteristics of the flowers of a given species and the
anatomy, physiology, and behaviour of their insect pollinators. Among
the characteristics of flowers that attract insects are: (i) the production
of <;pecific odour, (ii) colour, size and shape of the sepals and petals,
(iii) patterns of stripes or spots on petals, (iv) separation or
nonseparation of petals, and (v) shape of the flower. In pollination, both
plants as well as insects are mutually benefited. The plant is propagated,
and the pollinator gains a caloric reward (as food). Bees, butterflies,
moths, and thrips are the major pollinator insects.
1. Bees as pollinators and flower constancy. An estimated 80% of
the insect pollination of our commerical crops is performed by honey
bees, therefore, the economic importance of honey bees is 1 0-20 times
more as pollinator than its by-products. Honey bees and bumble bees
Insect-Plant Interaction
[ 261
are attracted to blue, purple and yellow flowers, but cannot see red
flowers. Also the bee-flower plants evolve flower constancy to restrict
the foraging bees to one plant species during single trips or for longer
periods of time. It increase the probability of fertilisation, in that pollen
deposited in a flower by a visiting bee is likely to be from the same
species rather than from an alien species of plants. Not only do
individual bees visit the same flower species for pollen on separate trips,
but entire colony may use one flower species for 10 to 1 1 days or until
the source is depleted.
Flower constancy is beneficial for both plants and the insects, and
there is likely to be a coevolutionary trend in that direction. The
general pattern seems to have been the evolution of floral structures
that favour certain types of efficient pollinators and discourage less
efficient ones. For example, bees provided the flower constancy, easily
learn to go to the food source. It reduces the time of search of flowers
and increase the frequency of flower visit per unit of time.
2. Fig insects and caprification. A very unique obligate symbiotic
relationship between plant and insect has been observed in case of
Ficus (Fig. trees) and their exclusive pollinator wasp , Blastophaga
(Agaonidae : H yrnenoptera). The fig flower consists of several tiny
imperfect flowers arranged inside a pear-shaped receptacle and the fruit
is the result of further growth of the receptacle. Many commercial fig
varieties produce no fertile pollen, and as fruit development cannot
proceed without fertilisation, pollen of the wild fig (caprifig) is used for
these varieties. To ensure pollination in fig orchards, flower branches of
the wild fig are suspended in the vicinity of cultivated fig trees, a
process known as caprification. The pollination is done entirely by fig
insects. The fig insects develop in a gall at the base of the caprifig
flowers; the female, in emerging from the gall, becomes covered with
caprifig pollen. The female visits a number of flowers, including
commercial fig where they pollinate the flowers, but lays eggs only in
caprifig.
3. Moths and butterflies as specialised pollinator. Most of the
moths and butterflies feed on nectar. The mouthparts (siphoning type)
formed from galeae, are well adapted for extracting nectar from plants.
The proboscis is carried coiled beneath the head at rest and is readily
uncoiled during sucking or siphoning food. Moths visit evening- or
night-blooming flowers whereas the butterflies visit to day-blooming
flowers. The flowers pollinated by butterflies are usually bright red or
orange and often have long narrow corolla with nectar at the bottom
which is accessible only to their specialised mouthparts. During nectar
collection they pollinate the flowers.
262 1
Insect-Plant Interaction
4. Pseudocopulation by bees/wasps with flowers. The
Ophrys
between the orchid,
relationship
with certain species of bees/wasps is a very
intimate floral-insect · association. The labellum or lip of the flowers of
Ophrys
resembles
mimics
the
species.
The
attempt
to
female
smell
of
male
copulate
bees/wasps
sex
5. Thrips
as
bees/wasps
are
inadvertently
them,
emergence
stage,
so
that
production
like
of the
as
get
form
that
attracted
to
and
colour,
particular
these
pollinating
even
bees/wasps
impostors
the
and
flowers
at
known as peudocopulation.
pollinators. The flowers of distant plant families are
by thrips,
example, the oviposition of
the
both
of
with
the same time. This phenomenon is
p ollinated
in
pheromones
ray
the
Asteraceae, Fabaceae
and
Solanaceae .
For
Microcephalothrips abdominalis coincides
petals of Wedelia chinensis in its late
larvae
synchronised.
emerge
with
bud
from
eggs, anthesis and nectar
;
Synchronisation in terms of flowering
periodicity and phenology of the pollinating thrips_ appears controlled by
the population-build of the thrips.
6.
Other insect pollinators. As
butterflies,
fig
insects,
and thrips are
mentioned
most
earlier,
valuable
bees,
insect
moth,
pollinators.
However, following insect groups are also valuable plant pollinators :
The
(a) Diptera.
are
hover
flies
(Calliphoridae),
most valuable
(Syrphidae),
Tachinidae,
pollinators
bee
flies
Chironomidae
belonging
(Bombyliidae),
and
Tabanidae.
to
this
order
blow
flies
Hover
flies
are common visitors to flowers and are most important pollinators even
in the poor environmental conditions when most of the bees become
inactive. Similarly, bee flies resemble bees as the body is covered with
hairs with a long slender proboscis.
Beetles
(b) Coleoptera.
are not as good
pollinators as bees,
moths
and butterflies but still occasionally visit flowers. Few plant species are
strictly
pollinated
by
beetles,
families Cantharidae, Meloidae,
e.g.,
mangolia
flowers.
Members
of
the
Buprestidae and Cerambycidae are good
pollinators. Beetle flowers attract the insects by odour and not by sight.
Ants and Plants
At least
10% of the plant species of the genus Acacia protect themselves
by harbouring certain species of ants of the genus
Pseudomyrmex .
The ants
gain protection from the plant by living in the swollen stipular thorns and
food
is
provided
by
the
sugary
secretion
of
petiolar
nectaries.
The
aggressive ants protect the plants by warding off herbivores. The ants also
eat
the
growing
tips
of
the
plants
grown
around
the
Acacia
thus
suppressing their population and make the plant less vulnerable to fire that
sweeps through this dry-tropics vegetation.
Insect-Plant Interaction
[ 263
Important Questions
l.
2.
3.
4.
5.
6
Describe the mechanisms by which plants protect themselves from insect attacks.
Give an account on the ways by which insects protect themselves from the plant
defenses.
Write an essay on 'tritrophic interaction' involving first three trophic levels.
Enumerate the physical defense mechanism of plants applied against phytophagous
insects.
Describe the host selection behaviour of phytophagous insects.
Write short notes on : (i) Mutualistic phytophagous insects (ii) Ir.sect pollinators
(iii) Mutualism between ahts and plants (iv) Mutualism between fig and fig insects
( v) Semiochemicals
19
Locusts and Termites
Locusts and termites both are very injurious to man as they destroy several
human commodities. Locusts are phytophagous and feed several plants
cultivated by man. Most of the locusts are solitary but few species are
gregarious and reproduce very fast. On the other hand, the termites are
wood feeder (xylophagous) or cellulose feeder (cellulophagous) and can
destroy any wood material like furniture, doors, standing crops etc. They
are social and live in highly sophisticated colony of thousands of
individuals. In forest ecology, however, they decompose the fallen leaves,
twigs and even trees and thus help in the nutrient cycling.
I
LOCUSTS
I
Locusts are insects belonging to the order Orthoptera. They are identical
in appearance to grasshoppers with which they share the family Acrididae.
The only difference between the two types of insects is that locusts can
exist in two different behavioural states (solitary and gregarious) whereas
grasshoppers do not. When the population density is low locusts behave as
individuals, much like grasshoppers. However, when the population density
is high locusts form highly mobile gregariously behaving bands of nymphs
or swarms of adults. It is this change from one behavioural state to
another, known as phase change, that makes locusts such devastating pests.
Phase change may be accompanied by changes in body shape and colour,
and in fertility, survival and migratory behaviour. These changes are so
dramatic in many species that the swarming and non-swarming forms were
Locusts and Termites
[ 265
once considered to be different species. The most important _locust species
is the desert locust Schistocerca gregaria (Fig. IA) which is supposed to be
international pest, influence various parts of Asia, Africa and Europe.
Other locust species highly injurious to mankind are the Bombay locust
(Patanga succincta- serious pest of crops in western and central parts of
the Indian Peninsula in past) and Indian migratory locust (Locusta
migratoria found all over India in its solitary phase). Both S. gregaria and
L. migratoria caused crop damage, often on devastating scale. Migration of
locust swarms can be added by storms and wind patterns so that, under
certain conditions, these insects may occasionally appear in large numbers
in Africa, Australia, Europe and Asia.
Locusts cause immense destruction to every kind of vegetation. It is
well known that in the past locust devastations had resulted in the
development of famines in several parts of the world due to shortage of
human food and fodder for cattle.
-
Bionomics of Locusts
The desert of Rajasthan is the main breeding place of the locusts
particularly desert locust in India. The monsoon months in this area are
July and August, a suitable condition for locust multiplication. In
September, desert become dry and temperature rises, a situation not
preferred by the locusts so that the newly bred swarms begin to escape the
area. They are carried partly eastward into the Punjab, U .P ., Bihar and
West Bengal, partly westward into the Pakistan, Iran, Baluchistan and
eastern Arabian countries, and sometimes southwards into the peninsular
areas. Such a migration continued until December. and January but most of
the locusts migrated eastwardly and southwardly are perished without
breeding. Those which survive breed in spring in Punjab and western U.P.
However, the swarms that are carried westward are able to breed during
January to March in the areas of winter-rainfall. The new generation
produced in these areas migrates eastwards into the Indian desert areas in
May and June. With the commence of monsoon rains in the desert summer
their breeding occurs.
[ I] Life-history
The breeding period of locust begins with the monsoon. The female lays
eggs in eggpods containing 60- 120 eggs, at a depth of 7- 1 5 cm in moist
sandy or loamy soil (Fig. l B ). A female may oviposit thrice at weekly
interval and lay about 500 eggs in her life-time. The eggs hatch into small
nymphs after 2 (in summers) to 4 (in autumn) weeks of incubation period.
The nymphs (also known as hoppers) have a tendency to congregate in
groups and begin to march from place to place eating up all vegetation
(polyphagous feeding habit) along their way. There are five nymphal instars
266 J
Locusts and Termites
A
c
Fig. I. Sch1stocerca m1graton,
(C) Eggs in eggpod
(A) Wings spread, ( B ) Locusts in the act of 0V1posit1on,
in the gregaria phase and the nymphs become adult in 4-6 weeks.
Metamorphosis is gradual (paurometabolous).
[ II] Phases in the life-history
There are two main phases in locusts that differ both structurally and
biologically. These are the gregarious phase and the solitary phase. The
solitary phase is characterised in its nymphal instars, by being variable in
colour. They are green, grey or brown with longer and crested pronotum
and longer hind femur. In gregaria forms, the nymphs are mainly black and
yellow or orange or pink markings. The adult has shorter and
saddle-shaped pronotum
and
shorter
hindfemur.
Biologically,
the
gregarious forms are highly active and have tendencies of congregation. In
adults, the gregaria forms occur in large, more or less dense swarms which
may fly over great distances under the influence of winds until
environmental conditions cause them to settle. The density of a locust
swarm can be at least 26 lac insects/km2 and sometimes over 50 Jae
Locusts and Termites
[ 267
2
1000 km, a locust swarm may
insects/km . Since swarms often cover over
easily number
5000 crore individuals !
Swarms originating in the
when
they
breed
there,
outbreak areas
may
give
rise
to
invade large regions
solitaria
or
gregaria
and,
forms
according to the local conditions. After a few years, however, the area
affected by swarms becomes smaller and the locust plague ends.
Control Measures
The locusts
may be controlled at all its stages of life cycle, viz., egg,
nymphs and adult. The eggs can be destroyed by digging them up either
by ploughing or harrowing the fields.
The hoppers and adults may
be
controlled by adopting following methods :
1. Trenching. Trenches of about 45 cm deep and 30 cm wide may
be
dug
at
some
distance
in
front
hoppers are driven to the trenches
of
marching
hopper
bands.
The
wherein they are buried alive.
2. Burning. Adults and hoppers gathered on bushes or trees are
killed
using
flamethrowers,
if
available,
otherwise
with
kerosene-oil
torches.
3. Poison baiting. Poison baits consisting of wheat or rice bran, an
insecticide and an attractant like molasses is spread around the bushes,
where hoppers rest
at
night or is
spread
in
the
infested
field in
the
day-time at the rate of about
20-30 kg/ha. The hoppers feed it and die.
4. Dusting or spraying of insecticides. Although in past BHC was
dusted and aldrin emulsion was sprayed over the hoppers to kill
but nowadays
India.
control
both
However,
in
the
the
the
insecticides
use
desert
of
areas.
only
are
dieldrin
Other
parathion and diazinon
are also useful.
air crafts
against
the
locusts.
The
flying
is
by
the
like
them
Government of
recommended
insecticides
of the insecticides
by
banned
for
malathion,
locust
methyl
Aerial spraying of the emulsion
on hopper infested areas pi:ove useful
swarms
application of diazinon. In Australia,
can
be
controlled
by
air-to-air
fipronil (Trade name : Adonis
8.5
ULV formulation, a phenyl pyrazole insecticide) and
fenitrothion at 2.5 g and 1 . 25 ai/ha, respectively are used for effective
UL- 8 .5
control
g/L
against the
locusts
infesting
a range
of habitats and vegetation
types.
An
effective
collaboration
control
among
the
of locusts
affected
is
possible
countries.
The
only
if there
Government
is
close
of India,
Pakistan and Iran regularly exchange information about the breeding and
swarm situation of the locusts . The Government of India have a permanent
Locust Warning Organisation established in
1 939 as a part of the Central
Directorate of Plant Protection, Quarantine and Storage. The staff of the
organisation constantly patrol and watch the sign of change in phase at the
early , stage with the o bject of their destruction.
268 J
Locusts and Termites
5. Biological control of locusts. In Australia, certain bioagents like the
naturally occurring fungus Metarhizium anisopliae is used against the locust.
Metarhizium is applied as dried spores suspended in oil and spores coming
in contact with the insect grow through the cuticle and into the insects body.
The fungal hyphae grow within the body and depending on temperature,
kill the insect within 1 -2 weeks. At later stages of infection the insects often
turn a characteristic pink colour and then green as the fungus sporulates.
TERMITES
The termites belong to the order Isoptera as fore- and hindwings of the
most of the species are similar in shape and size. They are small to
medium sized, soft bodied, social and polymorphic insects that live in
social groups or communities having a highly developed caste system. The
colony is composed of reproductive forms together with numerous
apterous, sterile soldiers and workers. Termites are mostly tropical or
subtropical, live in large communities in underground nests or termitaria or
in dry wood (used in the construction of building and furniture). The
subterranean termites are ground-inhabiting and a colony or nest may be
up to 1 8-20 feet below the soil surface to protect it from extreme weather
conditions. These termites travel through mud tubes to reach food sources
above the soil surface. These forms probably play an ecological role similar
to earthworms in that they aerate and add nutriment to the soil. In
addition to wood, termites feed on a variety of cellulose-containing
materials, fungi and dried animal remains. The digestion of cellulose is
carried out by flagellate protozoans or bacteria, which are mutualistic
inhabitants of the gut.
Bionomics of Termites
Termites are very significant pests, damaging wooden structures
(e.g., furniture, building materials, and wooden floors, railway sleepers,
wooden bridges, boats, books, large orchard trees like mango, apple,
coconut, guava etc.). There are two distinct categories of termites : those
forming mounds above the ground and those living underground. Ground
termites randomly and constantly forage for new food sources; and may
travel up to I 00 metres from their primary nest. A very common mould
forming species are Odontotermes spp. Colonies of Kalotermes (dry wood
termites) and 7.ootermopsis (damp-wood termites) exist entirely on wood.
Several species destroy standing crops like sugarcane, maize, sorghum,
ground nut, tea, cotton, potato etc. Reticulitermes construct earthen tubes
on concrete and are thus able to invade a structure even though it is not
Locusts and Termites
[ 269
in direct contact with the soil. The presence of earthen tubes is one of the
characteristics used to diagnose a termite infestation.
Ecologically, termites are good decomposer of dead wood and
vegetable products, they aid in agriculture by enriching the soil with
their faecal matter and by making the soil permeable to air and
moisture like earthworms. In addition, they constitute the food to several
animals like birds, reptiles, rodents etc. Natives of South East Africa
consume queens as a delicious dish.
[ I] Diversity of termites
Termites are divided into six families, however, about 75% of the recent
termites belong to one family Termitidae. Termitidae is thus largest family
with a wide range of food habit and colony structure. The worker caste is
well developed, in Microtermes beesoni, M. anandi, Macrotermes serrulatus,
Odontoterms banglorensis (sugarcane pest), Termes, Trinervitermes heimi
(sugarcane pest). Another family Kalotermitidae lives in dry wood and
have no worker caste, e.g., Neotermes, Kalotermes indicus, Cryptotermes
domesticus .
[ II] Colony structure and polymorphism
The termites are commonly confused with ants. This is probably the reason
that termites are sometimes referred to as white ants but they differ from
ants in several ways. Termites are soft-bodied and usually light coloured,
while ants are hard-bodied and usually dark coloured; the fore- and
hindwings of termites are similar in size and venation and are held flat over
the abdomen at rest, but in ants the hindwings are smaller than the
forewings and have fewer veins, and at rest are usually held above the
body. The antennae of a termite are miniliform while those of ants are
elbowed. There is no constriction between the thorax and abdomen in
termites like ants.
All termites are social insects and live in colonies ranging in size
from a few hundreds to as many as 70 lacs individuals. A colony' s
population is initiated and maintained by a queen that may .live for as
many as 50 years in some species. The colony reaches its maximum size
in approximately 4 to 5 years. Two or more castes may be present,
depending on the species, and all castes are composed of both males
and females. Conveniently, castes can be divided into reproductive and
sterile forms. There may be two kinds of reproductive forms in a colony
of a given species. The sterile castes live for 2-4 years.
1. Primary reproductive forms. These are macropterous forms, e.g.,
queen and king which are thought to comprise the original caste in
termite phylogeny; having dark, sclerotised bodies with completely
developed wings and compound eyes (Fig. 2A). The queen, usually a
270 J
Locusts and Termites
A
B
c
Ftg 2. Castes of termites (A) Sexual wmged adult. (B) Worker, (C) Soldier, (0) Section
of a royal cell with apterous queen, on the right chambers with fungus garden
pair in a colony, is monogamous. It becomes enlarged by largely growth
of its abdomen. It losts her wings after founding the colony.
2. Secondary reproductive forms.
These are brachypterous or
apterous forms, e.g., ones with shorter wings, less pigmentation, and
smaller compound eyes than the primaries. They substitute the primaries
when they die (substitute or complemental queen and king). They are
polygamous.
In all reproductive castes there is remarkable post-embryonic
growth, , esp�cially in the female. The inseminated female develops into a
queen which is 5-7 .5 cm long and 1 .0 - 1 .5 cm wide (Fig. 2D). The
increase in size is due to the enlargement of the abdomen only, the
head and thorax remain normal. The terga and sterna of the abdomen
do not grow, but the pleural membrane expand tremendously due to an
increase in the number and size of the ovaries and fat body. Because of
this the queen becomes large, inactive, egg-laying individual. The queen
formed from the macropterous female is largest. The queen normally
lives for 6- 1 5 years and lays a million eggs in its life. It was once
believed that the destruction of the queen would ultimately kill out the
Locusts and Termites
{ 271
community but this is not so because brachypterous or apterous queens
will form and continue the community.
3. Sterile castes. The sterile castes include workers and soldiers and
both castes are composed of male and female individuals.
(a) Workers. The colony may include 60,000 to 2,00,000 workers
which are soft-bodied, wingless, blind and creamy white (Fig. 2B). In
early stages, they are fed predigested food by the king and queen. Once
workers are able to digest wood, they provide food for the entire
colony. The workers perform all the labour in the colony such as
obtaining food, feeding other caste members and immatures, excavating
wood, and constructing tunnels. Workers mature within a year and live
from 3 to 5 years.
Workers may be dimorphic, in one the head and mandibles are
larger than in the other form as observed in Odontotermes . In case of
Termes, the workers are trimorphic with small intermediate and larger
sizes. Because of their gnawing habit the workers destroy crops, wood
and human belongings and cause tremendous damage to man.
(b) Soldiers or nasuti. Like workers, the soldiers are creamy white,
soft-bodied, wingless and blind, however, the head is enormously
elongated, brownish, hard and equipped with two jaws (Fig. 2C).
Soldiers must be fed by workers because they cannot feed themselves.
They are less numerous in the colony than workers and their only
function is to defend the colony against invaders. Soldiers mature within
a year and live up to 5 years.
The soldiers are of two types : (i) mandibulate soldiers having large
powerful mandibles but no frontal rostrum, and ( ii) nasute soldiers
( in Nasutetennes) having small mandibles and a median frontal rostrum
on the head. Soldiers function as defenders of the colony, mandibulate
soldiers with their mandibles and nasute soldiers by exuding a viscid
repellent fluid through the frontal rostrum. At times, soldiers in some
species defend the colony by plugging up holes with their heads. In the
simplest social structures, there are reproductives and soldiers, and the
immatures of both these castes function as workers. Termites have the
ability to change from one caste type to another during their immature
stages. This allows the colony to change the proportion of different
caste members as the need arises.
[ Ill] Life history of termites
The life-history and the origin of caste system in termites is extremely
complicated. New colonies are formed when at certain times of the year,
commonly in spring and fall, especially after a rain, winged primary
reproductives appear in huge number and swarm from the nest. After a
flight, the winged males and females return to the ground and shed their
(Z-57)
2 72 J
Locusts and Termites
�
A
B
c
J
Fig 3 Polymorphism m Ret1cul1termes (A) Eggs, (B) Nymph, (C) Female wmged form,
(D) Male
wmged
form,
(E) Workers, (F) Kmg, (G) Secondary reproductive (female),
(HJ Queen ( abdomen distended with enlarged ovanes), (I) Soldiers. and (J) Earthen tubes
from s01l across surface of concrete foundation to wooden structure.
wings. The wingless males and females pair off and search for sources of
wood and moisture in soil. The royal couple digs a chamber in the soil
near wood, enters the chamber and seals the opening. After mating, the
queen begins to lay eggs. In a mature colony, the queen may lay over
30,000 eggs/day. The eggs are yellowish white (Fig. 3A) and hatch after an
incubation of 50 to 60 days. Another method of nest foundation is
sociotomy. In this process colonies divide by the separation of immatures
and secondary reproductive forms (Fig. 3F, G) from the parent colony or
(Z-57)
Locusts and Termites
[ 2 73
by division of a migratory colony into two daughter colonies. Soon after
insemination, the female modifies as mentioned earlier attaining a size
nearly 20-30,000 times larger than workers (Fig. 3H). About 4,000 eggs are
laid per day. They are oval, elongated, smooth and pale coloured.
Development is gradual with incomplete metamorphosis. The nymphs
(Fig. 3B) undergo several moultings to attain adulthood (Fig. 3 C, D ) . The
mechanism of caste differentiation is not fully understood, however, it is
believed that it is based on complex interaction of hormones, pheromones,
availability of food supply and social behaviour, etc.
[ IV] Termitaria or termite nests
The termite nests vary from simple cav1t1es in soil or wood to vast
subterranean complexes or elaborate structures that project well above the
ground. In the African savannas, high temperature and low rainfall pose a
real threat to termites. To protect the colony Macrotermis bellicosus builds
a towering earth mound up to 7.5 m high, most of the above ground
portion being hollow to allow circulation of air. Similarly, the nest of
Nasutitermes triodae has been reported to reach a height of 6 m and a
diameter of 3.6 m at its base in Australia. The termitaria are provided with
very elaborate ventilation systems, design that provide for maintenance of
constant temperature; canopies that deflect rainwater and t ·lher structural
adaptations. The means by which the behaviours of individual members of
a colony are coordinated to produce such complex structure has_ long been
a source of surprise.
[ V] Communication in the colony
Termites communicate primarily by secreting chemicals called pheromones.
Each colony develops its own characteristic odour. An intruder is instantly
recognised and an alarm pheromone is secreted that triggers the soldiers
to attack. If a worker finds a new source of food, it fays a chemical trail
for others to follow. The proportion of termites in each caste within the
colony is also regulated chemically. Nymphs or immatures can develop into
workers, soldiers or reproductive adults depending on colony needs. Sound
is another means of communication. Soldiers and workers may bang their
heads against the tunnels creating vibrations perceived by others in the
colony and serving to mobilise the colony to defend itself. Mutual exchange
of foods enhances recognition of colony members.
[ VI] Food and feeding habits
The termites are basically xylophagous, i.e., they feed wood. However, they
may feed vegetation, faecal matter of termites, cast off skins and the dead
of the colony. Some termites developed a symbiotic association with fungi
of the genus Termitocytes, which the termites cultivate inside the nest on
(Z-57)
274 l
Locusts and Termites
fungus comb' (Fig. 2D) made of vegetable matter and their own faeces.
On the comb fungal hyphae grow producing white patches. Fungus gardens
are grown in chambers located near the centre of the nest, they
communicate with a royal chamber in which the king and queen is fed by
workers only on saliva and fungal hyphae. The eggs and nymphs develop
in fungal chambers or nurseries. The workers take care of nymphs feeding
them on fungus and vegetable matter which are partly predigested by them
with the help of certain flagellates (e.g., Trichonympha ) and bacteria. In
feeding, the symbiotic flagellates are also transferred to nymphs which
develop either into reproductive forms which can leave the nest and form
new colonies, or into sterile workers or soldiers.
[ VII] Economic importance
Termites often infest buildings and damage lumber, wood panels, flooring,
wallpaper, plastics, paper products and fabric made of plant fibers. The
most serious damage is the loss of structural strength. Other costly losses
include attacks on flooring, carpeting, art-work, books, clothing, furniture
and valuable papers. Subterranean termites do not attack live trees. The
termites also damage the standing crops like sugarcane, wheat, paddy,
groundnut, cotton, com, sorghum, chilli, brinjal, cauliflower, cabbage,
beans, potato etc. The crops are attacked from the time of transplantation
to harvest. Damage caused by termites can be very serious indeed. The
entire village of Sri Hargobindpur in the Punjab was abandoned in the
1950s because of the pervasive damage by the termite Heterotennes
indicola.
However, the termites feeding on wood are important agents in
decomposing branches, logs and tree-stumps and are thus also beneficial
to us, though the role of termites in cycling organic matter and soil
nutrients has not been sufficiently investigated. Termites also serve as
food for certain mammals like aardvark (Orycteropus afer) and aardwolf
(Proteles cristatus) of Africa, pangolins (Manis gigantea) of Africa and
Asia, anteaters (Myrmecophaga tridactyla) of South America. Other
termite feeders are ants, spiders, geckos and other lizards, shrews etc.
[ VIII] Evidence of termite infestations
Wood damaged by termites always has remains of mud tubes (Fig. 31)
attached to wood galleries or tunnels or in fields around the crops in a
irregular pattern. The tunnels may contain broken mud particles with fecal
materials. In the case of an active colony, white termites may be found in
infested wood. The presence of flying winged males, females or their shed
wings inside the building indicates an infestation. The presence of mud or
shelter tubes extending from the ground to woodwork or on foundation
(Z-57)
Locusts and Termites
f 2 75
walls also may indicate infestation. Workers travel periodically via shelter
tubes to their nest to regain moisture and perform feeding duties.
[ IX] Control measures
Termite control needs prophylactic treatment as it is difficult to predict the
intensity of attack at the beginning of the season and once the damaged
noticed there is no scope to check the damage. Therefore, following
measures should be adopted.
1. Prophylaxis. No home, new or old is safe from termites. By
building mini tubes, termites can cross concrete, brick, metal termite
shields, pre-treated wood, or even a professionally applied termite
barrier. Because termites need moisture and have low tolerance to air
and light, they usually live underground, attacking a home from below.
A loose mortar joint, a minute space around a drain pipe, or a
settlement crack in the concrete slab is all they need to gain entry.
Ground termites can create secondary nests above the ground called
"aerial colonies". These independent nests may survive independently
of the ground if a water source is available. Common interior water
sources include roof leaks, plumbing leaks, leaky showers or tubs, toilet
leaks, etc. Aerial infestations must be located for effective control.
Therefore, following preventive measures should be performed :
( 1 ) Avoid having any woodwork of the buildings within 45 cm of contact
with the ground.
(2) Use treated timber during building construction.
(3) Coat any untreated wood or exposed wood end cuts with an
appropriate termiticide like chlorpyriphos.
(4) Eliminate all wood-to-soil contact, remove any wood debris, and
reduce the wood moisture content to below 20%.
(5) Seal all cracks and crevices with cements.
(6) Perform treatment to the soil before construction with an appropriate
termiticide or a basaltic termite barrier.
(7) Termitaria in nearby area should be destroyed mechanically and queen
shouid be killed.
(8) Eliminate conditions conducive to infestation.
(9) For cultivated plants, highly susceptible crop cultivars should be sown
in the border lines as trap crop to attract termites towards them, so
that the main crop can be saved.
2. Insecticide application. There should be a continuous insecticide
barrier between the termite colony and wood in a building. Sometimes
there may be a secondary termite colony above the soil (in the roof or
other areas with a constant moisture supply) that requires additional
2 76 1
Locusts and Termites
treatment. Insecticide (e.g., chlorpyriphos) barriers may be established
during or after building construction. In an existing building, termite
treatments may involve any of the following procedures: mechanical
alterations and/or use of an insecticide to treat the soil, foundation and
wood. Methods vary with each house, depending on the type of
foundation or basement, construction materials, number and type of
porches, terrace, etc. By digging narrow trenches along walls and
drilling through horizontal surfaces insecticide can be applied where it
will kill termites within home and block the colonies re-entry.
Secondary and aerial colonies are controlled by correcting the
moisture problem to dry out the moisture-source area. When it is
desirable to rapidly reduce the secondary infestation, this can be done
by intergallery injection or surface treatment with a pesticide labeled for
these termites. Above-ground termite baiting systems that are placed
directly on the termite infested wood follow following procedures.
( 1 ) Locate kick-out holes.
(2) Lightly puncture kick-out hole.
(3) Inject appropriate insecticide (e.g., chlorpyriphos) in kick-out hole.
(4) Seal kick-out hole with cement.
(5) Prevent infection through education, detection and elimination of
conducive conditions (as mentioned above) which are the most
effective and cost efficient control measures.
(6) When activity is already present, apply liquid termiticide barrier and
baiting programme s.
(7) For termite infested crops, traditionally farmers apply BHC or aldrin
dusts in soil before sowing the seeds to control termites, but recent
recommendations for termite control are seed treatment with
chlorpyriphos or carbosulfan. The dose depends upon the type of crop.
Neem oil treatment was found promising. Neem leaves applied in seed
furrows not only significantly controlled termites but also help in
manuring the soil and proved superior than traditionally used dust.
Important Questions
I.
2
3.
4.
Describe i n brief the bionomics o f locust.
Suggest measures for control of locusts and termites.
Give an account on the colony structure and polymorphism in termites.
Write short notes on : (i) Prophylactic measure of termite control, (ii) Economic
importance of termites and (iii) Locust migration.
20
Household Insects and Their Control
Household insect pests are those that frequently visit our houses, those that
rest on sensitive parts of our body, those that make irritating noise and
those
that
damage
books etc.
our
stored
food
material,
cloths,
carpets,
furniture,
The following are some of the household insects.
1. Cockroaches (Dictyoptera : Blattaria)
1. Distribution. Cockroaches are most common household insects. They
are found in all parts of the world particularly in tropics. Several species
of cockroaches are found in India,
viz., Periplaneta americana (American
l A), P. australianse (Australian cockroach), Blatta orientalis
(Oriental or Indian cockroach, Fig. lC, D), Blatella germanica (German
cockroach, Fig. l B), Stylopyga rhobifolia. However P. americana and
B. orientalis are most common species frequently found in our houses,
cockroach, Fig.
restaurants,
hostels,
bakeries,
food
stores
and
even
in
railway
compartments and shipholds.
2.
Habit
remaining
storehouses,
bakeries,
and
hidden
hotels
public
habitat.
during
and
latrines,
The
daytime
cockroaches
in
cracks
restaurants,
under
prov1s10n
boxes
and
are
nocturnal
and cervices,
other
stores
in
and
neglected
insects
kitchens,
godowns,
articles,
in
main-holes of sewers etc. and come out at night for feeding purpose�.
They are very agile runners and usually depend on this ability, i nstead
of flying, to escape potential predators.
3. Appearance. The cockroaches are brown, brownish-black, or tan,
small to large sized (3-5 cm),
shiny, somewhat dorsoventrally flattened,
foul-smelling
insects
having
mandibulate
mouthparts .
Compound
eyes
278 J
Household Insects and Their Control
B
c
D
Fig. 1 . Cockroaches. (A) Periplanata americana, (B) Blatella germanica with ootheca,
(C) Blatta orientalis (male), (D) Blatta onentalis ( female).
and ocellus are usually well developed. Filiform antennae may have
more than 1 00 flagellar segments. Pronotum becomes enlarged and
ne¥ly covers dorsum of head like a hood. Forewings are thickened
tegmina while hindwings are membranous and fan-shaped. Legs are
generalised and cursorial. Abdomen bears a pair of styli (sing. stylus) on
9th sternum of males.
In females, ovipositor highly reduced and
concealed by 7th abdominal sternum. External genitalia of male are
asymmetrical.
4. Life history. March to September is active breeding season for
cockroaches. After copulation, the eggs are laid in deep brown coloured
ootheca . These are definite in shape and sculpture for the species. The
eggs in each usually number 1 5-40 (e.g., 1 6 in P. americana) arranged in
symmetrical double rows. The ootheca is formed over a period of
several days and is sometime cemented to the substratum (e.g., P.
americana). A single female lay up to 1 00 oothecae (generally 1 5-40) in
her life-time (about 2 years), each contammg 1 6 eggs (e.g., P.
americana). After 35 (in summers) to 1 00 (in winters) days of
Household Insects and Their Control
l 279
incubation period, first instar nymphs emerge out. The nymphal stage
persists for about 6 months (in summers) to 2 years (in winters) during
which the nymph undergoes I O to 1 3 moults or ecdysis. The post
embryonic development is gradual hemimetabolous type. Generally there
is one generation in a year, but under uniform and favourable
conditions there may be 2 or 3 generations.
5. Importance. The cockroaches are omnivorous and feed on a wide
variety of household goods, but the major charge against them is that
they are dirty, distasteful, and odoriferous creatures and are attracted to
such material as garbage, faeces, and foodstuffs consumed by humans.
They get into many kinds of food, consume part of it, discolour and
spot it with faecal material and leave behind a disagreeable odour.
Although there is a little evidence that points cockroaches as transmitter
of pathogens, circumstantial evidence is strong, and it is believed that
they may, in fact rival of house flies in their capacity for disease
transmission such as tuberculosis, cholera, leprosy, dysentery, typhoid
etc.
6. Control measures. Following prophylactic and control measures
should be adopted to get rid of cockroaches.
(a) Mechanical methods. The most important measure is good
house keeping, thorough cleanliness and preventing infestation by
keeping all pipelines, safety tanks, cisterns, main-holes tightly sealed. A
light infestation of cockroaches can be controlled by trapping.
(b) Chemical method. Baits containing suitable insecticides may be
spread in the infested area to kill them. Dusting or spraying of
chlordane (2.5% emulsion) or dieldrin (0.5% oil solution), or malathion
( 1 -5% spray or dust), daizinon (0.5 - 1 .% spray; 5% dust) is also
effective. These materials should be applied in dark corners of closets,
at the base of the walls in basements, under sinks, around drain-pipes,
in any cracks in the walls where cockroaches are likely to hide. Even
though all cockroaches have been eliminated from the house, it will not
remam clean for long, as it migrates from the neighbouring infested
houses. Therefore, anti-cockroach campaign should carry out like the
anti-rat campaigns by all householders in an area simultaneously and
persistently.
2. Crickets (Orthoptera : Gryllidae)
1. Distribution. Crickets are well known by almost everyone, as are their
chirping sounds. Two species of crickets, viz. Gryllodes sigillatus (house
cricket, Fig. 2A) and Acheta domesticus (house cricket, Fig. 2B) are more
common in Indian houses. Other house crickets are Gryllus testaceus and
Gryllodes melanocephalus which hide in cervices of kitchen and feed all
kitchen refuge and left-overs. Like grasshoppers, crickets are also abundant
280 J
Household Insects and Their Control
B
Fig. 2. Crickets. (A) Gryllodes sig1llatus (B) Acheta domesticus
in tropics as they survive well in hot dry places. Several species of crickets
are found in fields damaging trees and crops, e.g.,
Brachytrypes achatinus.
2. Habit and habitat. G. sigillatus
Gryllus campestris
(field
cricket),
places,
especially
inside
houses
is most abundant in damp warm
(under
logs,
stones,
boxes,
in
holes,
behind books and cervices, in kitchens), stores, groceries, etc. They are
nocturnal
cervices
in
or
habit
and
behind
remains
the
hidden
clothes,
wall
during
papers,
daytime
pictures
in
or
cracks
in
and
heap
of
firewood, under the housewares in the kitchen and come out in night
for feeding.
3. Appearance. The body of most of the crickets is dorsoventrally
flattened. Compound eyes are well developed.
setaceous
and
G. sigillatus
longer
than
body.
Antennae are filiform or
Mouthparts
are
mandibulate
type.
is a dull straw-coloured having deep brown small spots on
the body and legs and with a deep brown streak on the head. The body
is somewhat fusiform. The female is apterous. The forewings of males
are
of
harder
represented
consistency
by
small
attached
with
Females
possess
cerci.
the
posterior
a
A. domesticus
and
pads.
long
Long
are
called
paired
extremity
needle-like
of
tegmina.
unsegmented
the
abdomen
ovipositor
Hindwings
are
anal
are
in
projecting
cerci
both
sexes.
between
the
is darker species and its thorax is squarish.
The tibiae of fore legs of the crickets possess tympanic organ and
the
femur
of hindlegs
is
swollen
for jumping.
Males
of crickets
are
renowned singers and each species has a characteristic song and certain
sibling
species
of crickets
can
only
be
identified
by
the
sounds
they
produce. The sound is produced by the friction between two wings (see
chapter
1 5). The male crickets sing normal songs to please the females
Household Insects and Their Control
{ 281
for mating purpose and aggressive songs to warn other crickets entering
its territory.
4. Life history. The crickets breed throughout the year but they
breed faster during and after rainy season. The female lays eggs in
holes, up to 30 eggs in each hole in damp soil or in moist fabrics. The
eggs are creamy-white in colour, slightly curved, cylindrical in form and
blunt at both ends. After 8- 1 0 days of incubation period, eggs hatch
into first instar nymph. The nymphal period varies from 30-40 days.
They moults 8-9 times before reaching adulthood. The number of
generations varies with the prevailing temperature and moisture, but
normally there are 2-3 generations in a year.
5. Importance. The crickets are omnivorous, but the bulk of their
diet constitutes vegetable matter. The field crickets also used to visit
houses and consume cloths, food grains, books etc. It damages fabrics
like silk and woolen articles, books and sometimes make the food grains
non-consumable for human beings. Both nymphs and adults cause
damage to our belongings.
6. Control measures. As mentioned for cockroaches.
3. House Fly (Diptera : Muscidae)
1. Distribution. The house flies are cosmopolitan species, found throughout
the world. These are abundant in villages and in towns around human
habitation and dirty places. The common house flies which are found at
one time or other in every Indian house are Musca nebulo (Fig. 3),
M. dom estica (native of Europe), M. corvina, M. vicina and M. autumnalis.
2. Habit and habitat. House flies are diurnal active flying
detritivorous insects feeding upon human debris and other decaying
organic matter. Physically it does not harm the humans but its mere
presence becomes intolerable to a conscious person.
3. Appearance. Adult flies (Fig. 3A) are stoutly built somewhat oval
insect of about 5-8 mm in length. The body is dark grey coloured and
has four longitudinal lines on thorax and one black streak on abdomen.
Mouthparts are of sponging type having fleshy retractile proboscis, fitted
for sponging liquid food from exposed moist surfaces. Three segmented
antennae are aristate type. Head bears compound eyes and ocelli. The
flies bear only one pair of wings (forewing only), the hind wings are
modified to stubbed halteres. Legs are generalised and possess gustatory
receptors in the tarsi. Abdomen is 8 segmented in male and 9
segmented in female. In female, last 4 abdominal segments form
telescopic ovipositor.
4. Life history. After maturing in spring and summer they breed.
The female flies oviposit 2 to 2 1 egg masses (Fig. 3B) in any kind of
animal excrement or decaying matter such as organic manure, cow dung,
282 J
Household Insects and Their Control
Fig. 3. Life-cycle of house fly. (A)· Adult, (B) Eggs, (C) Newly hatched larva, (D) Mature
larva, (E) Pupa, (F) Pupal case or puparium.
decomposing fruits and vegetables, human faeces. Eggs are deposited in
a cluster about 1 2 mm deep from the surface of organic debris where
they get dark and protection. Each egg mass contains about 1 00- 130
eggs, which hatch within 24 hours. Eggs are pearly white, elongate, and
about 1 mm long. Larvae (maggots) are tapered exteriorly and are
creamy white (Fig. 3C) . Full-grown larvae are about 12 mm long.
Larvae are found in manure, rotting food, and other fermenting
vegetable matter. Maggots feed in decaying material for three to seven
days, move to the margins of the food source, and pupate. Before
pupation larvae moult twice. Puparia (Fig. 3F) are shorter than larvae
and more robust. They are dark brown and about 6 mm long when
mature. Pupae (Fig. 3E) in puparia develop in three to seven days and
adults emerge. The entire life cycle requires only I O to 1 2 days in
summer but take considerably longer in winter. The reproductive
potential is tremendous. It has been calculated that the progeny of a
single gravid female would easily produce 2 x 1020 flies in a one-year
period, filling 3 ,200 km2. Indeed house flies usually reproduce to the
limit of available food resources. This suggests that sanitation is one of
the best methods of limiting house fly population.
5. Importance. House flies are very annoying to both humans and
animals. In addition, house flies are known to be carriers of disease
Household Insects and Their Control
[ 283
organisms. For example, typhoid fever, anthrax, cholera, tuberculosis,
leprosy, diarrhea, and dysentery have been associated with mechanical
transmission by house flies. House flies also feed on the discharges from
eyes and wounds causing opthalmia in Egypt and Greece and yaws
(caused by the bacteria Treponema pertenue, and characterised chiefly by
the eruption of disfiguring skin lesions) in the tropics. They also serve
as intermediate hosts for parasitic tapeworms (helminths) or transmit
eggs of helminths, e.g., Hymenolepis , Echinococcus
(tapeworms),
Habronema (roundworm).
6. Control measures. Following prophylactic and control measures
should be employed to control the house fly population.
(a) Physical methods. Sanitation is one of the best methods of
limiting house fly population, therefore, garbage, stable manure, and
other decaying matters where they used to oviposit must be removed
from human habitats. Outdoor toilets should be avoided. Houses should
be neat and clean free of any filth. Food materials must be kept in
screened enclosure. Also, persons involved in selling food articles, fruits
etc. on roadsides or in bazar should be educated to employ hygiene
practices.
Poison baits (using mixture of formaline, sugar, milk soaked in
blotting paper or cotton swab) or electrified traps and screens are also
useful in decreasing the population of adult house flies.
(b) Chemical methods. If the garbage or manure accumulated near
human dwelling cannot be disposed off immediately, it should be
sprayed or dusted with insecticides like, lime, borax, crude oil, formaline
etc. Fumigation with dry neem leaves repels house flies from habitats of
human and livestock. Following chemicals should be sprayed/dusted if
needed: pyrethrums, malathion, DDT (restricted use).
4. Ants ( Hymenoptera : Formicidae)
Like termites, the ants are social and polymorphic insects and invades
houses. True solitary ants do not exist. They invade houses in search of
food and consume all kinds of human food particularly those that contain
sugar.
1. Distribution. The ants are cosmopolitan in distribution. Most of
the species inhabit in fields and may destruct crops and trees. However,
a certain number of species invade human dwellings. Common Indian
household ant is Monomorium indicum, M. destructor and M. gracillinum
enter houses in large number during rains. Other species of ants
commonly dwelling in houses are black carpenter ant ( Camponotus
compressus), small red ant (Solenopsis spp., carnivorous). Dory/us
orientalis (a large wasp-like blind ant that attacks the workers of other
ants like Pheidole indica), D. labiatus, Cremastogaster spp. and Form ica
284 J
Household Insects and Their Control
spp. are other household ants. Argentine ant (lridomyrmex humilis) is a
small common household insect distributed throughout North America.
2. Habit and habitat. Ants are social and polymorphic insects and
make their nests both inside buildings and in open fields. They can feed
every food article consumed by man. Different morphs of ants perform
different duties. Ants feed on a large variety of food like dead and
living matter, grains, vegetable substances, fats, fungi, meat and sweets.
Ants that are commonly seen crawling about are the workers. They
move in large numbers through the exposed smooth runways in the
fields or in rows on the smooth surface in houses.
Unlike termites,
they do not avoid light. Each worker carries a bit of food at a time to
the nest but large food material like a dead insect or a caterpillar are
carried by hundreds of the individual workers. The ants are very
industrious in habit.
3. Appearance. Ants are very common insect (Fig. 4) and can
easily be identified even by a layman. Usually they are red or black
apterous insects varying in size from 0.5 to 25 mm. Head is squarish
having geniculate or elbowed antennae. The characteristic feature of ants
is that the first or first and second segments of gaster are scale-like or
nodiform and well separated .
They are polymorphic and up to 29
morphs have been identified in one colony. The colony consists of
(i) workers or ergates, (ii) soldiers or dinergates, (iii) gyne or fertile
females or queens, and (iv) aner or fertile male.
(a) Workers or ergates. Workers are sterile and wingless females and
smallest members of the nest. They are characterised by reduced thorax,
smaller gaster and small eyes. The workers may be dimorphic or
polymorphic. The larger workers are called macrergates and smaller
ones are micrergates.
~
c
Fig. 4. Caste of an ant Formica sp.
(A) Queen, (B) M ale, (C) Worker.
Household Insects and Their Control
[ 285
(b) Soldiers or dinergates. These are modified workers with enlarged
head and powerful mandibles. The mandibles are used to grind hard
food materials as well as used to protect the nests from enemies.
(c) Fertile females or gynes or queens. These are the reproductive
morph of the nest. They are usually larger than other morphs of the
colony due to large abdomen that accommodates well-developed
reproductive organs. They are either primarily or secondarily apterous.
A colony of ant consists of several queens.
(d) Fertile males or aners. These are small, fertile and winged
individuals of the colony with a smaller head, reduced mandibles and
larger antennae. The sense organs, reproductive organs and genitalia are
well developed.
4. Life history. The queen ant, mated once in her lifetime during
her nuptial flight, breaks off and sheds her wings and establishes the
first nest and rears her first brood. She draws her nourishment from the
now useless flight muscles and stored up fat. When the first larvae
appear, they are fed with a special nutritive secretion of the salivary
gland and as soon as the first workers appear, they go forth into the
world foraging. They take overall duties of the rearing of the brood,
foraging, nest building, cleaning, nursing, fighting, etc. The queen
survived up to 1 5 years and devotes her energies solely to egg laying.
The population of a single colony varies considerably from a few
thousand to over 5,00,000 individuals. The ant nests or formacaria is
established in a bewildering variety of situations such as underground,
inside hollow stems, fruits, thorns, galls, among leaves, under stones etc.
The tropical Oecophylla smaragdina webs leaves of various trees with
silken threads into a nest. The larvae secrete the silk and are used by
workers as a kind of living thread ball. Most of these ants tend aphids,
membracids and others for honeydew.
5. Importance. Sugar and sugary materials are usually destroyed by
most of the species of ants in houses. C. compressus sometimes attacks
stored grains and woodwork of old buildings. M. indicum makes nest
out of doors, near the walls of buildings, etc. and always trouble the
housekeepers invading every sort of food, woodwork and masonry.
M. grocillinum attack flour and fats inside houses. S. molesta feeds
meat, butter, cheese and other protein foods kept in kitchen. Some ants
bites and stings human beings causing irritation. Majority of ants also
tends aphids and coccids and exhibit mutual relation with those insects.
The aphids provide honeydew as food for them while ants protect them
from their natural enemies.
The ants also serve as food for several animals like giant ant-eaters,
scaly ant-eaters, hedgehogs, lizards, frogs etc.
286 1
Household Insects and Their Control
6. Control. Very few species of ants are pest and need control
measures while maj ority of the species are usually harmless to humans.
Following prophylactic and control measures should be adopted to
control the ants.
(a)
Mechanical methods. The most important measure is good
house keeping, thorough cleanliness and preventing infestation. All food
materials should be kept in insect-proof covers. All holes in the floor
should be properly sealed.
(b) Chemical method. Suitable baits containing insecticides may be
spread in the path of the ants. Dusting or spraying of deodorised
kerosene oil in the infested area is useful. Application of chlordane
(2.5% emulsion), dieldrin (0.5% oil solution), malathion ( 1 -5% spray or
dust), daizinon (0.5 - 1 . % spray; 5% dust) is also effective. These
materials should be applied in dark comers of closets, at the base of
the walls in basements, in any cracks in the walls where ants are likely
to come out.
5. Furniture Beetles ( Coleoptera : Bostrychoidea)
1. Distribution. Members belonging to several families of the beetles like
Bostrychidae, Lyctidae, Ptilinidae, Anobiidae invade and destroy the wood
materials used in house construction, furniture and packing materials. All
these families are cosmopolitan but well distributed in tropical and
temperate countries. Lyctus africanus (powderpost beetle, Lyctidae) is- the
common powder-pest beetle in India attacking every kind of wooden article
that contain sap wood. Dinoderus ocellaris and Synoxylon sudanicum are
common bostrychid furniture beetle distributed throughout in India and
other tropical countries. Anobium punctatum (Anobiidae, Fig. 5) is major
furniture beetle of European and Asian countries.
2. Importance. All furniture beetles are pest of wooden materials
used in construction of houses, furniture, packing boxes, etc. A.
punctatum is very destructive to furniture, rafters and flooring, and its
larvae bore into the solid wood. D. ocellaris and S. sudanicum are
destructive to furniture and other household materials made up of
bamboo in India. L. africanus attacks packing cases, planks, boards,
battens, furniture, tool handles and plywood containing sap wood. The
larvae of these beetles eat the wood tunneling through timbers in
successive generations until the interior is completely reduced to fine
flour-like powder. Small shot holes are seen externally.
3. Habit and habitat. All furniture beetles feed upon either sap
wood or both sap and hard wood . The grubs feed the wood and pack
their tunnel with exceedingly fine flour-like frass. The holes are from 1 .5
to 6 mm in diameter. D. ocellaris bore into felled bamboo plants and
also bamboo furniture.
Household Insects and Their Control
A
[ 287
B
Fig. 5. Furniture beetle Anobium punctatum. (A) Adult,
(B) Larva.
4. Appearance. The adults of most of the species are small, 2-5 mm
in length, hard shelled, brownish, elongate, and cylindrical or short and
stubby, and with varies sculpturing on body and wings. They are not
often seen in adult stage.
5. Life history. The eggs are laid singly in short tunnels specially
made for this purpose by the adult or on the rough surface of the
timber. The freshly laid eggs are elongate, elliptical, opalescent white,
1 .2 mm long and 0. 1 mm broad. Incubation period lasts within a week
and young larvae hatch out and bore into the wood, making deep
irregular tunnels, some up to 38 cm in length. The full grown larva with
large head is about 1 5 mm long and pupates inside a cell at the end of
the tunnel. The freshly emerged adult continues to bore and feed for
some time before it finally emerged out of the wood either by making a
hole on the surface (L. africanus) or through the original entrance hole
after travelling back a long distance (D. ocellaris). Depending on the
environmental conditions and availability of the food, there can be 3 to
4 generations in a year. However, under adverse conditions one
generation may take as many as 8 years.
6. Control. Surfacing the wood with spirit polish, varnish, paint, tar,
etc. protect the wood from insect attack. Unpainted furniture may be
painted with benzene or carbon tetrachloride with naphthalene dissolved
in it. Wood must be treated with copper sulphate or zinc chloride
before making furniture. Drying the wood at 82°C for half an hour also
kills the insects. Fumigation with methyl bromide can be done wherever
feasible.
(Z-57)
288 1
Household Insects and Their Control
6. S ilverfish (Thysaneura : Lepismatidae)
1. Distribution. Silverfishes are cosmopolitan in distribution partic;ularly in
tropical and temperate region. Lepisma saccharina (Fig. 6)
is very
common silverfish in India.
2. Importance. Silverfish feeds on a large variety of starchy food
such as starched clothes, rayon fabrics, bindings of books, papers on
which paste or glue has been applied and thus destroy neglected books.
3. Habit and habitat. The silverfishes are found inside houses,
libraries etc. as well as under tree bark, dry leaves, stones, refuse etc.
In houses it is common among neglected books, papers, cardfiles,
behind pictures hanging on the wall and usually destroy them. They
prefer damp warm places next to the soil about basement rooms and
porches and is less abundant in upper stories of houses. In kitchen they
like to feed vegetable matter high in carbohydrates, such as flour and
oatmeal.
4. Appearance. The full-grown insect is wingless, about 8 to 1 2 mm
in length. and shining silvery in colour. The body is more or less
spindle-shaped, depressed and covered with scales which give it a silvery
shiny appearance. Antennae are long and filiform. The posterior end of
the body is provided with three long, many segmented, thin and fine
appendages, one median caudal filament and two lateral cerci.
5. Life history. The female deposits 7 - 1 2 eggs in cervices and other
concealed moist places. The development is slight and the young ones
(nymphs) are very similar to adult. They undergo 5-7 moults to reach
adulthood taking about three months during favourable conditions,
however, under varied environmental conditions the development takes
about 2-3 years. The moulting continues into the adult stage.
6. Control. The silverfish has a special liking for glue, therefore,
they may be killed by a poison bait consisting of oatmeal, sodium
fluoride or white arsenic, sugar and common salt. These insects may
also be controlled by spraying with 2.5% chlordane, 1 % lindane, 0.5%
dieldrin, 1 % pyrethrins, 2% malathion, 0.5 per cent Diazinon. These
sprays, or equivalent amounts of dusts, should be applied where the
insects are most abundant.
7. Carpet Beetle ( Coleoptera : Dermestidae)
1. Distribution. There are several species of carpet beetles which are
distributed throughout the world. Two species Attagenus piceus and
A . glorisae are commonly known as black carpet beetles whereas species of
Anthrenus flavipes (Fig. 7) and A. coloratus are called spotted carpet
beetle. A . flavipes are most common species in India.
- t7 'i?l
Household Insects and Their Control
{ 289
A
Fig. 6. Silverfish Lepisma sacchanna,
Fig 7. Carpet beetle
A nthrenus flavipes
(A) Adult, ( B ) Larva.
2. Importance. The carpet beetles severely damage fabrics or
articles containing dried animal products like wool, hair, fur, leather,
skin, feathers, horn, bristles, silk and dry insect specimens or stuffed
animals in the museum. The damage is mainly caused by the larvae,
which feed on every sort of dried animal product.
3. Habit and habitat. The carpet beetles mostly inhabit in household
fabrics and dried animal products and consume it. The larvae live a
large part of the time in secluded places about the rooms, out of reach
of house-cleaning operations. They also invade stuffed museum
specimens as well as dried insect specimens.
4. Appearance. A. flavipes is oval, 3 mm long, with red, yellow,
white and black varicolored markings. The head is directed downwards
and is partially concealed while the antennae are short and clubbed.
5. Life history. The female deposits eggs on the food materials or
in dark secluded places such as clothing, furniture, cracks about
baseboards, and other undisturbed places. The eggs are elongate, oval,
0.7 mm long and about half as broad. The egg bears spine-like
projections at one end that serve to attach it to the surface upon which
it is deposited. It hatches in a week or two and the young larvae
immediately attack such food as they can find. The freshly hatched larva
is creamy white, elongate, oval and clothed with dark hairs (Fig. 7B).
The larvae unlike parents, avoid light and feed voraciously. The larval
period is about 6 months at 35"C. The pupal period lasts for about 1 -2
weeks. After moulting to the adult stage, the insects may further remain
within the larval skin from a few days to a few weeks. The life-cycle
may be completed in less than a year to over three years depending on
the prevailing climatic conditions and food availability. The adult beetles
are abundant during the hot weather The adult may live from 2-6 weeks
(Z-57)
290 J
Household Insects and Their Control
but never dam�e household goods in this stage. They are active,
photophilic and ate often found about windows and out-of-doors upon
flowers eating the pollen.
6. Control. As the larvae live in neglected places, to avoid their attack,
regular cleaning of materials is essential. Godowns and other storage
places of animal products should be fumigated with carbon disulfide or
hydrogen cyanide. Use of paradichlorobenzene crystals or naphthalene
balls inside boxes, storewells etc. protects the clothings. The sun-drying,
beating and brushing of clothings in hot weather may give relief. Spray of
chlordane or pyrethrum in de-odorised kerosene oil should be applied to
cracks in floors and baseboard.
8. Cloth Moths ( Lepidoptera : Tinaeidae)
1. Distribution. Species of Tinea , Tineola and Trichophaga are common
cloth moths distributed throughout the world. Tinea pellionella (Fig. 8) and
Trichophaga abruptella are the most common cloth moths in India.
2. Importance. Only larval stages are injurious to our commodities
made up of wool, hair, feathers, furs, leather. Animal and fish meals
and milk powders are also damaged by the larvae. The fabrics injured
by the cloth moths have holes eaten through them by caterpillars.
3. Habits and habitats. The adult moths do not feed and only
larval stages feed upon skin products. The larvae live in silken cases
and therefore, the presence of such cases or lines of silken threads
indicate the presence of the caterpillar. Small, buff-coloured moths may
be seen over the infested goods when such goods are exposed to sun.
They are very active if the materials are left undisturbed for some time
or stored in dark places.
4. Appearance. T. pellionella is greyish-yellow with dusky spots on
the middle of forewings. Head bears rough hairs. Hindwings are
narrower.
Fig. 8. The cloth moth Tinea pel/ionel/a.
(C) Caterpillar in the case.
fZ-57)
(A)
Adult
moth,
(B)
Caterpillar
and
Household Insects and Their Control
[ 291
5. Life history. The adult moths (Fig. 8A) do not feed and mate
soon after emergence. The female oviposits about 75- 1 50 eggs on a
variety of materials that can be consumed by the larvae . Eggs are oval
in shape, slightly broader at one end, creamy-white. Within a week the
eggs hatch into dull white or pale yellow caterpillars which begin to
feed. The caterpillar constructs a parchment-like silken case (Fig. 8C)
which it drags along. The fully grown larva measures about 25 mm in
length (Fig. 8B) . Pupation takes place inside larval-case. Pupal period
lasts in 10- 1 5 days. The complete life-cycle requires about 6 weeks.
6. Control. The control measures suggested for carpet beetle may
hold good for this also.
9. Booklice ( Psocoptera : Psocidae)
1. Distribution. Booklice are distributed throughout the world. Liposcelis
divinatorius is almost cosmopolitan in distribution. There may be hardly any
house or warehouse or library not infested by these small insects.
2. Importance. They are usually not very injurious, but may damage
considerable where their breeding materials remain undisturbed for a
long period. They eat away the paste from the book bindings and
sometimes cause considerable damage to dry-zoological and botanical
specimens.
3. Habits and habitat. The booklice Liposcelis are found in dark
and damp places inside houses, mills, food-stores, warehouses, libraries
and museums. The domestic species may feed on stored food products,
natural history specimens, straw and chaff in barns and warehouses, or
occur in hay stores. They usually feed the paste of book-bindings. They
are often gregarious in habit. Sometimes they seriously infest tea packed
in chests and stored in damp godowns, particularly in Assam.
4. Appearance. The booklice are small insects about 1 .0 mm in
length, with a light brown, grey or pale-yellow colour with rather soft,
Fig. 9.
Book lice L cposce/is d1vinatorius.
292 1
Household Insects and Their Control
stout bodies and usually delicate, membranous wings. However, species
of Liposcelis are apterous (Fig. 9). They can walk and run fast.
5. Life history. The female deposits 20- 1 00 eggs which are
ellipsoidal or bluntly rounded at one end. They may be scattered singly
or laid in groups at damp places near their food and may sometimes be
covered over with a silken web. The young nymph undergoes five
moults, but the number may vary according to the climatic conditions,
before reaching the adult stage. Though domestic species breeds
continuously, outdoor species have one to three generations per year.
6. Control. A thorough cleaning and sunning of the infested articles
is sufficient when the infestation is mild. When the infestation is severe,
heating the room to a temperature of 50°-60"C for several hours
effectively destroys the pest. Alternatively, fumigation with sulphur
dioxide or hydrocyanic acid gas should be carried out.
Other household i nsects like termites, store-grain pests, mosquitoes,
fleas, human head and . body louse and bedbugs are dealt in separate
chapters i n greater details.
Important Questions
1.
2.
3.
Write an essay on household msects
Suggest control measures for house hold msects particularly cockroaches, crickets, ants
and house flies.
Write short notes on · (i) Carpet beetle, (u) Ants, (iii) Furniture beetle, (1v) Silverfish.
21
Insects Inj urious to
Man and Livestock
The insects attack humans and livestock essentially the same way. The
medical entomology and veterinary entomology are two branches of applied
entomology that deal the biology of insects in reference to humans and
animals of economic importance, respectively. The insects affect the health
of both in two ways: directly as the causative agents of disease and
discomfort and indirectly as the transmitters or vectors of disease-causing
pathogens like bacteria, viruses, protozoans, helminthes, etc. Insects cause
disease and/or discomfort by several ways as combination of feeding
activities (e.g., mosquitoes, black flies, bed bugs), physical injury (e.g.
predaceous bugs, blister beetles), secretions (e.g., honey bees, wasps),
invasions and infestations, and psychological disturbances. Diseases like
malaria, filaria, cholera, typhoid, dengue, plague, kala-azar, sleeping
sickness, oriental sore, etc. are transmitted by insects. Some of the more
important insects of public health importance and pests of livestock are
briefly described.
INSECTS INJURIOUS TO HUMANS
Few insects like mosquitoes, bedbugs, lice, fleas, flies etc. are directly or
indirectly involved in the welfare of humans causing their health problems.
Insects Injurious to Man and Livestock
294 ]
1 . Mosquitoes
Three genera of mosquitoes, Anopheles , Culex and Aedes cause maximum
sickness among human beings. The mosquitoes feed by sucking the blood
of man and transmit a number of deadly diseases in this process.
[ I] Species of mosquitoes
Anopheles spp. transmit a sporozoan protozoa Plasmodium spp. causing
malaria, Culex spp. transmit microfilariae of a nematode Wuchereria
bancrofti causing filaria or elephantiasis and Aedes spp. transmit viruses
causing dengue fever and yellow fever in human population. Mosquitoes
occur in damp and marshy areas throughout the world, but are most
abundant in tropics and subtropics.
The mosquitoes are nocturnal insects and they remain hidden in
dark places during daytime. At night they come out from their hides
and actively fly about in search of food. However, at most places they
can bite at any hour of day or night and make life uncomfortable. Only
female mosquitoes suck the blood from the humans (anthrophagous
mosquitoes)
and other animals
(zoophagous mosquitoes)
through
piercing and sucking type of mouthparts. Male feeds on nectar and
other plant secretions. Antennae of males are plumose whiie that of
females are pilose. The population of mosquitoes increases tremendously
during raining season but declines in summers and winters. This is
because the female lays eggs in water and life cycle is also completed in
water. Thus water filled in pools, ponds, lakes, ditches, small pits etc.
serve as ideal place for their breeding. They are very good fliers and
may cover upto 20-30 km in one night.
1. Anopheles. Anopheles mosquitoes are distributed throughout the
tropic regions of the world, only few species occur in temperate regions.
(a)
Importance.
The
species
of
Anopheles
particularly
A . maculipennis group (E urope), A . culicifacies (India), A . m inimus
(Assam to China), A. gambiae (Africa) and A. albimanus (Central and
South
America)
are
carrier
(primary
host)
of
different
species
of
Plasmodium of one or more forms of malaria. The plasmodia (P. vivax,
P. falciparum, P. ovale, P. malariae) live in blood and liver, destroying
the red blood cells and hepatic cells, respectively causing anemia
accompanied by characteristic alternating chills, fever, and sweating.
Once introduced to a human body by the mosquito, the parasite
multiplies very rapidly (asexual phase) reaching as many as 3 billion in
the blood of one patient; but it cannot pass from that person to another
without the help of mosquitoes, in the bodies of which a necessary part
of its life cycle (sexual phase) is completed. Malaria which alone was
Insects Injurious to Man and Livestock
[ 295
responsible for about 1 0,00,000 deaths every year till 1 947 that causes
loss of about a thousand crores of rupees in the country's labour
output. After launching national programmes such as National Malaria
Control Programme (1953) and the National Malaria Eradication
Programme ( 1 958), the inCidence of malaria in India has been reduced.
However, estimated number of people infected and clinical cases are
267 and 1 07 millions per year, respectively. The estimated mortality is
1-2 million per year particularly in tropical Africa, Asia and Central
and South America in spite of recent advance techniques of their
control measures. The total number of affected countries and number of
people considered at risk are 1 03 and 2 1 00 million, respectively. Over
the last few years, epidemics or atypical increases in malaria incidence
have been reported from several areas, including the Amazon region,
Ethiopia, Madagascar, Sri Lanka and the Solomon Islands and some
parts of northern India.
(b) Appearance. The Anopheles mosquitoes are relatively smaller
(about 3 mm) with one pair of spotted wings. In siting posture, body
remains elevated at 45° from the surface of substratum and proboscis
remains straight in line with long axis of the body. Also, the
5-segmented maxillary palps are equal to the proboscis both in males
and females; in males it is club-shaped.
(c) life cycle. Female lays 40- 1 00 eggs at a time in somewhat clear
water which are laid singly. Eggs are black, somewhat boat-shaped with
lateral airfloats floating horizontally on the water surface (Fig. I A). The
dark-coloured larvae or wrigglers lack respiratory siphon (Fig. lB).
During breathing, the entire body lies horizontally at water surface. It is
a surface feeder. The larvae after fourth moult become inactive and sink
down to the bottom and metamorphose into comma-shaped pupae or
tumbler which are greyish green and motile (Fig. l C). After 2-7 days,
pupa metamorphoses into adult (Fig. l D).
2. Culex. Culex pipiens fatigans is an almost tropicopolitan mosquito
of great economic importance.
(a) Importance. Along with other species C. pipiens fatigans is a
carrier of Wuchereria bancrofti which causes elephantiasis. During severe
infection, the microfilariae block the lymphatic vessels and glands
causing lymphatic obstructions causing accumulation of lymph in certain
organs (legs, arms, genital organs etc.) due to which they swell
fantastically.
(b) Appearance. The Culex mosquitoes are about 4 mm in length with
one pair of clear uniform wings. In siting posture, body remains parallel to
the surface of substratum and proboscis remains bent at 45° from long axis
of the body. The maxillary palps are 5-segmented and somewhat equal or
Insects Injurious to Man and L ivestock
2 96 J
Fig
I . Different stages of mosqmtoes
(A) Egg, (B) Larva,
(C) Pupa,
(D) Adult of
Anopheles maculipennis, (E) Egg raft. (F) Larva, (G) Pupa, (H) Adult of Cu/ex p1piens
fatigans; and (I) Egg, (J) Larva, (K) Pupa, (L) Adult of Aedes aegypti.
Insects Injurious to Man and Livestock
[ 297
larger than proboscis and terminally pointed in males; and only
3-segmented, quite short and terminally club-shaped in females.
(c) Life cycle. The female lays 1 50-300 eggs at a time in somewhat
dirty water. Eggs are laid in floating rafts keeping vertically upright at
right angles upon water surface. Eggs are greyish, cigar-shaped and
without lateral airfloats (Fig. l E). Larvae are light coloured with
respiratory siphons (Fig. IF). During breathing at water surface, body
remains submerged in water at 45° and suspended by tail end from the
surface. The larvae are bottom feeder. The larvae after fourth moult
become inactive and sink down to the bottom and metamorphose into
comma-shaped pupae or tumbler which are greyish in colour and motile
(Fig. I G). After 2-7 days, pupa metamorphoses into adult (Fig. I H) .
3 . Aedes. Aedes aegypti i s one o f the commonest mosquitoes o f the
tropics and subtropics of the world, and occurs largely along the coasts
and the courses of the larger rivers.
(a) Importance. A. aegypti and related species are able to transmit
the virus of dengue and yellow fever from one human being to another.
Dengue fever (= breakbone fever) is an infectious tropical disease
characterised by fever, extreme pain in the joints and muscles, vomiting,
and a skin eruption. The causative agent is a filterable virus which kills
white blood cells. Dengue is endemic in some parts of the tropics; it
has occurred in epidemic form in both tropical- and temperate-zone
countries, as in Latin America and the Caribbean in 1 995. It is seldom
fatal and usually runs its course in 6 to 7 days, but convalescence is
usually slow. No specific treatment for dengue is known. Yellow fever is
a non-contagious and infectious disease caused also by virus and
characterised by high fever and jaundice. This type of the disease, which
occurs only sporadically in human beings, is known as jungle yellow
fever.
(b) Appearance. The Aedes mosquitoes are about 4 mm in length
with one pair of wings. Like Culex, in siting posture, body remains
parallel to the surface of substratum and proboscis remains bent at 45°
from long axis of the body. The maxillary palps are 5-segmented and
somewhat equal or larger than proboscis and terminally pointed in
males; and only 3-segmented, quite short and terminally club-shaped in
females.
(c) Life cycle. The eggs (Fig. 1 1) are laid singly in depressions, along
high water lines, and in almost any container of rain water, water tanks
of coolers etc. The light coloured larvae possess a short and broad
siphon tube for breathing (Fig. 11). During breathing at water surface,
body remains submerged almost vertically in water and suspended by
tail end from the surface. The larvae are bottom feeder. The larvae
after fourth moult become inactive and sink down to the bottom and
298 1
Insects Injurious to Man and Livestock
metamorphose into comma-shaped pupae or tumbler which are greyish
in colour and motile (Fig. 1 K). After 2-7 days, pupa metamorphoses
into adult (Fig. l L).
[ II] Control measures
The following general methods may be used to control the population of
mosquitoes.
1. Elimination of breeding places. Mosquitoes breed in stagnant
water, therefore, it is recommended that at least those water bodies
should either be removed (like used tyres, glasswares, cans, urns etc.
kept on roofs of the buildings which are filled with water during rain)
or cleaned or temporarily dewatered regularly
(e.g., water in coolers,
cisterns, sewage lines, septic tanks etc.); it is effective against Aedes . As
far as possible small ditches and depressions that may hold water
should be filled with mud. In urban areas, most of the gutters and drain
lines are choked due to garbage mostly packing materials including
plastics and need their regular removal. Therefore, people must be
educated to develop good civic sense to avoid throwing any material m
those drains.
2. Destruction of larvae. The mosquitoes may be killed easily in
their larval forms with the application of following methods : (i)
repeated spraying of kerosene oil as the oil film formed on' the surface
of water not only prevents breathing of larvae but is also toxic to them;
(ii) spraying Panama larvicide (a mixture of caustic soda, resin and
phenol in water) or temephos in water bodies; (iii) application of
insecticides like fenthion, fenitrothion and malathion on the surface of
water has been found to be effective; (iv) breeding larvivore fishes like
minnows and Gambusia in water bodies where drainage and dewatering
methods are impossible, as in lakes, ponds, rivers, streams and
water-storage reservoirs; (v) using water dispersible formulation of a
bacteria Baccilus thuringiensis var. israelensis serotype H 1 4, strain 1 64
which is highly effective against early instar mosquito larvae when
applied at 0.5 g/m2 of water surface.
3. Destruction of adults. Adult mosquitoes may be killed by weekly
spraying with 0.2% synergised pyrethrins around human surroundings.
Due to development of resistance, chlorinated hydrocarbons are usually
not recommended. The spraying of malathion aerosols is found effective
against adults in outdoors.
4. Personal protection. Although personal protection of mosquito
bites is not a control measure, yet it prevents the spread of diseases
transmission. It can be achieved by (i) wearing clothing which limits the
amount of exposed skin, (ii) screening the houses by fine mesh nets
(iii) using mosquito repellents like mosquito cream, citronella, odomas,
Insects Injurious to Man and Livestock
[ 299
indalone, allethrin (burning in coils or mats), (iv) using mosquito nets in
bedroom while sleeping, the bed nets may be impregnated with
permethrin, alphacypermethrin, cypermethrin or deltamethrin 2.5% or
Ar-cyhalothrin @ 25 mg/m2 or cyfluthrin at 50 mg/m2 .
2. Sand Flies ( Diptera : Psychodidae)
The sand fly, Phlebotomus is small to minute (about 3 mm), usua.lly very
hairy, mothlike flies that, when at rest hold the wings rooflike over the
body or together above the body. The adults occur in shady places in
the vicinity of water; they are sometimes extremely abundant in drains
or sewers.
[ I] Importance
The sand flies are blood-sucking insects and like mosquitoes only female
fly suck the blood. They occur in the southern states of America and in the
tropical countries. They act as vectors of several diseases in various parts
of the world: pappataci fever or three-day fever (caused by a virus
transmitted by P. papatasi which occur throughout northern India) which
occurs principally in the Mediterranean region and in southern Asia;
kala-azar (visceral leishmaniasis caused by Leishmania
donovani
transmitted by P. argentipes (Fig. 2), most prevalent in India in B ihar state)
and oriental sore (cutaneous leishmaniasis caused by Leishmania tropica
transmitted by P. argentipes very common in Bengal and Assam) which
occur· in South America, northern Africa, and southern Asia; oroya fever
(caused by a bartonella organism), which occurs in South America; verruga
disease in Peru is also transmitted by sand fly.
[ II] Life cycle
The female lays 20-30 eggs at moist places, preferably in damp
moist crevices in walls, stones and rocks with humus as substratum, on
which the young larvae feed. The eggs are elongate and dark brown.
Development is complete and the eggs hatch in about 10 days into
minute larvae. The full grown larva is 2-8 mm long and provided with
elongate caudal bristles. The pupa usually carries the larval exuvia at its
anal extremity. The life cycle takes about five weeks for completion
under warm and moist conditions. The life cycle takes about five weeks
for completion under warm and moist conditions.
[ Ill] Control measures
Accumulation of bricks and stones kept for building construction,
particularly those which are damp, should be removed to prevent breeding
of sand flies. Residual spray of suitable insecticides like pyrethrum and
malathion nearby human dwellings provide better results.
3 00 ]
Insects Injurious
Fig 2. Sand fly, Phlebotomus argent1pes.
Fig. 3.
to
Man and L ivestock
Bed bug, C1mex lectularis.
3. Bed Bug ( Hemiptera : Cimicidae)
Bed bugs are secondarily apterous blood sucking ectoparasite. It is
distributed throughout the world and is an important household insect.
There are three species of bed bugs that feed upon human blood, such as
Cimex lectularis, C. hemipterus (
rotundus) and C. boueti. C. lectularis is
distributed in America, Europe, Siberia, Northern China. Australia and
northern India. C. hemipterus occurs throughout India, Myanmmar,
Malayasia, southern China and central Africa. The bed bugs inhabit dark
and damp buildings, hotels, hostels, rest houses, barracks, cinema theaters
etc. They live in cracks in the walls and floor, cervices in the beds and
furniture etc. They are nocturnal and suck the human blood while sleeping.
Occasionally they come out in daytime also. The warmth and the body
odour of humans attract them. Bed bugs can live for several months to
year without food. They have stink gland in the abdomen secretion of
which give a peculiar odour.
=
[ I] Appearance
The bed bugs are small, oval and flattened, redish-brown in colour and
measure about 5 mm in length and 3 mm in width (Fig. 3). Head bears
compound eyes, 4-segmented antennae and opisthognathous mouthparts
which are well adapted for piercing and sucking. Metathorax is generally
covered by vestigial forewings, hindwings are completely absent.
[ II] Importance
In addition to unbearable foul smell and irritating bites that cause
disturbance in sleep, the bed bug also possess micro-organisms that are
Insects Injurious
to
Man and Livestock
[ 301
supposed to be cause of transmission of certain diseases like kala-azar,
plague, relapsing fever, typhoid, tuberculosis etc.
[ Ill] Life cycle
After taking a full meal, the mated female lays about 200-500 eggs, singly
or in batches, 2 or 3 eggs per day, in cracks and cervices of cots and
furniture, in holes etc. The development is gradual and eggs hatch into
small nymphs after 6- 1 0 days of incubation period. The young nymphs are
very small ( 1 - 1 .5 mm), flat, active, delicate, semitransparent and pale in
colour. After few hours the young nymphs are able to pierce the human
skin to suck the blood. After five successive moults, the nymphs reach the
adulthood. Generation period is about 7 weeks under suitable conditions
but in absence of food it may take 2 years.
[ IV] Control measures
The following measures are recommended to manage the population of
bedbugs: (i) the houses should he well-ventilated and damp-free; (ii) cots,
beds and bed sheets, pillows, mattresses, furniture, etc. should be cleaned.
regularly; (iii) badly infested furniture should be washed with boiling water;
(iv) temephos or emulsion of kerosine, benzene and petrol should be
'
sprayed on the wall of room and furniture ; (v) walls of rooms and furniture
should be painted to seal cracks and crevices; and (vi) the bedbugs can be
killed also by the fumigation of rooms with sulphur.
4. Fleas ( Siphonoptera : Pulicidae)
The adult fleas are exclusively blood sucking ectoparasites of living birds
and mammals. They can jump a horizontal distance of nearly 33 cm. The
rat flea, Xynopsylla cheopis is widely distributed in the tropics and in India.
It is widely prevalent in plains and also in hilly regions. Its primary host is
the common house rat. Other species X. brasiliensis and X. astia are also
found in India. The human flea Pulex irritans is distributed in tropics.
[ I] Importance
Fleas transmit several fatal diseases to humans, e.g. bubonic plague and
murine endemic typhus. Plague is a disease of wild rodents; and causative
agent, plague bacillus Pasteurella pestis, is transmitted from rodent to
rodent by fleas. When rat population is killed, the fleas leave the
cold bodies of the dead rat and attack humans and develop the symptoms
of plague. In regions of high population concentration and poor
sanitary conditions, the potential may exist for epidemics of this disease,
the black death.
Insects Injurious to Man and L ivestock
302 J
c
D
Fig. 4 Rat flea, Xynopsylla cheop1s. (A) Eggs, (B) Larva, (C) Pupa m case, (D) Adult.
[ II] Appearance
The fleas are secondarily apterous, laterally compressed exopterygote
insect with highly sclerotised integument (Fig. 4D). The head sets closely
with into thorax and bears comb formed by a row of powerful spines on
the latero-ventral border of head. The mouthparts are adapted for piercing
and sucking of blood. The short antennae are concealed in grooves when
at rest and serve as copulatory claspers. Being parasitic, eyes and ocelli are
.
absent. The hindlegs are strong, coxae are enlarged and are adapted for
gliding easily among hairs and for jumping or leaping. The body is covered
with backwardly inclined spines and bristles. The thorax is compact and
segments are free.
[ III] Life cycle
The female fleas generally require blood meal before oviposition. She lays
about 6-8 eggs (Fig. 4A). The eggs are placed either in the hairs of the
host or its burrow. A female lays about 200 eggs in her life-time. The blind,
eucephalous and apodous vermiform larvae (Fig. 4B) feed on a variety of
organic matters. Pupation takes place in silken cocoon-like case (Fig. 4C).
The pupae are exarate. The adults are generally able to remain quiescent
in the cocoon for Jong periods of times and exit in response to vibrations
. of the substrate and other stimuli associated with a potential host. Under
favourable conditions the entire life cycle is completed within a month.
Adults are able to survive for prolonged periods of starvation.
[ IV] Control measures
Since rats serve as primary host of fleas, there must be a campaign for the
eradication of rats. Dead rats must be burnt. The breeding places should
be treated with suitable insecticide like propoxur, dieldrin, chlordane,
diazinon, parathion and malathion.
Insects Injurious to Man and Livestock
[ 303
5. Lice (Siphunculata : Pediculidae)
The
human
lice
are
secondarily
apterous
ectoparasite
of mammals.
Pediculus humanus is a notorious species infecting humans. Two subspecies
are distinct : (i) head louse P. humanus capitis found on the skin and in
hairs of the head and (ii) body louse, P. humanus corporis found in the
clothes. The head louse is slightly smaller and darker than the body louse
and has also somewhat stouter antennae. They occur in all countries and
upon all races of people. The body louse is less abundant in the tropics,
probably because of the lesser amount of clothing worn as well as of high
temperature.
[ I] Diseases transmitted
Human louse transmits several diseases such as relapsing fever, epidemic
typhus fever, French fever, etc. of which epidemic typhus fever, caused by
Rickettsia prowazeki (a micro-organism, transmitted not by feeding but
when crushed on injured tissue of skin) is most important.
[ II] Appearance
The female (Fig. 5) is dark brown to black, about 2.5 mm to 4.2 mm long,
males are somewhat smaller than females. The body of head louse is
flattened, The head is orthognathous or prognathous, conical and bears
3 to 5 segmented antennae. Mouthparts are highly modified and adapted
for piercing and sucking of blood. Eyes large, convex and almost always
distinctly pigmented. Thoracic segments are indistinct. The legs are
clinging type, tarsi are unseqmented and bear strong curved claw
specialised for grasping hairs. Body less densely clothed with thinner setae,
arranged in rows.
[ III] Life cycle
A single female of head and body louse deposits about 50- 1 00 and 200-300
eggs, respectively, 8 - 1 0 eggs each day. The eggs are attached by their
Fig S. Head louse, Ped1lUlus huma11us capl11s.
(Z-57)
304
1
Insects Injurious to Man and Livestock
posterior ends by a hard cementing secretion to the hairs in case of head
louse and in seams of the clothing in case of body louse. The young nymph
hatches on the 6-8 day after deposition of the eggs. Three nymphal instars
are observed each having a distinct chaetotaxy. Sexual maturity is attained
after 20-30 days of hatching. The two subspecies freely interbreed under
experimental conditions and remain fertile through several generations.
[ IV] Control measures
Infected hairs should be combed and lice collected in this way should be
killed mechanically. The infested head should be regularly shampooed
having suitable insecticides such as pyrethrin or allethrin. There is a
commercial preparation 'Medicar' for killing head louse. Clothing infested
with body louse should be washed in boiling water containing any
detergent. Use of chlorinated hydrocarbons such as BHC, DDT etc. should
be avoided as they cause severe health problem.
6. House Flies
The detail account of house fly has already been given in chapter 20.
The above described species of insects attacking humans may also
infest our livestock such as mosquitoes, sand flies, house flies, etc.
There are some more insects that either annoy the humans or cause
direct injury to them, e.g., crab louse (Pthirus pubis), wasps, bees, etc.
INSECTS INJURIOUS TO LIVESTOCK
Like humans, the domestic animals also suffered with attack of a variety of
insects. Certain insects directly damage the animals while others transmit
diseases and functional disorders in them. Due to the insect infestation, the
productivity of affected animals is impaired and the utility of farm animals
is reduced resulting in great economic loss. Most of the insects attacking
domestic animals belong to Diptera, Mallophaga, Siphonaptera and
Siphunculata
(=Anopleura).
1. Buffalo Gnats or Black Flies (Diptera : Simuliidae)
B uffalo gnats are strongly built dark flies whose females are vicious biters
and suck blood of cattle. Species of Simulium such as S. aureohirtum,
S. dam nosum, S. indicum , S. sim ile are almost cosmopolitan in distribution
and are pestiferous in some parts of the India, Africa, South-east Asia,
Canada, USA and Latin America.
(Z-57)
Insects Injurious to Man and Livestock
[ 305
[ I] Importance
S. indicum commonly known as 'potu fly' is a troublesome cattle pest in
parts of Himalayas. Another species S. colum bascense is at times a great
scourge of man and domestic animals. It often appears in enormous
swarms and the flies attack orifices of the body entering the ears, nostrils,
margins of eyes, etc., in great numbers, and their punctures produce an
inflammatory fever often resulting in death. The adult flies occur in the
neighbourhood of streams and rivers or any flowing water. Only few
species transmit filarial disease (onchocerciasis caused by a nematode
Onchocerca volvulus transmitted by S. damnosum) in man.
[ II] Appearance
B uffalo gnats are small, usually dark-coloured with short legs, broad wings,
and humpbacked appearance (Fig. 6A). Mandibles are elongated
particularly in females for biting and sucking the blood.
[ III] Life cycle
The female lays eggs either on herbage or stones, above or beneath the
surface of the water. Females of S. maculatum submerge to a depth of 30
cm during oviposition and the eggs are laid on vegetation and are coated
with a gelatinous secretion. The larvae are invariably aquatic and require
swiftly flowing water, like streams (Fig. 6B). They are somewhat
club-shaped and swollen posteriorly. They pupate in cone-shaped cases
attached to objects in the water (Fig. 6 C,D). The life cycle completes
within 5-8 weeks.
[ IV] Control measures
Burning of pieces of dry wood mixed with damp shrubs and foliage, or
agricultural wastes so as to obtain a constant supply of smoke, is a
practicable and effective method of protecting livestock from the attack of
gnats. Application of insecticides in water bodies should be avoided as they
may be fetal to aquatic fauna.
A
Fig. 6. Buffalo gnat, S1mulium simile
case.
(A) Adult, (B) Larva, (C) Pupa, (D) Pupa
m
pupal
(Z-57,
306 1
Insects Injurious to Man and Livestock
2. Horse Flies (Diptera : Tabanidae)
Horse flies are found throughout the world, with different species
dominating specific regions. The common species found in India are
Tabanus rubidus and T. striatus. Other species of horse flies are Chrysops
dispar and Haematopota javana. Like gnats, only female horse flies suck
the blood which continues to ooze from the wounds even after the fly has
left the animal. Males feed only on nectar and honeydew. These insects
sometimes attack man also.
[ I] Importance
Horse flies are significant livestock pests in all areas of the world. Females
inflict deep wounds in animals, causing considerable blood loss that the
flies sponge up with their specialised mouthparts. Under heavy infestations
(more than 50 flies on an animal), they suck about 1 00 ml of blood per
animal per day. Fly attacks result in both reduced weight gain and milk
production. The flies transmit anthrax (caused by a bacterium, Bacillus
anthracis, death is caused by toxemia), tularemia (caused by a bacterium
Francisella tularensis) and surra (caused by the trypanosome Trypanosoma
evansi). Members of the genus Chrysops, commonly called deer flies, often
annoy and attack humans and may transmit Loa Loa in human beings.
[ II] Appearance
Adult horse flies are about 1 0-25 mm long with large bright green
iridescent eyes (Fig. 7). The eyes are contiguous in males but well
separated in females. Head is somewhat hemispherical in shape. Adult flies
are dark brown to black, with stripes on the abdomen and wings mottled
with dark patches.
[ III] Life cycle
The eggs are cylindrical or spindle shaped, from 1 to 2.5 mm long and are
found in layers of 300 to 1 000 on the leaves and stems of plants growing
in water or marshy places. The eggs hatch in 5-7 days. Newly hatched
larvae burrow into the mud and begin feeding. Larvae (maggots) are
tapered at both ends and are white to tan. Many species have black bands
around each segment of the body, and some species are 50 mm long when
full-grown. The larvae feed on vegetable matter or on small animals. The
horse flies overwinter in larval stage. When fully grown, larvae move to dry
soil to pupate. The pupae are markedly elongate and cylindrical. After 5
to 35 days adults emerge and mate, and females begin taking blood meals.
Life cycle completed within 4-5 months. Some species produce 2-3
generations per year, whereas others may produce only one every two to
three years.
(Z-57)
Insects Injurious to Man and Livestock
[ 307
[ IV] Control measures
The egg clusters should be periodically collected from the water bodies
and destroyed. An emu�sion containing 1 % pyrethrins and 1 0% piperonyl
butoxide applied at 4 gm per animal with a hand sprayer twice a week is
effective. Burning of damp agricultural wastes to provide continuous smoke
protect the animal form insect bites.
Fig. 7. Horse fly, Tabanus striatu5.
Fig. 8 The ox-warbel fly, Hypoderma lmeata.
3. Warble Fly (Diptera : Oestridae)
Warbel flies are pests of domestic animals such as cattle, sheep and goats.
They are well distributed in USA, UK and India. In India, the ox-warbel
fly, Hypoderma lineata is prevalent in arid regions such as Punjab, western
Uttar Pradesh and Rajasthan. In some area it may infest 50-90% cattle.
The goat-warbel fly H. crossii is well distributed in Punjab and certain
parts of Uttar Pradesh.
[ I] Importance
The adult warbel flies do not bite or injure the cattle when they oviposit
but they are very annoying to cattle. The larvae are parasitic and live in
tissues of the hides of cattle and seriously affecting their health. The holes
made in the skin by escaping larvae reduce the value of the hide when it
is made into leather.
[ II] Appearance
The warbel flies are recognised by their vestigial mouthparts, the entire life
of the adult being exclusively devoted to procreation. The adults are large,
stout-bodied flies that resemble bees (Fig. 8) and are fast flier.
[ III] Life cycle
The female deposits about 500-800 eggs in small batches on the legs of the
cattle near about hoofs. The incubation period is about three days and the
Insects Injurious to Man and L ivestock
308 J
newly hatched larvae penetrate the skin and disappear into the internal
tissues of the animal. The larvae constantly burrow inside the tissue and
after remaining in the mucous membrane of the oesophagous of the host
for some days, they appear on the back where they develop in swellings or
' warbels' just under the skin. This whole journey takes about seven month.
The larvae drill minute holes in these warbels for respiration. When full
grown ( 1 6-26 mm), they escape through these holes and drop into the soil
for pupation. The pupal period lasts after three weeks and adult flies
come out.
[ IV] Control measures
Control measures against warbel flies usually comprise the application of
larvicidal dressings (a mixture of tobacco dust and lime) on the warbel
tumours and washing with powdered derris root and soft soap.
Removal
or burning the hairs close to hoofs in order to destroy the eggs during the
oviposition season, i.e., monsoon months also pryvent the infection.
4. Screwworm Fly (Diptera : Calliphoridae)
The screwworm fly,
tropical
and
Cochliomyia hominivorax
subtropical
regions
was menace for livestock in
of the New
World.
The
sterile-insect
technique, conceived with this insect, was observed successful against the
fly. Before eradication, in 1 935, 1 2,00,000 cases of infestation and 1 ,80,000
livestock deaths
were reported
in
the USA
and livestock
losses
were
estimated over $ 100 million. At present it is restricted only in Central and
South America.
[ I] Importance
The screwworm
is a serious pest of livestock and wildlife. The species
infests cattle and other wild mammals when they develop open wounds, on
their bodies as with dehorning, castration, or fly and tick bites. The females
oviposit on these wounds, and developing larvae (maggots) feed directly on
flesh and wound exudate. The odour from infested animals attracts more
female flies, which lay additional eggs on the wounds. Infested animals may
die if not treated.
[ II] Appearance
Adults are active year-round in warm climates. They are metallic blue to
bluish-green, with an orange to brown head and three dark stripes on the
top of the thorax (Fig. 9). The arista is markedly plumose.
[ III] Life cycle
Adults feed on manure juices and wound exudates. Females oviposit from
1 0 to 500 eggs in about 1 6 hours, however, a single female can oviposit up
Insects Injurious to Man and Livestock
[ 309
to 3,000 eggs. E ggs are 1 mm long and white. After hatching, the maggots
feed gregariously on live flesh and fluids of the open wound. After 4-9 days
passing_ through three stadia, the full grown maggots which are white and
about 1 6 mm long h, drop to the ground and pupate in soil. Puparia are
dark brown and about 10 mm long. During winter pupae hibernate in soil
for about two months, until warmer weather. Adults emerge in 7 to 54
days, depending upon temperature. Flies mate two to five days after
emergence.
[ IV] Control measures
The American entomologist, E. F. Knipling, in 1 937, developed a method
to sterilise male flies. Male pupae were sterlised in the laboratory with
Cobalt- 60, and the newly emerged males were released into the
environment. Sterilised males, by advantage in numbers, would outcompete
normal wild males for females. Because females are monogamous they
produced infertile eggs. By this technique the screwworm was eradicated
from most of the regions of North America.
Fig_ 9. The screw-worm fly,
Cochlwmyza homm1vorax_
Fig. 1 0 Louse fly, Hippobosca
maculata.
5. Louse Flies (Diptera : Hippoboscidae)
Louse flies are parasitic on cattle and dogs.
[ I] Importance
Louse flies suck the blood of mammals. Hippobosca maculata is a fairly
large flies and attack cattle, horses etc. while its another species
H. capensis, commonly known as kukunnakhi, is smaller and infes� s dog.
Both species are tropicopolitan in distribution and both the species are
widespread in India. As they suck a considerable amount of blood, they
hamper the health of the cattle and pet dogs. They are not known to
transmit any disease into their hosts.
310 1
Insects Injurious to Man and L ivestock
[ II] Appearance
The body of Hippobosca species is brownish in color and is flattened and
leathery (Fig. 1 0). The head is sunken into the anterior part of thorax.
Proboscis is retractile with a sheath formed of labial palpi. Legs are short
and shout with tarsal claws. The wings are provided with white spots.
[ III] Life cycle
The females are viviparous and deposits full-grown larvae on the ground or
on other objects, where they pupate after a short period. The pupa is
black, seed-like and measures about 5 mm in length. The pupal period is
of about 25-30 days.
[ IV] Control measures
Application of fenitrothion 50% EC @ 1 00 ml in 10 litres of water or
deltamethrin 2.8% EC 2 ml/litre water has been found to be effective. The
animals should be sprayed individually. Use of pyrethrum powder on the
tissues. Such insects include: house flies, mosquitoes, sand flies, fleas
which are dealt in the first section of the chapter. Other insects of
interests are: Haematopinus (H. suis
hog louse, H. tuberculatus buffaloes), Musca crassirostris (blood sucking fly- host cattle), Stomoiys
calcitrans (stable fly - blood sucking pest of cattle), Gastrophilus
intestinalis (horse bot fly), etc.
-
Important Questions
I.
2.
3.
Descnbe in bnef the h fe cycle of and control measures for two insects transmittmg
human diseases.
Give an account of certain insects infesting our cattle and suggest their control
measures.
Write short notes on: Hippobosca, Ph/ebotom us, Ped1cu/us, Xynopsylla.
22
Insects Transmitting
Diseases 1n Plants
(Aphids and Whiteflies)
There are a number o f insects that transmit pathogens o f several plant
diseases such as viruses, bacteria, phytoplasma and fungi. These insects are
known as vectors. Most of the insect vectors belong to the order
Hemiptera, particularly aphids, leaf hoppers, whiteflies, mealy bugs, thrips
and some beetles. Among Hemiptera, aphids predominate others. Out of
620 plant viruses known, about 200 are transmitted by the aphids. Next to
aphids, whiteflies transmit several plant viruses, e.g. , Bemisia tabaci
transmits Bhendi vein-clearing disease, dolichos yellow mosaic, tomato
leaf-curl, papaya leaf-curl, etc. Following table summarises the insect
vectors transmitting plant virus diseases.
Insect vector
Virus
Aphids
Aphis gossypii
Aph1s cmccivora
Myz.us persicae
Pentaloma mgronervosa
Rhopalos1phum maidis
Papaya mosaic, cucumber mosaic, chilli mosaic
Cowpea mosaic, papaya mosaic
Cucumber mosaic, potato mosaic, tomato mosaic
Banana bunchy top, cardamom mosaic
Sugarcane mosaic, maize dwarf mosaic, barley yellow
dwarf
Whiteflies
Bem1sia tabacz
Th rips
Thnvs tabac1
Bhend1 yellow vem mosaic, dohchos yellow mosaic,
tobacco leaf-curl, tomato leaf-curl, papaya leaf-curl
Tomato snotted wilt
312
J
Insects Transmitting Diseases in Plants
APHIDS
1. B iology of Aphids ( Hemiptera : Aphididae)
Aphids (Hemiptera : Aphididae), popularly known as plant lice or
ant-cows are tiny plant sap sucking homopteran insects varying in size
between 0.7 to 7.0 mm in length. They form one of the major groups of
phytophagous
insects
due
to
their
polyphagism,
polymorphism,
parthenogenesis,
viviparity and fast development,
host alteration,
transmission of plant viruses etc. In suitable conditions their power of
multiplication is astronomical so that they rapidly attain pest status in
agro-ecosystem.
[ I] Distribution
Aphids are cosmopolitan in distribution. Compared to warm tropical
conditions, cool temperate climate holds more aphid species. Out of
estimated world fauna of over 4700 species of aphids, about 787 species
are known from India infesting over 1 200 species of plants. The
stratigraphic distribution varies from plains up to an altitude of 4500
metres. In India, the north-east states harbour the largest proportion of
aphids. The temperature range as well as the number of plant species
greatly influences aphid species diversity. Aphids fly rather slowly and
heavily but with the help of the wind they occasionally make astonishing
extensive migrations. Air current may carries them to altitude of about
I 000 m. On calm, warm and humid autumn days thousands of them float
in and out among one another, all borne in the same direction by the
gentle wind. Most of the aphids are air borne twice a year, once in winter
(November to January) and the other during spring (March to April).
[ II] Host-plant specificity and behaviour
Aphids exhibit high degree of plant specificity. Each species within a genus
feeds on certain plant species within a clearly defined group of genera.
Primary and secondary host-plants of host-alternating aphids are
taxonomically distinct. Aphids on agricultural crops tend to have a wider
host range than related economically unimportant species. Majority of pest
aphids restricts their feeding to species within one plant family. The most
polyphagous cosmopolitan aphid, M. persicae is known to infest about 460
species of plants.
Most aphid species show some degree of gregarious behaviour. It
helps to protect them against natural enemies. If one aphid is attacked
by a ladybird beetle, for example, the information is rapidly
communicated to the whole cluster as each aphid disturbs the next. This
Insects Transmitting Diseases in Plants
[ 313
allows the winged ones to fly off and the apterae to walk round to the
other side of the leaf by the time the beetle has finished eating its
original prey.
[ III] Importance
The aphids attack all parts of the plants including roots. Some of them
directly damage the plants by sucking their nutrients which causes general
devitalisation of plants. They also indirectly affect the health of the plant
by their copious secretion of honeydew that occludes the stomata! openings
of the leaves and thus hamper their normal physiological processes like
photosynthesis and respiration. Deposition of honeydew on leaf surface
also allows the growth of black mould which in tum proves detrimental to
the plant life. Aphids are also most important plant virus vector. Out of
620 plant viruses known, about 200 are transmitted by the aphids. M.
persicae alone transmits more than 1 00 different viruses to its polyphagous
feeding habits. Most of the non-persistent viruses [e.g., Tobacco Mosaic
Virus (TMV)] are transmitted only by aphids. Few semipersistent viruses
[e.g., Beet Yellow stunt Virus (BYV)] and persistent viruses [e.g., Potato
Leaf Roll Virus (PLRV)] are also transmitted by them. Due to infestation
of almost all parts of plant surfaces, the aphids are of great agricultural
significance and now-a-days they are being considered as most serious
pests of agriculture and horticulture. Usually in absence of primary
agricultural crops, i.e., when harvesting of one crop is done, they tide over
in unfavourable season on other economic crops. in lieu of these latter
crop plants, they just thrive upon many wild plants. Globally, more than
250 species are pests of both agricultural and horticultural crops. The
major pest aphids are : Aphis gossypii (cotton aphid), A. craccivora (black
bean aphid), Brevicoryne brassicae (cabbage aphid), Diuraphis noxia
(Russian wheat aphid), Lipaphis erysimi (mustard aphid), Myzus persicae
(green peach aphid), Acyrthisiphon pisum (pea aphid), Rhopalosiphum
maidis (corn aphid), Sitobion avenae (grain aphid), Uroleucon compositiae
(safflower aphid), etc.
[ IV] Morphology of the aphids
The aphids are either yellow, brown, red, green, black, pink and purple
and various shades of these colours, which matches with the colouration of
the leaf, flower, fruit and stem of the host-plants on which they feed. This
affords them certain amount of camouflage. The body of the aphid is
usually divisible into the head, thorax and abdomen, however, m some
species it is very difficult to divide the body due to tendency of fusion of
the segments. The head is usually dorsoventrally flattened. The number of
antenna! segments varies between one and six. The last antenna! segment
has a stout base and a short to very long slender terminal portion. the
3 14 J
Insects Transmitting Diseases in Plants
processus terminalis. The head also bears three-faceted eyes. The proboscis
is laid back beneath the body when not in use. It may be so long, especially
in the species that live on trees that it sticks out beyond the end of the
abdomen. The prothorax or the entire thorax may be fused variably with
the head. Each thoracic segment bears a pair of legs having usually five
segments: coxa, trochanter, femur, tibia and 2 tarsomeres. The alatae bear
a pair of wings, these are of similar consistency but the forewing is always
longer and broader than the hindwing. The abdomen consists of nine
segments, the 9th being the cauda. Wax-plates or pores may be present on
the dorsum of the thorax and abdomen. Dorsolaterally on the abdominal
segments 5 or 6 usually occur a pair of siphunculi or cornicles of variable
shape and size. The secretion of the cornicles has a defensive function.
They either disturb the small predators and parasitoids or induce dispersal
of aphids feeding nearby.
[V] Life cycle
The life history of aphids is very complicated. The aphids have several
biological peculiarities such as prolific breeding, polyphagy, advanced
degree of polymorphism, host alternation and high potential for rapid
evolutionary changes because of parthenogenesis and polyvoltinism. Some
aphids are anholocyclic (continuously parthenogenetic), while others living
in temperate climates are holocyclic (sexual generation alternates with
parthenogenetic reproduction). In a year' s time, numerous generations may
succeed one another, for even at moderate mean temperatures the nymphs
which moults four times at most, complete their development in little more
than 10 days. A genralised life-cycle pattern is illustrated in figure 1 .
0
�
<::)�
�1'
��
O"v.
�
�
�
��
�'5
�
Fig. I . Generalised life-cycle of apluds
Insects Transmitting Diseases in Plants
I 31 5
Most of the Indian aphids are parthenogenetic and virginoparous for
most of the year but are capable of sexual reproduction with production
of eggs. They develop in parthenogenetic female without fertilisation.
Even embryos inside parthenogenetic females may contain embroys, i.e.,
a mother can have in its ovarioles developing embryos which in tum
also contain embryos, the future grand daughters. Thus, there is a
telescopic generation due to parthenogenesis and viviparity in aphids.
This results in reduced postnatal development periods and generation
time.
There is a more or less regular cyclic or anholocyclic alternation of
parthenogenetic oviparous and viviparous generations associated with
polymorphism, changes of food plants and mode of life. Several
generations often succeed each other, in which the males are extremely
rare or are totally absent. Individuals of the same generation often differ
considerably from one another. Some have fully developed wings others
have atrophied wings and still others are apterous.
Low temperature, short day length and physical condition of host
plants are regarded as important factors governing the production of
hills
fundatrices
fundatriginae
(?)
(?)
WINTER
holocycly with an
alternating anholocycly
migrantes
(immigrants)
oviparous females
viviparous m o rphs
(in mid and lower h ills)
AUTUMN & WINTER
Brassica
campestris)
(on
over summering
anholocycly with a
possible holocycly
viviparous morphs
(upto 45
generations or
more on all cruciferous
plants in 5-6 months)
plains
Fig
2
Possible hfe-cycle of L. ery.Hmt
m
India
316 J
Insects Transmitting Diseases in Plants
sexuales. Complete life histories of Indian aphids are not known. In
figure 2 the possible life cycle of mustard aphid, L. erysim i in India is
illustrated.
Aphids are remarkable on account of their peculiar mode of
development and the polymorphism exhibited in different generations of
the same species. Females may exhibit up to eight discrete phenotypes
which are genetically identical individuals. They differ in morphology,
physiology, numbers, timing of production, progeny sizes, developmental
periods, longevity, host preferences and ability to locate and utilise the
alternative
host
plants.
The
associated
phenomena
concerning
reproduction are : ( 1 ) parthenogenesis; (2) oviparity and viviparity; and
(3) the occurrence of generations in which the sexes are very unequally
represented, males often being wanting and frequently rare. With regard
to structure the phenomena are: (a) the production of totally different
types of individual of the same sex either in the same or different
generations; (b) the production of individuals with perfect and also
atrophied mouthparts; and (c) the production of individuals of the same
sex but differing as to the gonads. Associated with habits are : (i) host
alternation, involving migration to totally different plant hosts;
(ii) different modes of life of the same species on the same host; and
(iii) different habits of individuals of the same generation. In extreme
cases almost all the above phenomena may occur associated with the
annual cycle of an individual species.
The most usual life-history of a generalised aphid is as follows. The
eggs laid during the previous autumn by sexual females hatch by the
commencement of spring and give rise to apterous parthenogenetic
viviparous females after passing through usually four nymphal instars
within a few days. The latter produce a new generation of similar forms
among which a few winged (alate) females may occur. A variable
number of generations of this kind are produced throughout the summer
and winged viviparous females often become common. The latter are
concerned with the migration and dispersal of the species and are
produced in varying numbers in different generations. At times these
winged females appear in swarms and cover the vegetation. Those
individuals which find 5upporting food plants similarly reproduce on
their own account. Towards the end of summer or in the autumn their
progeny, and also those of the apterous forms that remained on the
original plant, give rise to sexual males and females.
[ VI] Polymorphism in aphids
During the life cycles of a typical migratory aphid, the following sequence
of polymorphism (Fig. 3A-H) is met with :
Insects Transmitting Diseases in Plants
[ 317
�
A
B
D
E
G
F
H
Fig. 3. Different morphs of a typical aphid. (A) Egg, (B) Ftrst instar nymph of fundatnx.
(C) Fundatrix, (D) Apterous viviparous female, (E) Nymph of alate viviparous female, (F)
Oviparous female, (G) Alate viviparous female, (H) Male.
1.
Fundatrices.
These
are
usually
apterous,
viviparous,
parthenogenetic females which emerge in spring from the overwintered
eggs. The sense organs, legs and antennae are not so well developed as
in succeeding apterous generations. The reduction of the parts 1s
apparently correlated with increased reproductive capacity.
2. Fundatrigeniae. These are apterous, parthenogenetic, v1v1parous
females which are the progeny of the fundatrices and live on the
primary host.
3. Migrantes. These usually develop in the second, third or later
generations of fundatrigeniae and consist of winged parthenogenetic
viviparous females. They develop on the primary host and subsequently
fly to the secondary host.
4. Alienicolae. These are parthenogenetic, viviparous females
developing for the most part on the secondary host. They often differ
markedly from the fundatrices and migrantes; many generations may be
produced comprising both apterous and winged forms.
3 18 1
Insects Transmitting Diseases in Plants
5. Sexuparae. These are parthenogenetic viviparous females which
usually develop on the secondary host, the alate forms migrating to the
primary host at the end of the summer. The sexuparae terminate the
generations of alienicolae by giving rise to the sexuales.
6. Sexuales. These usually appear only once in the life-cycle and
consist of sexually reproducing males and females, the latter being
oviparous. The females with rare exceptions are apterous, and
distinguishable from the apterous viviparous generations of the same sex
by the thickened tibiae of the hind legs, and the greater body length.
With non-migratory species the terms migrantes and alienicolae are
not applicable. In these cases, the winged and wingless viviparous
females are more conveniently referred to as fundatrigeniae alatae or
apterae as the case may be, and either one or the other may give nse
to the sexuparae.
[ VII] Control measures
High reproductive rate and multiple host sequences provide optimal
conditions for aphid population development. The varied habitats, seasonal
population development and intra- and inter-crop and wild host movement
present an extremely complex and difficult challenge requiring new
approaches for formulating control and suppression methodology for
aphids.
There is really no easy way of controlling these vector insects. In
the past adults were easily killed with insecticides but pesticide
resistance in their populations is a common problem. These insects have
become resistant to chemical insecticides quite rapidly and the wisdom
of relying only on chemical insecticides is questioned. Therefore, an
integrated approach becomes essential to manage their population. This
approach combines the cultural and biological practices with application
of selective insecticides.
1. Cultural control. Cultural control is more of prophylactic in
nature than of curative. Following are the important agronomical
practices that directly or indirectly affect the aphid biology and keep
their population at low level.
(a) Planting time. This practice is more meaningful if planting of
crop is done on the basis of information on the population dynamics of
aphid(s) as this is purely based on the phenological asynchrony of the
crop with aphid. It is now established that early sown crop either
escapes aphid attack or has_ less degree of infestation. Brassica
campestris var. toria escapes attack of Lipaphis erysimi if sown in mid
September. Other cruciferous oilseed crops suffer less if they are sown
between middle of October to first week of November depending on the
ecoclimatic belt. Safflower when sown early escapes the attack of
Insects Transmitting Diseases in Plants
[ 31 9
Uroleucon carthami, particularly at early stage of the crop. Similarly,
lentil planted in early November also showed higher population of A.
craccivora as compared to crop sown in late November or early
December.
(b) Manual removal of infested twigs. It is essential to nip the early
infestation of aphids in the buds or twigs as aphids after appearance
settle on the twigs and multiply from where they disperse to adjoining
plants and field.
(c) Crop geometry. Plant density have direct influence on the plant
growth as well as yield of the crop. Each plant competes with other for
nutrients, moisture, sunlight etc. Dense population may be congenial for
some insects whereas it may be unfavourable to others.
(d) Intercropping. It includes mixed intercropping: row intercropping,
strip
cropping,
relay
cropping
and
passageway
intercropping.
Intercropping is preferred over monoculture to avoid risk of crop
failure, better utilisation of farm resources and Jabour, and to protect
the crop from insect pests. Intercrop reduces the attraction of pest to
the host, adversely modify the microclimate of the pest habitat which
may result in impeded dispersal, increased emigration and reduced
survival of the pest in the intercrop. It has been shown that infestation
of A. gossypii is less in pure crops of green gram, black gram and
sunflower as compared to main crop in combinations with cotton. When
beans are intercropped with older or densely populated maize, fewer
plants of former were infested by A. fabae. Similarly, intercropping of
groundnut with pearl millet reduced the incidence of A . cmccivom on
main crop.
(e) Water management.
Water management is one of the most
important factors responsible for proper growth and development of
plant and higher yield. Under drought and/or rainfed conditions plant
looses turgidity as well as sap pressure which may result in reduction of
feeding, reproduction and survival in aphids. These conditions also
stimulate dispersal of aphids. Drought condition increases the solute
concentration and sap viscosity to such an extent that feeding by aphid
is drastically hampered. Population of L. erysimi increases on mustard
crop, B. brassicae on cabbage and A. craccivora on lentil
and
groundnut under irrigated conditions. Mustard crop should be irrigated
twice to avoid heavy aphid infestation.
(f) Fertility management. There are 20 essential plant elements
which are needed for the growth and development of the plants. Out of
these, N, P and K are major nutrients. In general, high nitrogen supply
results in increased tissue softness and water content as carbohydrates
making the plant more susceptible to attack by aphids.
Presence of
higher level of phosphorus makes the plant less susceptible for aphids.
(Z-57)
320 J
Insects Transmitting Diseases in Plants
Thus manipulation of these major nutrients can be used to manage the
insect population under control. Higher proportion of N:P:K (80:40:30)
showed higher population of L. erysimi whereas 40:80:40 ratio reduced
aphid infestation. Similarly, high N:P:K (225:90:45) increased population
of B. brassicae on cauliflower.
(g) Removal of alternate hosts. Important aphids like, M persicae,
A . cmccivom, A . gossypii and others are polyphagous in nature and
thrive well on cultivated as well as on wild plants. These wild plants
and weeds provide suitable habitat and food for the aphid during off
season. Such plants should be removed to check the initial population
ready for attack on the main crop . D e struction of yellow flowering
weeds has been found useful against M. persicae in potato field.
(h) Trap crop. Trap crop is generally used to ward off the insects from
the main crop. It prevents the insects from reaching the main crop. Trap
crop is more attractive and susceptible than the main crop. The planting
of trap crop is done in such a time that its susceptible stage coincides with
peak activity of the insect. Mustard as trap crop has been found very useful
in the management of L. erysimi and B. brassciae on cabbage when planted
in mustard 2 : cabbage 9 ratio.
(i) Distance from other crop. Closely related or crops grown for
different purposes should be planted distantly so that insects from one
crop may not be able to reach other crop where physiological
conditions suitable for aphid deteriorates. Toria and Sarson should be
sown away from mustard and other long duration brassicas. Seed plot of
potato should be away and located upwind from the commercial potato.
(j) Rogueing and avoidance of ratooning. Rogueing of aphid infested
plants and avoidance of ratooning have been found very useful in the
management of banana aphid, Pentalonia nigronervosa, a vector of
bunchy top virus and A. gossypii which transmits cucumber mosaic virus.
2. Biological control. Biological control of aphids in the fields has been
successfully achieved in several parts of the world because their predators
and parasitoids have great potential in managing their populations in spite
of certain limitations. The braconid parasitoids Aphidius colem ani,
A . sm ithi, A . matricariae, Diaeretiella mpae, Ephedrns cemsicola, Aphelinus
mali etc. and the coccinellid predators such as Coccinella spp.,
Cheilomenes spp ., syrphid flies, etc. have successfully been used to control
the population of several aphid species infesting varieties of crops
worldwide.
3. Chemical control. As far as possible chemical control of aphid
pests should be avoided as most of them are fatal for honey bees and
other beneficial insects particularly the parasitoids and predators of
aphids. Only when the use of insecticide becomes inevitable, then
following insecticides may be used to control the aphid population in
(Z-57)
Insects Transmitting Diseases in Plants
[ 32 1
different agroecosystems
chinimix (5%),
chlorpyriphos
(0.04%),
dichlorvos (0.20%), dimethoate (0.06%), endosulfan (0.07%), M.l.P.C.
(50%), malathiol'l.' (0.20%), methyl-o- demeton (0.05%), monocrotophos
(0.08%), phosphamidon (0. 1 0% ), quinalphos (0.05% ). Other insecticides
like
bifenthrin,
cyfluthrin,
cypermethrin,
dicrotophos,
ethiofencarb,
fenvalerate,
furadan,
imidacloprid,
methamidophos,
parathion,
permethrin, pirimicarb also give satisfactory results.
WHITEFLIES
Among the whiteflies, Bemisia tabaci (commonly known as sweetpotato
whiteflies) is one of the most injurious insect pest spreading several kinds
of plant viruses.
[ I] Distribution
There are more than 1 1 00 whitefly species in the world. B. tabaci attacks
more than 500 species of plants worldwide and is one of the more
pestiferous of the group. It infests and spreads plant viruses on the
following crop plants: cauliflower, cabbage, waxgourd, cucumber, edible
gourds, eggplant, fig, guava, lettuce, ghia torai, pumpkin, rose, soybean,
squash, sweetpotato, tomato, watermelon, beans, pea, etc. The B. tabaci has
been reported as a serious pest of cultivated crops in tropical and
subtropical areas including Africa, Asia, Central America, North and South
America, and the West Indies where it is also known as the tobacco
whitefly and cotton whitefly.
[ II] Habit and habitat
Adult females oviposit preferentially on young foliage and crawlers do not
move any significant distance from their eclosion site thus, immature stages
tend to be distributed vertically on the plant with older stages found on
progressively older leaves. Like aphids, sweetpotato whiteflies are also
attracted to t� yellow colour. Short-range movements within and between
'cultivated and . weed host plants are known to take place regularly. There
is also evidence bf long-range whitefly migration. The direction of whitefly
flight is primarily determined by the wind. They land on particular plants
mostly by chance, electing to stay on suitable hosts and move away from
those that are not.
[ III] Importance
The whiteflies are recognised as an important pest on many crops. Like
aphids, three types of damage may be caused by them: direct damage,
indirect damage and virus transmission. Direct feeding damage is caused
by the piercing and sucking sap from the foliage of plants. This feeding
(Z-57)
322 1
Insects Transmitting Diseases in Plants
causes weakening and early wilting of the plant and reduces the plant
growth rate and yield. It may also cause leaf chlorosis, leaf withering,
premature dropping of leaves and plant death. Indirect damage results by
the accumulation of honeydew produced by the whiteflies. This honeydew
serves as a substrate for the growth of black sooty mould on leaves and
fruit. The mould reduces photosynthesis and lessens the market value of
the plant or yields it unmarketable. A small population of whiteflies is
sufficient to cause considerable damage by transmitting viruses. Plant
viruses transmitted by whiteflies cause over 40 diseases of vegetable and
fiber crops worldwide. Among the 1 100 recognised species of whiteflies in
the world, only three are recognised as vectors of plant viruses. The
sweetpotato whitefly B. tabaci
is considered the most common and
important whitefly vector of plant viruses worldwide. It is also the only
known whitefly vector of viruses categorised in the geminivirus group. In
the past decade, whitefly-transmitted plant viruses have increased in
prevalence and distribution. The recent impact has been devastating with
yield losses ranging from 20 to 100 per cent, depending upon the crop,
season, and prevalence of the whitefly. Some diseases associated with
sweetpotato whitefly include: Lettuce necrotic yellows, irregular ripening of
tomato, silver leaf of squash, cotton leaf curl, tobacco leaf curl, and cassava
mosaic.
[IV] Appearance
Adult sweetpotato whiteflies are small, approximately 25 mm in length,
with a pale yellow body and two pairs of white wings and covered with a
white waxy powder. At rest, wings are held in an inverted V position. Their
compound eyes are red.
[V] Life cycle
Female whiteflies deposit pear-shaped eggs (Fig. 4A) )into the mesophyll
or inner tissue of the leaf from the lower surface. Eggs are attached to the
leaf by a stalk-like process. Eggs are white when first laid, and become
brown prior to hatching. They are generally laid on the underside surface
of the younger, upper leaves of the plant. Females lay from 28-300 eggs
dependin� on host and temperature. Egg densities can be as high as 2
eggs/mm .
Whiteflies have six life stages: the egg, four nymphal stages (Fig.
4BD), and the adults (Fig. 4F). The development time from egg to
adult range from 15- 70 days depending upon temperature and plant
host. Development occurs in temperatures ranging from 1 0 to 32"C
however, the optimal temperature is 27"C. Overlapping generations occur
throughout the year.
The first nymphal stage is called crawlers and the last stage is often
referred to as the pupa (Fig. 4E). After hatching the crawlers move a short
·
(Z-57)
Insects Transmitting Diseases in Plants
A
8
c
D
I 323
E
F
Ftg. 4. Different developmental stages of whitefly, Bemisia tabaci. (A) Egg, (B-D) First to
third mstar larva, (E) Pupa, (F) Adult.
distance and settle to feed. Once settled, the subsequent three nymphal
stages are scale-like and sedentary. Nymphs are creamy white to light green
and oval in outline. The total nymphal period lasts about 2-4 weeks.
Adults usually emerge from their pupal cases in the morning hours
and may copulate a few hours later. Oviposition occurs from 1 to 8
days after mating. Adult life span ranges from 6-55 days depending on
temperature.
B. tabaci may reproduce parthenogenetically. Virgin
females produce only sons.
[ VI] Control measures
The whiteflies resemble aphids in most of the reproductive biology and
therefore, the procedure of their control is more or less the same as
adopted for the aphids. Following are the control measures by which the
population of whiteflies may be regulated eco-friendly :
1. Cultural control. Barriers such as row covers that effect
phototactic responses of whiteflies, have shown some promise in delaying
or reducing disease incidence, but are not useful when whitefly
populations and virus innoculum levels are high. Control of weeds
adjacent to cultivated fields, the use of trap crops, and implementation
of crop free periods may be effective in reducing vector populations in
certain cropping systems. Following are the some cultural practices that
should be adopted to prevent attack of whiteflies.
(a) Sequential plantings. New sweetpotato should not be planted
near fields that are presently experiencing sweetpotato whitefly problems.
Doing so would lead to early establishment of the pest and can lead to
serious losses.
(b) Alternate hosts of whitefly. Weed and volunteer crop hosts should
be removed from the field well before new plantings are established.
(c) Crop transplants. Crop infestations can begin from infested
transplants. Extra care should be taken in controlling whitefly m
seedling trays before transporting them to an uninfested field.
324 1
Insects Transmitting Diseases in Plants
(d) Post-harvest practices.
after crops
have
been
Whiteflies
abandoned.
have been disconnected.
It is
continue
This
occurs
to
develop
even
on
plants
if irrigation
a good practice to spray
lines
and plow the
plants immediately after last harvest.
It can
(e) lntercropping t>f alternative host plants.
be an alternative
method for the reduction of pests in certain situations. Beneficial insects
are often increased and their activity enhanced on intercrops. Cucumber
planted in alternating rows
30 days before tomato delayed infection of
the tomato with the whitefly-vectored tomato yellow leaf curl virus.
It
(/) Host plant resistance.
management
component
for
has
potential
suppression
as
of
an
integrated
sweetpotato
pest
whitefly
populations and may provide a more bio-rational approach for reducing
the
impact
of
sweetpotato
whitefly
transmitted
viruses
and
plant
disorders than reliance on pesticides.
2. Biological control.
whitefly.
The species
sweetpotato
whitefly.
whiteflies .
These
anthocorids
spiders
of
Similarly
include
and mirids;
and mites.
Several
are
true
ladybird
many
bugs
beetles,
of these
attack
Eretmocerus
and
there
various
Some
parasitoids
Encarsia
are
the
predators
especially
lacewings,
sweetpotato
genera attack the
that
attack
predatory
bugs,
syrphid flies,
opportunistic
predators
ants,
of adult
whitefly, others are general feeders of whiteflies, still others are specific
predators
of
whiteflies.
The
Verticillium lecanii, Paecilomyces
Beauveria bassiana have been
fungi
famosoroseus, Peacilomyces farinosus,
and
demonstrated to be pathogenic for whiteflies.
3. Chemical control. The
whiteflies
immature
larvae
is
difficult
forms
and
insecticides
resistance
to
primarily
because
on
underside
pupae
located
have
effectively
has
developed
conventional
achieve
the
lower
in
the
controlled
rapidly.
Current
chemical
of
the
of the
plant
this
control
of
distribution
of
leaves,
canopy .
pest
reliance
in
on
with
the
the
older
A number of
the
past
chemical
but
control
must be considered to be a temporary measure untjl a satisfactory IPM
programme
can
be
developed.
The
insecticides
mentioned
for
aphids
may also be used to control whiteflies.
Important Questions
1.
2.
3.
4.
5.
Describe the generalised biology o f aphids.
Give an account of measures for controlling the aphids.
Describe the distribution, economic importance and life cycle of the whitefly Bemisia
tabaci.
Write . an essay on the insects transmitting viral diseases in plants.
Write short notes on : (i) Polymorphism in aphids, (ii) Mystery of aphid biology.
23
Insect Injurious to Crops
Modem
agriculture
is
continuously
facing
insect
problem
since
its
inception. Although less than 1 % of known insects are injurious to crops,
about 30% of all crops are damaged annually by insects inspjte of pouring
400 million tonnes of pesticides. All the crops cultivated by us are attacked
by a variety of insects before and after harvest. Each and every part of the
plants including roots, stems, leaves, flowers, fruits and seeds are cherishly
consumed by insects. India faces an annual loss of about 1 500 crores of
rupees due to damage caused by insects to agriculture. Following are the
description of some of the important insect pests infesting our cash crops
like cotton and sugarcane; oleiferous crops like mustard and ground nut;
cereals like paddy, wheat and maize, sorghum; vegetables like cabbage,
brinjal, tomato, potato, cucurbits, etc.; fruit trees like mango, apple, citrus,
coconut etc . ; pulse crops like pigeonpea, pea, gram etc . ; tobacco crops etc.
I
INSECT PESTS OF CROPS
Pests of Cotton
Cotton is the most important natural textile fibre in the world. The four
species of cotton
G. barbadense
viz., Gossypium hirsutum, G. arboreum, G. herbaceum
and
are grown under variable agro-climatic conditions ranging
from 8-32 °N !attitude and 70-80 °E. In the year 2000, target of cotton
cultivation was IS million hectare and production of 1 90 lakh bales : To
increase the productivity, Government of India has launched
'All India
Coordinated
established
Cotton
Improvement
Project'
(AICCIP)
and
Insect Injurious to Crops
326 1
research institutes such as Central Cotton Research Institute, Nagpur. It is
a cash crop in several parts of the country such as Punjab, Gujarat,
Maharastra and Andhra Pradesh. Insect pests are one of the primary
factors hindering the successful cultivation of cotton crop. Out of 1 326
species of insects recorded on cotton world over, only 1 62 attack cotton
crop in India. However, out of them only few are the most serious pests of
national importance which cause about 50-60% losses in seed cotton yield.
Following are the major cotton pest in India: Aphis gossypii (cotton aphid),
Helicoverpa armigera (American bollworm, see pests of pulse crop), Earias
vittella and E. insulana (spotted bollworms), Pectinophora gossypiella (pink
bollworm), Amrasca biguttula biguttula (cotton leaf hopper), Bemesia tabaci
(cotton whitefly, see chapter 22), Myllocerus undecimpustulatus maculosus
(cotton grey weevil), Thrips tabaci, Scirtothrips dorsalis (cotton thrips),
Pempherulus affinis (cotton stem weevil), Rabila frontalis (red boll worm),
Dysdercus cingulatus, D. koenigii (red cotton bugs), Oxycarenus
hyalinipennis (dusky cotton bug), Sylepta derogata (cotton leaf roller), etc.
Biology of few cotton pests are given below
[ I] The cotton aphid : Aphis gossypii
( Hemiptera : Aphididae)
1. Distribution. Generally distributed throughout temperate, subtropic, and
tropic zones, the cotton aphid occurs in all cotton-producing areas of the
world. In India it is recorded not only from the states of the country where
cotton is grown but also in non-cotton growing states where it infests a
number of crop plants and weeds (Fig. I ).
2. Host plants. In India, A. gossypii is highly polyphagous and sucks
the sap of more than 400 species of plants both cultivated as well as
wild. It is a potential pest of cotton, cucurbits , solanaceous vegetables,
pulses, groundnuts, guava, citrus, coffee, cocoa, peppers, okra, and many
ornamental plants including Hibiscus spp .
etc. Often the aphids are
attended by ants.
3. Importance. The nymphs as well as adults both suck the plant
juice and thus deprive the plants with nutrients so that they become
A
Fig. 1. Cotton aplud Aphzs gossypiz. (A) Wingless adult, (B) Winged adult.
Insect Injurious to Crops
[ 32 7
week. Severe infestation results in curling of leaves, stunted growth and
gradual drying and death of young plants. Black sooty mould develops
on the honeydew excreted by the aphids over the leaves which hamper
the
photosynthetic
ability
of the
plants.
If honeydew
falls
onto
open
cotton, the growth of sooty mould cause blackening of the cotton thread
reducing its quality and brings a low price for the grower. It is also a
vector of the persistent viruses of cotton.
4. Appearance. Apterous A. gossypii is a greenish brown soft bodied
aphid measuring 0.9- 1 .8 mm (apterae) or 1 . 1- 1 .8 mm (alatae),
small
however, the colour of apterae is very variable. Large specimens may be
dark green, almost black, but the adults produced in crowded colonies
at high temperature may be less than
to almost white.
1 mm long and very pale yellow
The siphunculi are dark.
Life cycle. The life cycle of A. gossypii is very complicated. It is
5.
a. polymorphic and adults of both apterae (wingless) and alatae (winged)
viviparous
are
and
reproduce
by
parthenogenesis.
The female deposits
80- 100 nymphs (8-22 nymphs/day) which become adults in 7-9 days on
cotton
after
overcrowded,
passing
the
through
number
of
four
moults.
winged
When
adults
the
increases
population
so
that
is
they
migrate from one plant to others. The detail life cycle of aphids are
given in chapter 22.
6. Control measure. As described in chapter 22, it is very difficult
to control the population of aphids because of their high reproduction
rate.
Some
control
procedures
are
also
described
therein.
In
cotton
(0.03% ), methyl parathion
(0.025%), methyl demeton (0.025%), profenofos (0.05%), monocrotophos
(0.04%), phenthoate (0.05%), pbosalone (0.05%) and triazophos (0.02%)
agrosystem
provide
foliar
spray
of
dimethoate
sufficient protection.
: Pectinophora gossypiella
(= Platyedra gossypiella) (Lepidoptera : Gelechiidae)
[ II] The pink bollworm
1.
Distribution.
The pink bollworm, native of India, is at present
distributed all over the world where cotton is grown such as USA, Africa,
Australia, and Asia. In India, it is found in all cotton growing states like
Punjab, Gujarat, Maharastra, Andhra Pradesh, and Tamil Nadu. In other
states it infests other malvaceous plants (Fig. 2).
2.
Host plants. Cotton is the major food plant of P. gossypiella.
Apart from this it also infests lady' s finger, hollyhock, okra and other
malvaceous plants.
3.
Importance. The species is a serious cotton pest throughout the
world, and in some areas, it can cause total crop destruction. Early in
the growing season, the larvae feed in the squares, attacking developing
flower
structures.
Usually,
this
damage
is
not
severe.
Later
in
the
Insect Injurious to Crops
328 1
B
�
Fig. 2. The cotton pink boll
(C) Adult
,
A
c
'l\Qrm, Pectinophoro gossypiel/a. (A) Full grown larva,
(B) Pupa,
growing season, however, larvae feed in the bolls on lint, carpel tissues,
and seeds. A single boll may contain up to 10 caterpillars. �e
infestation results in the seeds being destroyed in addition- to retardation
of lint development. Further infested bolls open prematurely al!d expose
it to invasion by saprophytic fungi. The seeds from- damaged bolls show
lower germination power. Despite quarantines, the species continues to
expand its range of distribution.
4. Appearance. Adults have a 12 mm wingspan and are greyish or
dark brown, with inconsistent black markings. The antennae are filiform
and the hindwings are deeply fringed. The adult moths are nocturnal
and fly around after dusk. It feeds on nectar.
5. Life cycle. The larvae overwinter in hollowed-out cottonseeds or
in plant debris in the field. In early spring, larvae pupate and in about
1 0 days, adults emerge. Adults are active at night. Females lay up to
200 eggs on the host plant near the bolls or in between bracts or on
buds and flowers, mostly in cluster of 2- 1 0 eggs. Eggs are oval,
flattened, striated, about 0.5 mm long, and white. Before hatching, they
become red. After 4-25 days, the eggs hatch, and developing larvae
undergo three or four moults (IO to 14 days). Young larvae are white,
with a brown head, whereas older larvae (fourth instars) display a
distinctive pink colouration. Full-grown larvae are approximately 1 3 mm
long. Larvae then leave the boll or remain in damaged seed to pupate
in a thin silken cocoon. Pupae are brown. The pupal period is about
6-20 days. Generation time is approximately 25 to 30 days, and there
may be up to 6 generations per year. Both short-cycle larvae and
long-cycle larvae occur in northern India.
H ibernation during winter
takes place in the larval stage. In south India the insect is not known to
hibernate in any stage of its development.
6. Control measures. Following prophylactic and control measures
should be adopted to minimise the infestation of this pest :
(a) Heat treatment of the seeds. Since the larvae hibernate inside
seeds, the seeds should be dried in sun in May-June for 4-5 hours to
Insect Injurious to Crops
l 3 29
kill them. Before sowing the seeds should also be kept in seed heaters
at 60"C to kill the surviving larvae, if any.
Such seeds, if identified,
should also be taken out.
(b) Removal of infested parts of the plant. Infested bolls should be
picked and destroyed. As far as possible, ratooning practice should be
avoided as such plants may carry infested bolls and re-infest the crop.
(c) Seed treatment with fumigants. Seeds should be fumigated with
methyl bromide at 1 .5 kg/100 m3 or with aluminium phosphide at 1 8
tablets/ 1 00 m3 for 24 hours.
(d)
Planting of resistant cultivars. As far as possible, resistant
varieties of cotton should be planted. Such as G-27, Abhadita, Glot- 1 0,
DHY- 286, MCO- 7, Sujata, Digvijay etc. The early maturing varieties may
escape insect infestation.
(e) Use of pheromone trap. The pheromone traps (both stick or
funnel traps) containing (Z,Z)-7, 1 1 -hexadecadienyl acetate are found very
·
effective in catching the adults at night.
(j) Biological control Recently introduction of a larval braconid
parasitoid Microchelonus blackbumi, and an egg parasitoid Trichogramma
chilonis and a predator Chrysoperla camea in the cotton fields has
resulted a great success in the control of P. gossypiella.
(g) · Application of insecticides. When the aforementioned procedures
fail to check the attack of P. gossypiella following insecticides should be
periodically ( 1 5 days interval) sprayed with care : Fenvalerate and
permethrin
@
1 00- 1 50 g a.i./ha, cypermethrin @ 80 g a.i./ha,
deltamethrin @ 1 2.5- 1 5 g a.i./ha, phosalone
35 EC @ 1 .5-2.5 I/ha,
carbaryl 50
WP
@ 2.5-3.0 kg/ha, endosulfan 35 EC @ · l .5-2.0
I/ha, monocrotophos 40 SC
@ 1 .0- 1 .25 I/ha, profenofos 50 EC @
0.75- 1 .0 kg a.i./ha, thiodicarb 75 WP @ 625 g/ha.
[ III] The spotted boll worms : Earias insulana and Earias vittella
(Lepidoptera : Noctuidae)
1. Distribution. The spotted bollworms are caterpillars of small moths
Earias vittella and E. insulana and are distributed throughout old world
countries. E. insulana, being able to tolerate high variation in temperature
and humidity colonise besi in northern Punjab and Pakistan, while E.
vittella is more common in south-eastern Punjab and other cotton growing
areas of India having mild climate. E. vittella is also distributed in some
parts of south-east Asia, whereas E. insulana is widely distributed in west
Asia and North Africa (Fig. 3).
2. Host plants. The main host plant of spotted bollworms is cotton,
but also infest lady' s finger, okra, hollyhock and several other
malvaceous crops.
Insect Injurious to Crops
330 J
•
A
'�
B
D
Fig, 3, The spotted bollworm, Earias Vltella. (A) Eggs, highly enlarged, (B) Full grown
larva, (C) Pupa, (D) Adult.
3. Importance. The initial infestation generally occur on 6 week old
crop in which the caterpillar cause drooping and drying of shoot due to
its feeding by boring into it. In the later stages the larvae feed on
buds, flowers and bolls. As a result, flower buds and fruits drop
prematurely. Fruits remaining on the plants get deformed and often
show exit holes of the larvae. One caterpillar is able to destroy several
bolls until pupation. The infested bolls produce poor lint having very
low commercial value.
4. Appearance. The adult E. vitella is small ( 12 mm in length and
25 mm across wing-span) having pale white forewings with broad
greenish bands in the middle whereas in E. insulana the forewings are
completely greenish.
5. Life cycle. The female moth lays 2-3 eggs on bracts, leaf axils
and veins on the under surface of the leaves at night. A single female
may lays up to 300-400 eggs. The eggs are crown-shaped, sculptured
and deep sky blue in colour. The incubation period is about 3 days.
The newly hatched larvae bore into the growing shoots or bolls
consuming the plant tissue. The full-grown larva is about 15 mm in
length and is brownish white with a number of black and brown spots
on the body, hence, called spotted bollworm. The last instar larvae
come out the boll and pupate in tough silken cocoons either on plants
or in soil or among the fallen leaves and rubbish material. The pupal
period is about 2 weeks in summer, 3 weeks- in autumn and 6- 1 2 weeks
in winter. In sununer, total life cycle completes within a month.
6. Control measures. Removal and destruction of infested shoots,
fruits and shed materials prevent re-infestation. Alternative food plants
grown nearby the cultivated fields should be destroyed. Only resistant
cultivar of cotton should be planted. Pheromone traps are also helpful
in catching adult moths that are destroyed mechanically. On heavy
infestation, foliar spray with fenvalerate and permethrin @ 1 00- 1 50 g
a.i./ha, cypermethrin @ 80 g a.i./ha, deltamethrin @ 1 2.5- 1 5 g a.i./ha,
phosalone 35 EC @ 1 .5-2.5 l/ha, carbaryl 50 WP @ 2.5-3.0 kg/ha,
Insect Injurious to Crops
[ 331
endosulfan 35 EC @ 1 .5-2.0 I/ha, monocrotophos 40 SC @ 1 .0- 1 .25 I/ha,
profenofos 50 EC @ 0.75- 1 .0 kg a.i./ha, thiodicarb 75 WP @ 625 g/ha,
or dusting of 2% carbaryl dust or 0.05% malathion dust or dichlorvos
(DDVP) or 5% fenitrothion at the two weak intervals provide protection
of the crop from the spotted bollworms. If vegetable crops are infested
by the pest, all fruits should be plucked before insecticide applications.
_
[ IV] The red cotton bug Dysdercus cingulatus and D. koenigii
(Hemiptera : Pyrrhocoridae)
1. Distribution. The red cotton bugs Dys.dercus spp., also known as cotton
stainers, are tropicopolitan and are distributed throughout Indian
sub-continent, Philippines, Australia etc. (Fig. 4).
2. Host plants. Cotton is the main food plant of the red cotton
bug. H owever, other malvaceous crops such as lady's finger, hollyhock,
hemp and others having succulent, juicy and oily seeds are also infested
by them.
3. Importance. The nymphs as well as adults suck the sap from the
leaves and bolls thus they deprive the plants from nutrients. As a result
the bolls open irregularly. They also cause staining of the lint and make
the seeds unfit for sowing. A bacterium Namataspora gossypii enters at
the site of injury and stains the cotton fibre.
4. Appearance. B oth the nymphs and adults are medium-sized (male
1 2 mm, female 1 5 mm) deep red bug and have white bands on the
abdomen and black markings on wings. The mouthparts are at the apex
of the head (prognathous) and are adapted for sucking the plant juice.
5. Life cycle. The adults pass the winter. During spring, the female
lays the spherical yellow eggs in the soil in a loose mass of 70-80 eggs.
The red nymphs hatch out in about 7 days and feeds gregariously and
voraciously on the cotton bolls. Female nymphs are larger than male
ones. There are five nymphal instars before adulthood. The life cycle is
·
Fig. 4. The adult red cotton
bug, Dysdercus cingulatllS.
332 1
Insect Injurious to Crops
completed between 45-90 days. The adults survived for 3 months in
winter and a fortnight or so in summer. The bug breeds on cotton
from August to November and pass winter in adult stage under leaves
or debris. D uring spring onwards (last week of March to July) it feeds
on lady' s finger and hemp.
6.- Control measures. Following prophylactic measures should be
employed to prevent the bug infestation: Cotton fields after the
harvesting of the crop should thoroughly be ploughed to expose the
eggs for sun dry; in cotton fields, lady' s finger plants should be pJanted
as trap crop; and resistant varieties of the cotton should be cultivated.
In case of severe infestation, spray of 0.05% malathion, endosulfan 35
EC 1 .0 I/ha, phosphamidon 100 EC 0.25 I/ha, fenitrothione 1 00 EC 1 .0
I/ha protect the crop from the pest.
[ V] The grey weevil : Myllocerus undecimpustulatus maculosus
(Coleoptera : Curculionidae)
1. Distribution. It is distributed throughout in India, Sri Lanka, USA and
some other countries on one or other host plants.
2. Host plants. About 20 food plants are recorded as host for the
M. undecimpustulatus maculosus, but only few such as cotton, okra,
sorghum, soybean, pigeonpea and Hibiscus spp. are mostly suffered from
its infestation.
3. Importance. The adult weevils feed on leaves, nibbling the leaves
from the margins and eating away small patches of leaf lamina while the
grubs feed on plant roots and damage the crop.
4. Appearance. The adult weevil measures about 7-8 mm in length,
and generally whitish-grey in colouration. All the femora of the weevil
are spined.
5. Life cycle. The cotton grey weevil is active from April to
November and passes winter in the adult stage, hidden in debris. The
female lays on an average 360 eggs over a period of 24 days. The eggs
hatch in 3-5 days. The young grubs feed on the roots of cotton and
other plants. The grubs complete their development in one to two
months. Pupation occurs- ·in the. soil inside earthen cells and takes about
one week. The life cycle is completed in 6 to 8 weeks during the active
period. The adults live for 8 to 1 1 days in the summer and four to five
months in the winter.
6. Control measures. Cultural practices may be of value. Frequent
hoeing and 'interculture' disturb and kill the grubs of the cotton grey
weevil. The weevil has a marked preference for pigeonpea (Cajanus
cajan) which can be sown as a trap crop. The chemical treatment may
be of little or no economic value because of the prohibitive expense and
the limited period of vulnerability as the larvae are protected while
Insect Injurious to Crops
[ 333
feeding under the ground. However, soil fumigation by methyl iodide or
methyl bromide appears to be effective.
[ VI] The cotton leaf hopper : Amrasca biguttula biguttula
(Hemiptera : Cicadellidae)
1. Distribution. The cotton leaf hopper, Amrasca biguttula biguttula
(=
Amrasca devastans) is distributed throughout many states of India, South
and Southeast Asia, and in the Mariana Islands. In India, it is more
abundant in Punjab, Tamil Nadu, Andhra Pradesh, Karnataka and
Maharastra (Fig. 5).
2. Host plants. It commonly infests cotton, sunflower, okra,
sun-hemp, potato, tomato, niger, brinjal etc.
3. Importance. In India, A. biguttulla biguttulla causes severe damage
to cotton and sunflower. The nymphs as well as adults are fast moving
and found in large number on ventral leaf surface of the plant. Both
suck the plant sap and also inject toxins contained in saliva into the
plant tissues. Damaged leaves curl at the edges and develop brown dead
spots with a yellow halo at the edges of the leaves. Severely affected
leaves may desiccate and fall off. The floral heads, bracts and petal are
also infested. Its incidence begins with the germination and continues till
harvest. Stunted growth, hopper bum and crinkled leaves are the
common symptoms of hopper attack.
4. Appearance. The adults fly readily. They measure 3 mm in length
having greenish yellow body. The forewings have a black spot on each
on the apical margin and two black spots on the vertex of the head.
The nymphs are also green and walk diagonally.
S. Life cycle. The female lays 30-40
eggs inside leaf veins
particularly the midrib. After 4- 1 1 days of incubation period, eggs hatch
into pale green nymphs. Nymphal period varies according to the weather
conditions from 7-2 1 days during which it moults 5 times. High nitrogen,
low plant density and humid conditions favoured its multiplication. In a
year 10- 1 1 generations occur.
A
B
c
Fig. 5. The cotton leaf hopper, Amrasca bigutu/la b1gutulla.
(A) First instar nymph, (B) Second instar nymph, (C) Adult
334 J
Insect Injurious to Crops
6. Control measures.
The extract of following phytoproducts
controls the cotton hopper to a major extent: garlic, chillies, ginger and
tobacco leaves in the ratio of 1 , 1 , 1 and 3 are required. For the
preparation of this extract garlic ( 1 kg) is soaked in 1 00 ml ' kerosene
oil overnight. Next day, a paste is prepared with it adding considerable
amount of water. Also, a paste pf chilli and ginger is made with water.
The leaves of tobacco is boiled in water for 45 minutes and is filtered.
Other ingredients are mixed with this extract which is diluted with 60
litres of water. Khadi soap solution is added as emulsifier at the rate of
1-2 ml/litre. Now this preparation is ready to spray which is sufficient
for one acre crop. When the crop is severely infested, then 0.05%
endosulfan or 0.02% phosphamidon or 0.03% dimethoate should be
sprayed.
Pests of Sugarcane
The sugarcane (Saccharum officinarum) is the principal cash crop of India.
It is infested by about 200 insect species out of which 1 2 species inflict
severe damage. If the conditions are favourable for insect multiplication
these insects may appear in the form of epidemic causing enormous loss to
sugar industry. Following insects are of economic importance for the
sugarcane crop: Scirpophaga nivella (the sugarcane top borer), Emmalocera
depressella (the sugarcane root borer), Pyrilla perpusilla (the sugarcane leaf
hopper),
Aleurolobus barodensis (the sugarcane whitefly), Chilo
infuscatellus (the shoot borer), C. sacchariphagus indicus (the internode
borer), Odontotermes obesus (termite), Holotricha spp. (white grubs),
Melanaspis glomerata (the sugarcane scale insect), Kiritschenkella sacchari
(the sugarcane mealy bug), Sesamia inferens (the pink borer), Acigona
steniellus (Gurdaspur borer), Melanaphis sacchari (sugarcane aphid),
Hieroglyphus banian (grasshopper), etc. Biology of only first four insects
are given below :
[ I] The sugarcane top borer : Scirpophaga (= Tryporyza) nivella
(Lepidoptera : Pyraustidae)
1. Distribution. The top shoot borer is distributed throughout south Asian
countries where sugarcane is cultivated such as India, Pakistan, China,
Taiwan, Philippines, Thailand, Sri Lanka and Union of Myanmar. In India,
it is more destructive in northern states like Uttar Pradesh, Bihar and
Madhya Pradesh (Fig. 6).
2. Host plants. The main host plant is sugarcane but it may live on
munja and other wild grasses.
3. Importance. From March to September the damaging tendency of
the pest is at its worst. Only the caterpillar is destructive. Upon
hatching from the eggs, it makes hole in the midrib of leaves and then
Insect Injurious to Crops
[ 335
B
A
Fig. 6. The sugarcane top borer, Scirpophaga nivella
(A) Larva, (B) Pupa, (C) Adult.
travels the central shoot and consumes the growing plant tissues of the
top 4-6 internodes. As a result, the upper part of the shoot dries up
and charred forming 'dead-hearts ' . Its infestation is detected not only by
the presence of dead-hearts but also by the presence of small holes in
leaves and galleries in the midribs. The formation of side shoots which
give rise to a bunchy top is another symptom of top borer infestation.
First two generations infest young plants causing their death. Due to its
infestation, not only the yield of the crop is highly decreased (up to
20-30%) but the sugar content in the cane juice is also highly reduced.
4. Appearance. The adult insect has a white body with almost silvery
white wings. The abdomen of male is pointed, while that of female is
stout and blunt. The males are smaller than females, the latter are
about 20-2.5 mm across the wings. The anal segment of female is
covered with a tuft of yellow, orange or brownish silken hairs.
5. Life cycle. The female lays 300-500 eggs either singly or in groups
of 5- 1 0 on the inner side of the leaves. The eggs are elongate, oval and
are covered with buff coloured hairs. The eggs hatch in to larvae after
5- 10 days of incubation period. The newly hatched larvae, about 2 mm
in size with black head, make its way to the top shoot of the cane
through midrib of the leaf. The larva is yellowish white in colour. It
becomes full-grown passing through five instars within 35-45 days. The
full-grown larva, 30 mm in length, forms a characteristic chamber with
an emergence hole just above the node. The hole is plugged with 4-5
membranous and circular septa. The larva pupates within this chamber.
The pupal stage ends in 7- 1 2 days. The adult moth survived for 4-5
days. There are 5-6 generation in a year. The larvae of last generation
do not pupate and unaergo diapause to over winter in north India.
(Z-57)
336 J
Insect Injurious to Crops
6. Control measures. The egg masses should be collected and
destroyed early in the season, i.e., April to May. Biological control by
release of a ichneumonid wasp Isotima javensis @ 1 25 femaleslha in
coastal area of Tamil Nadu and inundative release of a chalc1d wasp
Trichogramma chilonis @ 50000 eggs/ha in Andhra Pradesh and
northern India have been recommended. Resistant cultivars of the
sugarcane should be planted such as Co 4 1 9 , CoS 767, CoJ 67, Co
1 158. Soil application of carbofuran at 2 kg a.i./ha or phorate at l kg
a.i./ha is also recommended. On severe infestation, malathion 50% EC,
1250 ml or endosulfan 35% EC 800 ml or pholithian 100% EC 300
ml/ha may be sprayed over the standing crops.
[ II] The sugarcane root borer : Emmalocera depressella
(Lepidoptera : Pyraustidae)
1. Distribution. The sugarcane root borer is distributed mainly in north
India, viz., Uttar Pradesh, Bihar and Madhya Pradesh. However, at a lower
scale it is found throughout in India and Pakistan (Fig. 7).
2. Host plants. The main host is sugarcane but it also infests maize,
sorghum, millets and munja.
3. Importance. The caterpillars consume the base of newly sprung
shoot of the sugarcane from April to June. It results the formation of
dead-hearts. The central middle portion of the infested plant starts
withering in the third week of infestation and the plant dries out within
two months.
4. Appearance. The head of the adult moth is pale-pink while wings are
pale or dirty brown. It is about 20 mm across the wings. Its hindwings are
larger in width than forewings. It has a dark lengthwise strip on each wing.
The abdominal tip of the male is tapering while that of female is cylindrical.
5. Life cycle. The female deposits 200-300 eggs, singly or in batches
on the under surface of the leaves. The eggs may be laid on stems or
even on the ground. The eggs are creamy oval and scale-like. The eggs
A
B
c
Ftg 7. The sugarcane root borer, Emmalocera depressella. (A) Larva in situ, (B) Pupa,
(C) Adult female.
(Z-57)
Insect Injurious to Crops
[ 337
hatch after 4-7 days and soon after emergence, the first instar larva
bores into the base of the shoot or below the soil surface as a result
dead-heart is formed . The larval period is about 35-45 days during
which the larva attains a maximum growth of 25-30 mm. Before
attaining pupation period, the full-grown Jarva moves above the soil
surface in the stem and makes an exit hole and constructs a silken tube
in which it pupates. Pupation period lasts for 9- 1 4 days. The entire life
cycle takes about two months to complete. The last instar larvae of fifth
generation u1;1dergo diapause to pass winter.
6. Control measure. The infested plants should be stripped off. In
infested areas, tendency of keeping ratoon crops should be dropped.
Resistant crop varieties should be planted. After harvesting, the part of
the cane under soil should be collected and burnt to destroy the
diapausing caterpillars. The soil may be treated with endosulfan @ 30
kg/ha. Soil application of carbofuran at 2 kg a.i ./ha or phorate at I kg
a.i./ha is also recommended.
[ III] The sugarcane leaf hopper : Pyrilla perpusilla
( Hemiptera : Lophopidae)
1. Distribution. The sugarcane leaf hopper, Pyrilla perpusilla is distributed
throughout in India where sugarcane is cultivated. It usually severely
damages the cane in Uttar Pradesh, Bihar, Punjab, Madhya Pradesh and
Maharastra. Outside India, it is reported from Sri Lanka, Republic of
Myanmar and Thailand (Fig. 8).
2. Host plants. Sugarcane serves as the primary host of the pyrilla,
however, the insect is able to thrive well on a variety of food plants
such as wheat, barley, oat, maize, millets, paddy, wild grasses, etc.
Occasionally, it is also seen in the fields of lady ' s finger, cucurbit
vegetables, and certain legumes.
A
8
Ftg 8. The sugarcane leaf hopper, Pyrilla perpus11/a. (A) Nymph, (BJ Adult.
(Z-57)
Insect Injurious to Crops
338 1
3. Importance. The nymph and adults, both damage the crop by
sucking the sap from the foliage depriving the plants with nutrients. As
a result. the foliage of the canes become pale yellow and dry up. Like
aphids,
the
pyrilla
also
excretes
honeydew
upon
which
sooty
mould
develops turning the leaves black. Due to this, the photosynthetic ability
of the plant is hampered affecting adversely on the yield of the crop.
Sugar
content
infested
may
crop.
decrease
from
Additionally,
if
7-9% in healthy crop to 2-5%
sown,
such
canes
do
not
in
germinate
properly. Most of the damage caused by the pyrilla occurs during April
to
October.
4. Appearance.
The straw-coloured adult insect has two pairs o f
wings which are folded o n the abdomen in shape o f a roof. The length
of the body is about
prominent red eyes.
8- 1 0 mm. They possess long pointed snout with
The female has
a pair of pads on the abdominal
end of the body. The adults are very active flier.
5. Life cycle. The pyrilla breeds throughout the year. The female
1 0-65 eggs which are covered
lays eggs in large clusters each containing
with
white
These
fluffy
eggs
are
filaments
laid
on
secreted
the
inside leaf-sheaths during winter.
eggs
in
one
generation.
greenish in colour.
and
by
the
underside
A
The eggs
of
anal
tuft
leaves
of the
during
single female may
are
oval,
shining
The eggs hatch into nymphs after
females.
summer
lay
and
up to
750
and pale-white
or
7 days in summer
22 days in winter. The freshly hatched nymphs are cream coloured,
soon
turning
into
pale
brown,
and
have
a pair of characteristics
anal
filaments. Nymphal period varies with climatic conditions. In summer it
is about
6-8 weeks but in winter it is about 1 7- 1 8 weeks as winter is
passed
in
adults.
Male
nymphal
stage.
After
five
moults,
the
nymphs
change
into
5-7 weeks while females for 5-8 weeks. In
monsoon, the life cycle is completed within 6-9 weeks. In a year, about
4 overlapping generations occur.
6. Control measures. Following cultural practices should be
employed
to
survived
minimise
for
the
pyrilla
attack
on
sugarcane:
(i)
the
egg
masses should be collected and destroyed. (ii) the cane-trash should not
be
burnt
after
harvesting
amount of nitrogen
as
fertiliser
it
in
kills
soil
their
should
natural
be
enemies,
(iii)
kept moderate as
the
high
nitrogen content in soil makes the leaves succulent and attractive for the
hoppers, (iv) resistant varieties
of su garcane should be planted, (v) the
tendency to keep ratoon crops should be dropped, (vi) for its biological
control,
its egg parasitmd, Tetrastichus pyrillae and the ectoparasitoid
Epiricania melanoleuca ( 1 5000 cocoons/ha) should be introduced
in the infested fields, (vii) if 20-30% nymphs and 40-60% adults are
moth,
parasitised,
(Z-57)
application
of
any
insecticide
should
be
avoided,
and
Insect lnjw ious to Crops
{ 339
(viii) if the crop is heavily infested before monsoon, endosulfan 35 E C
@ 1 .5 ml/I o f water should be sprayed.
[ IV] The sugarcane whitefly : Aleurolobus barodensis
(Hemiptera : Aleyrodidae)
1. Distribution. The sugarcane whitefly, Aleurolobus barodensis has
assumed senous pest on sugarcane in Bihar, Gujarat, Haryana, West
Bengal, Orissa, Karnataka, Maharastra, Punjab, Tamil Nadu, Uttar
Pradesh, Uttaranchal and Andhra Pradesh (Fig. 9).
2. Host plants. The main host p lant of this whitefly is sugarcane,
however, it may feed on Saccharum moonja, wheat, barley and wild
grasses.
3. Importance. The nymphal stages damage the crop by sucking the
plant sap of the leaves with the help of piercing and sucking
mouthparts. In the month of July-November, they cause severe damage
to ratoon crops. The plants turn pale in colour and the l eaf apices
remain unopened. The sugar content in cell sap decreases up to a great
extent.
4. Appearance. The adult insects are small, 3.0 mm in length,
fragile and pale yellow in colour. The female is bigger and stouter than
male and sluggish in nature. They copulate just after emergence.
During November to D ecember, the females lay
5. Life cycle.
nearly 65 creamy-white conical eggs on under surface of the leaf in a
linear fashion close to the mid rib. The incubation period lasts after 5-7
days and the eggs hatch into small nymphs which are oval in shape and
pale yellow in colour with three pairs of legs. The young nymphs (0.36
mm) take position on the under surface of the leaves and begin to suck
the plant juice. During development, the nymphs undergo four moultings
to attain adulthood. Three consecutive nymphal instars take about 25
days but the last instar needs 10- 1 5 days. The last instar nymph
undergoes pupation converting itself as a pseudopupa as true pupa arc
c
D
Fig 9 The sugarcane whitefly, A/eurolobus barodens1s (A) Eggs, (B ) Nymph
(C) Pseudopupa. (0) Adult
340 J
Insect Injurious to Crops
not found in the life history of Herniptera. After 8- 10 days, adults
emerge out. The longevity of the adult is not more than 2 days. The
entire life cycle completes within 25-48 days. Nine generations have been
recorded in south India.
6. Control measures. The procedure mentioned for pyrilla is also
applicable for whiteflies.
Pests of Paddy
At present, India is producing about 90 million tonnes of rice ( Oryza
sativa) per year. Rice is a staple food of about 65% of Indians. It is grown
in about 42.5 million hactares. The crop suffers maximum due to
infestation of a wide range of insect which alone cause 30% yield loss every
year in spite of all control measures. Out of about 80 species of insects
infesting paddy crop, 20 species severely damage the standing crop. These
insects include bugs (Leptocorisa acuta, Brevennia rehi), leaf hoppers
(Nephotettix virescens, N. nigropictus, Nilaparvata lugens), stern borers
(Scirpophaga incertulas, Sesamia inferens, Chilo suppressalis), gall midges
( Orseolia oryzae), thrips (Stenchaetothrips biformis), termites, army worms
(Spodoptera mauritia), hispa (Dicladispa armigera),
grasshoppers
(Hieroglyphus banian), etc.
[ I] The rice stink bug : Leptocorisa acuta ( = L. varicornis)
(Hemiptera : Coreidae)
1. Distribution. The rice stink bug Leptocorisa acuta, also known as rice
earhead bug or gandhi bug is a tropicopolitan species and is distributed
throughout rice growing countries of Asia. In India, it is reported from
almost all states where rice is cultivated (Fig. 1 0).
2. Host plants. Rice is the main host plant of L. acuta, however,
maize and millets are other crops which are infested by the bug. It also
survives on grasses.
3. Importance. Adults and nymphs both suck the sap of developing
rice grains at the milky stage and cause considerable yield loss. Sucking of
the grain sap by this bug causes ill-filled/partial filled and chaffy grains and
also enhances subsequent fungal and bacterial infection. The yield loss
varies from 10-60% depending upon the crop varieties and land types. The
'
economic threshold level is 5 bug/rn 2.
4. Appearance. L. acuta is green, light brown or mixed yellow in
colour with a slender body. The male measures 13-14 mm and female
1 6- 1 9 mm . in length. Head is triangular and bears 4 segmented antennae.
Legs are long. The abdomen of both male and female is constricted slightly
in the middle. The abdomen of female is a little bit inflated. There are
stink glands on the eitherside of the abdomen that emit a foul odour,
Insect Injurious to Crops
A
[ 341
B
c
Fig. 10. The rice stink bug, Leptocorisa acuta. (A) Eggs on paddy leaf, (B) Middle age
nymph, (C) Adult.
hence called stink bug or gandhi bug. The adults survive for 30-55 days.
The female oviposits after 3-4 days of mating.
5. Life cycle. The females lays 250-300 eggs during night in 2 or 3
straight rows of 10-20 eggs along with the midrib on the upper surface of
the leaf blade. Eggs are 2 mm long, disc shaped or dorsally flat and
elliptical, with surface slightly granulated and shining. Incubation period is
6-7 days. First instar nymph is very small, nearly 2 m!Il long, pale green in
colour which grows to deepen green through different instars. After
passing through five instars within 1 5-20 days depending upon the
availability of the food, it attains adulthood.
6. Control measures. Removal of the alternate hosts from the
nearby paddy fields minimises bug incidence. No variety of rice is found
to be resistant against this pest, therefore, we have to still rely on
synthetic pesticides. Using malathion dust or spray applications 1 5 days
after flowering is effective. Neem based products like achook or
nimbicidine ( I %) spray also controlled the population of bugs
effectively. Use of chlorinated hydrocarbons as pesticides should be
avoided as its residues make the fodder unfit for cattle.
[ II] The paddy s tem borer : Scirpophaga (= Tryporyza) incertulas
(Lepidoptera : Pyralidoidea, Pyraustidae)
1. Distribution. Scirpophaga incertulas commonly known as paddy stem
borer or yellow stem borer or yellow rice borer is distributed throughout
India where rice is cultivated such as Andhra Pradesh, Assam, Bihar,
Gujarat, Himachal Pradesh, Haryana, Jarnmu & Kashmir, Kerala, Madhya
Pradesh, Maharastra, Orissa, Punjab, Sikkim, Tamil Nadu, Uttar Pradesh
and West Bengal. Apart from India, the borer is also observed on rice in
other southeast Asian countries, viz., Afganistan, Bangladesh, Bhutan,
342 J
Insect Injurious to Crops
B
�
�·�
A
c
Fig. 1 1 . The nee stem borer, Scirpuphaga mcertulas (A) Larva, (B) Pupa, ( C) Ad•ilt.
Myanmar, China, Japan, Indonesis, Malaysia, Nepal, Pakistan, Philippines,
Sri Lanka, Taiwan, Thailand and Vietnam (Fig. 1 1 ).
2. Host plants. S. incertulas is a monophagous insect pest and
paddy is considered to be the only host plant. However, there are
certain reports that it also survives on Bermuda grass, jungle rice,
torpedo grass, kodo, millet, sugarcane, wheat and maize.
3. Importance. The stem borer larvae tunneling in the stems and
feeding on the soft tissues cause injury to the paddy crop. Due to such
feeding at the vegetative stage of the plant, the central leaf whbrl
remains unfold, turns brownish and dries up while the lower leaves
remain green and healthy. This condition is known as 'dead hearts' . The
affected tillers dry out without bearing panicles. If infestation begins
after the formation of panicles, no grain is formed inside panicles. Such
empty panicles are white and hence called 'white ears' and are visible in
the field in erect posture. The estimated loss of paddy due to the stem
borers ranges from 30 to 95% in India.
4. Appearance. The adult moth is 1 3- 1 6 mm long and measures
22-30 mm in their wing expanse. The male is smaller than the female
and is light brown with numerous small dark spots near the tip of the
forewings. The female is straw coloured and the colour deepens towards
the tip of the forewings with a single dark spot at the centre. The anal
end of the abdomen of the female is covered with tufts of yellowish
silken hairs. The moths are active in evening and mate in night and are
highly phototactic and can easily be collected in light traps.
5. Life cycle. The female lays 200-300 eggs in masses usuall y on the
upper surface of the leaves towards the tip which are covered with a
buff coloured tuft of hairs. In each mass there are 15-80 eggs, which
are creamy white, flattened, oval and scale-like. Before hatching, the
eggs darken to a purplish tinge. Incubation period ranges between 5-8
days. In winter it takes even more time. The l arvae pass through 4-7
instars with a total larval period of 30-40 days. The newly hatched
larvae move upwards and feed green tissues for 2-3 days after which
Insect Injurious to Crops
{ 343
they bore into the stem. The full-grown larva is yellowish white and 25
mm long with an orange head. The larvae undergo diapause from
November to January. Pupation takes place inside the paddy stem, straw
or stubble. Before pupation, the full-grown larva cuts exit hole in the
internode for adult emergence and webs 1 or 2 horizontal septa to
make it water proof. Thereafter, the larva webs a silken cocoon and
pupates inside it. The pupa is 1 2 mm long, pale in beginning and
gradually turns dark brown. The pupa developed in to adult in 6- 10
days. In a year there are 4-6 generations.
6. Control measures. The stem borer management in paddy includes
various control components which can be integrated to develop a
package of practice for IPM against them.
(a) Varietal resistance. Resistant to moderately resistant cultivars of
paddy should be cultivated such as IR20, Sasyyasree, Ratna, Manika,
Tambha, Samanta, Sarathi, Bhuban etc., however, the growth of
varieties depend upon the land types.
(b) Biological control. The introduction of egg parasitoids such as
Trichogramma japonicum, T. chilonis and Tetrastichus schoenobii results
5-97% egg parasitism m fields. There are several other natural enemies
like braconid and ichneumonid larval and pupal parasitoids, coccinellid
predators, spiders, birds, dragonflies etc. which act as mortality factor of
the borers.
(c) Cultural control. Cultural control can only be successful when
employed at community level like biological control. The incidence of
the stem borers may be minimised by selection of early maturing crop
varieties, use of balanced N, P and K fertilisers, proper irrigation etc.
(d) Physical control. Hand picking of egg masses and setting of light
traps using pheromones for mass collection of moths, though have limited
value, help in reducing their attack.
(e) Chemical control. At present, use of insecticides is the first line
of defense against stem borers. The
proper insecticide should be
applied by evaluating the extent of incidence, time of their foliar
movements etc. Following chemicals have been evaluated as effective
insecticides against S. incenulas and other borers: granular formulation
of carbofuran, isazophos, diazinon, phorate etc. @ 1 .0 kg a.i./ha;
sprayble formu- lation of monocrotophos, chlorpyriphos and quinalphos
@ 9.5 kg, phosphamidon @ 0.3 kg and triazophos @ 9.25 kg a.i./ha.
[ III] The rice striped borer : Chilo suppressalis
(Lepidoptera : Pyralidoidea, Crambidae)
1. Distribution. The rice striped borer, Chilo suppressalis (= Chilo simplex,
C. oryzae) also known as Asiatic rice borer or striped stalk borer is
distributed throughout the South Asian rice growing countries such as
344 1
Insect Injurious to Crops
Fig 12. The nee striped botrer, Chilo suppressalzs. (A) Larva, (BJ Pupa
1 ( 1 A d ult
India, Nepal, Pakistan, Bangladesh, Myanmar, China, Japan, Tatwan.
Malaysia, Philippines, Thailand and Vietnam. In India, it is distributed
throughout the country (Fig. 1 2).
2. Host plants. The main host plants of rice striped borer are
paddy and com but it also infests kodo millet, pearl millet, common
reed, sugarcane, sorghum, wheat, tomato, brinjal, Chinese cabbage,
garden radish, goose grass etc.
3. Importance. The newly hatched larvae immediately start boring
into the plant tissues. With the advancement of growth and development
of the larvae the central shoot withers and the larvae gradually migrate
to the neighbouring stems. Larvae after hatching on a matured crop
normally enter either to the third or fourth leaf sheath and remain
there for about a week before migrating to adjoining plants. A single
caterpillar may damage up to 8- 1 0 plants. It alone causes 4-6 % loss to
paddy crop.
4. Appearance. The moth is about 1 3 mm long with a wing expanse
of nearly 23-28 mm. The male moths are smaller than the females. The
head, thorax and outer wings are pale yellow or straw coloured. There
is a row of black dots at the tips of the forewings and the scales on the
forewings are grey-brown and scattered. Hindwings are white to
yellowish brown, face distinctly projected forward beyond eyes producing
a prominent comeous point and a ventral ridge.
5. Life cycle. Soon after emergence the adults mate for 30 minutes
to 3 hours. After one day of emergence the female moth oviposits near
the base of leaf sheath during evening hours repeatedly at an interval of
1 to 3 days throughout its life span of 4-8 days. Eggs are disc-like, pale
yellow and overlap in the egg mass and are not covered with hairs. The
female lays about 300 eggs in several batches. The incubation period
ranges from 4 to 10 days depending on temperature . The newly hatched
Insect Injurious to Crops
[ 345
larva is about 1 .2 mm long and is sparsely covered with fine setae. Five
longitudinal rows (three dorsal and two lateral) of purplish brown
stripes are present on the abdomen of the larvae, due to which it is
called striped borer. The larvae pass through 5 to 8, usually 6 instars .
Full-grown caterpillar is 26 mm long and 2.5 mm wide and has a
yellowish brown head. However, under poor nutrition and adverse
conditions as many as 9 stadia have been recorded. The newly hatched
larvae are positively phototropic for four hours, then negatively
phototropic until pupation. The entire larval period lasts for 30 to 40
days. Prior to pupation the larva makes an emergence hole. The
matured larvae do not construct cocoon and pupate within the rice
stalks either at the middle or basal internodes depending upon the
moisture condition in the stalk. In addition to the stubbles, some larvae
also pupate in the harvested straw. Because of differential microclimatic
condition in stubble and straw uniform larval and pupal development do
not take place and this results in asynchronised adult emergence. The
pupae are brownish, approximately 7 mm long and 3.5 mm wide. The
adults usually emerge after 6 days. Life cycle is completed in 4 1 to 70
days and there are usually 4-6 generations in a year.
6. Control measures. Since the larvae pupated in the stubbles, the
crop must be harvested from the level of soil to eliminated future
generation. Also, after harvesting the fields should be ploughed and
filled with water to destroy hibernating larvae and pupae. In addition,
the control measures described for S. incertulas may also be applied to
control C. suppressalis.
[ IV] The rice grasshopper : Hieroglyphus banian
(Orthoptera : Acrididae)
L Distribution. The grasshopper are found throughout in India and
adjacent countries like Pakistan, Afganistan, China, Sri Lanka, ·Bangladesh,
Myanmar, Thailand etc. (Fig. 1 3).
2. Host plants. H. banian is a polyphagous grasshopper and infests
a variety of crops such as cotton, maize, pearl-millet, sorghum, rice and
sugarcane.
3. Importance. H. banian is considered as a serious pest of paddy
crop in northern India. Although the grasshoppers are occasional and
sporadic pests but their outbreak has been reported from many parts of
India in past tsee chapter 1 9). The nymphs feed on germinated
seedlings of paddy which wither away and the adults feed on the leaves
and shoots but sometimes cut the earheads.
4. Appearance. Adult hoppers are dull green or yellowish green with
brownish black lower surface. Adult females measure 34-55 mm in
length, whereas, males are only 28-40 mm. Head is hypognathous with
346 J
Insect Injurious to Crops
Fig 1 3 . The nee grasshopper, H1eroglyphus baman.
filiform antennae and large eyes. There are 2-3 black markings running
laterally on eitherside of the thorax. Hindlegs are jumping type.
Brachypterous forms are also observed.
5. Life cycle. Females deposit eggpods in to the soil from October
to December. E ach eggpod contains 30-35 eggs in the wet sandy soil
at 3-5 cm depth. The eggs are yellowish and covered with a gummy
substance that hardens into a waterproof coating. Mortality of the eggs
is very high if the temperature is above 40°C and rains are insufficient.
The eggs remain in the soil till rains begin during the following June July. The nymphs hatch out after the onset of the rainy season. Newly
hatched young hoppers are brownish-yellow and afterwards tum to dull
green . The nymphs which hatch in the eggpods buried in compact soil
at 5 cm or more depth fail to come out. After passing through 5-7
instars nymphs attain adulthood. Developmental period of females is
more than males. Adults mate after 1 -3 days after emergence and
survived for 1 -6 months. There is only one generation of the insect in a
year.
6. Control measures. The adults as well as nymphs should be
collected by sweeping and destroyed. The infested fields after harvesting
of the crop should be deeply ploughed to expose the eggpods to sun as
well as for predators (entomophagous insects, birds) for egg destruction.
When population of the insect is more than 20 hoppers/m 2, foliar
application of carbaryl or monocrotophos @ 0.5 kg a.i./ha would be an
effective control (also see chapter 1 9).
[V] The rice hispa : Diceladispa ( = Hispa) armigera
(Coleoptera : Chrysomelidae)
1. Distribution. Diceladispa armigera is distributed throughout India, viz.
Punjab, Himachal Pradesh, Jammu & Kashmir, West B engal, Assam, Tamil
Nadu, Andhra Pradesh, Orissa, Uttar Pradesh. Apart from India, it is also
reported from Nepal, China, Pakistan, Sri Lanka, Malaysia, Indonesia,
Loas, Bhutan, Thailand, Vietnam, Papua New Guinea (Fig. 1 4) .
2 . Host plants. Rice i s the main host plant but in its absence it
sustains itself on the sugarcane, sorghum and wild grasses.
3. Importance. The rice h1spa is a sporadic and occasional leaf
feeding pest and occurs in most of the rice tracts. Infestation of this
Insect Injurious to Crops
A
[ 34 7
B
c
Fig. 1 4. The nee h1spa, D1celad1spa amufiera
(A) Larva, (B) Pupa, (C) Adult
pest have increased in recent years due to introduction of high yielding
varieties and improved agronomic practices. Incidence of D. armigera
usually occurs before flowering. Both adults and larvae feed on the
green portion of the leaves causing characteristic linear patches along
the vein. The yellowish grubs mine into the leaves presenting blister
spots. The adult feeds by scrapping the green matter and remove
chlorophyll first between the veins of the lamina giving the appearance
of white parallel streaks on the leaves. The field infested with hispa
gradually turns yellow as the leaves dying and the plants withering. The
hispa prefers young plants so pest attacks begins in the nursery itself.
Average loss to the crop yield varies from 6-65%.
4. Appearance. The adult beetle is 5 mm long, shiny and bluish
black in colour and characterised by several short spines over body.
5. Life cycle. The female lays eggs after 3-4 days of emergence and
continues up to a month. A single female may deposit 30-300 eggs. The
eggs which are oval and about 1 mm long are laid singly, each egg
being inserted in the epidermal tissue in the upper part of the leaves,
not far from the point. The incubation period ranges from 4-5 days.
The newly hatched grub is pale yellow, dorso-ventrally flattened and
about 2-4 mm long. The grub feeds on the mesophyll of the leaf, eating
it away and producing a yellow spot. The grub may easily be seen if a
spotted leaf is held up to the light. A single grub may consume about
2
1 25 mm of leaf area per day. Larval stage lasts for 7-12 days passing
through four instars. Pupation takes place within the larval mines in a
period of 4-6 days. The pupa is flat brown and exarate. Upon
emergence from pupal case, the adult beetle cuts its way out of the rice
leaf and becomes external feeder. Females survive for 30-50 days. Total
life cycle is completed in 1 5-25 days. There are 6 generations in a year
in coastal area, however, in Punjab and Uttar Pradesh it completes 2-3
generations during paddy season.
348 J
Insect Injurious to Crops
6. Control measures. Infested leaf tips should be clipped off and
destroyed while transplanting. If the nursery beds are flooded, the
beetles float and can be swept together with brooms and then
destroyed. The adult beetles may also be swept by using cloth bags.
Weeding off the alternate host plants in the fields, bunds, and adj acent
areas minimise the incidence of the attack. Since the life stages of the
rice hispa is highly safegaurded, only few pesticides are effective in
controlling the pest, for example, application of phorate l OG @ 10
kg/ha in nursery and monocrotophos or quinalphos or chloropyriphos @
0.5 kg a.i./ha in the field.
[ VI] The rice swarming caterpillar
( Lepidoptera : Noctuidae)
:
Spodoptera mauritia
1. Distribution. The rice swarming caterpillar is distributed in South and
South East Asia and Australia region where rice is cultivated. In India it
is a major pest of rice particularly in north India (Fig. 1 5).
2. Host plants. S. mauritia is a polyphagous insect and feeds on
almost all crops. The primary hosts are rice, sugarcane, brassica
vegetables, cotton and grasses.
3. Importance. Th� swarming caterpillars cause severe damage to
rice plants in nursery beds. They appear suddenly in masses and move
like an anny from field to field so that seedbeds or the direct seeded
fields look as if grazed by cattle. Generally a transplanted crop is not
severely affected . Newly hatched larvae cause the plants to look sickly
with withered tips and cut leaves but larvae more than 10 days old feed
v oraciously and cause almost complete defoliation of the plants. They
feed mostly at night and migrate from field to field and extensive losses
are often caused within a week. Their migration is facilitated by the
absence of standing water in the field.
4. Appearance. The adult insect is medium sized greyish black moth
with a white blotch on its forewings which are irregularly waved. The
hindwings are whitish in colour.
5. Life cycle. The adults are a::tive from July to September. The
female lays egg in batches on the lower surface of rice and other grass
leaves and covered with greyish hairs from its anal tuft. A single moth
lays about five to six egg clusters each containing 1 50-200 eggs.
Individual egg is pearly white, round and has a ridged surface. The
incubation period ranges from 5-9 days. Hatching usually occurs during
the morning hours and the newly hatched larvae are very active. They
feed by scraping the green matter from the leaf tips and rest within the
rolled edges of the young leaves where they almost invisible.
Occasionally they suspend themselves from the plants with a silken
thread which they spin and drift by wind to other plants. The larvae
Insect Injurious to Crops
{ 349
•
.
A
,d4if!f41•
B
c
Fig. 1 5. The Spodoptera maurltla. (A) Egg, (BJ Larva, (C) Pupa, (DJ Adult.
undergo five instars in an average of 22 days to full-grown. Those
beyond third instar are strictly nocturnal and hide during the day time.
However, during cloudy weather they also remain active during the day.
The full-grown larva is about 38 mm long and is dark to pale green
with dull dorsal and subdorsal stripes. Pupation takes place in the
earthen cells, slightly below the ground level. The pupa is dark brown
and about 1 3 mm long. The pupation stage lasts for 1 0- 14 days. The
adult moths are nocturnal, hides during the day in crevices in the soil
or under other cover but is very active after dark. Generally it is not
attracted to light. The moth mates 1 -2 days after emergence. The female
begins to oviposit
shortly after mating. It is a strong flier and can
move great distances for oviposition. Usually they have the tendency to
congregate and oviposit in the same area. The first generation moths
usually appear when the seeds are germinating in the seedbeds or direct
sown fields. Usually 4-20 day old seedlings in flood seedbeds or in
direct sown fields with standing water are preferred for oviposition.
Plants older than 20 days and growing in dry fields are rarely infested.
The moths die shortly after oviposition.
6. Control measures. Light trap can be used to monitor the
emigration of this insect to the rice field. Flooding the rice field and
removal of alternate host such as grasses for clean cultivation have
shown to reduce the pest populations. Exposure of the larvae to natural
enemies and weather related factors can also control this pest. Planting
of sunflower and castor plants as trap crops around and within the
fields attracts the adults to lay their eggs. This trap crop has to be
inspected regularly to remove the eggs or larvae that have emerged .
There are a number of biological agents that could reduce the
population of this pests if employed such as Telenomus remus (egg
parasitoid), Apanteles ruficrus, A. kazak, Campoletis chlorideae, Hyposoter
didymator
(larval
parasitoids),
Canthoconidia furr:ellata and
Canocephalus sp. (predators), Serratia marcescens, Bacillus thuringiensis,
Nomuraea rileyi and polyhedrosis virus (pathogens). The heavy
dependent on insecticides for the control of this pest in non-rice field
350 J
Insect Injurious to Crops
has caused the pest to develop resistance to almost all the available
insecticides. In nee,
this pest has been shown resistance to
cypermethrin, fenvelerate, endosulfan, quinalphos, monocrotophos and
methomyl. Therefore, biological control needs to be given greater
priority alternative to chemical control.
Pests of Wheat
The wheat is an important cereal crop grown all over the world. The area
under wheat cultivation rn India is about 24 million hectares with annual
wheat production of 65 million tonnes. The productivity is 25 q/ha which is
very low as compared to other countries such as USA and Russia. One of
the major constraints for getting high yield of wheat is in�ect pests that
damage the crop. Apparently, 1t seems that the wheat crop is pest free
crop, but in reality it i� attacked by a number of insect pests such as
termites, grasshoppers. gujia weevil (Tanymecus 111dirns), grey weevil
(Myl/ocerus discolor). pink stem borer (Sesamia inferens), aphids
(Macrosiphum miscanthi. Sitobion avenae,
Rh(lpalosiphum maidis,
Schizaphis graminum), thnps (Anaphoth rips flavicenctus), army worm
(Mythimia separata), cut wonns (Agrotis spp.), pyrilla, etc. The biology of
S. inferens 1s given below.
[ I] The pink stem borer : Sesamia inferem
( Lepidoptera : Noctuidae)
1. Distribution, The pmk stem borer Sesamia inferens is distributed
throughout the rice cultivating countries of the world such as India,
Pakistan, Nepal, Bangladesh, Bhutan, Myanmar, China, Taiwan, Indonesia,
Japan, Kampuchea, Korea, Laos, M alaysia, Philippines, Singapore,
Sri Lanka, Thailand and Vietnam. In India, it infests wheat crops in
Rajasthan, Madhya Pradesh, Uttar Pradesh, Delhi, Haryana, Punj ab and
Gujarat (Fig. 1 6).
2. Host plants. This is an extremely polyphagous species that attacks
various cereal crops. In India, besides wheat, other crops like sugarcane,
maize, jowar, rice, barley, oats and some species of grasses have been
recorded as its alternate hosts.
3. Importance. The damage is caused by the caterpillars, which
bore into the stem after hatching and cause death of the cereal shoot
known as 'dead-hearts' . lbe caterpillars rrugrate from one plant to
others injuring several plants in their life. This pest is common during
the dry pre-monsoon period. The older plants are not killed but the
grain yield is very poor.
4. Appearance. Moths are moderately robust with pale yellow brown
body. The head and thorax bear tufts of thick brown hairs. The forewings
are brown to light brown in colour with dark brown markings. From a
Insect Injurious to Crops
�
[ 35 1
A
Fig. 16. The pink stem borer,
(DJ Adult.
c
Sesamia mferens
D
(A) Egg mass, ( B ) Larva, \ C J Pupa.
central point in the forewing, a few grey-black lines resembling a band
spread towards the wing tips, ending in a thin terminal line of dark spots.
The hindwings are white with light yellow scales along major veins.
5. Life cycle. The eggs are beadlike and are deposited in rows
between the leaf sheath and stem and are not covered with hairs. The
femaie also lay eggs on the soil surface near the base of the plant. A
female lays as many as 300 eggs in five masses. The incubation period
is 4-9 days in summer and 9-25 days in winter. The freshly hatched
larvae are pale yellow with the anal plates dark brown and usually do
not feed in groups. They bore into young seedlings and feed on the
central tissues. The full-grown caterpillar measures 35 mm in length and
3 mm in width with an orange red head. Its body is purple pink on top
and white below. The larvae after 3 1 -3 8 days of development pupate
inside the stem or between the leaf sheath and stem. Before pupation
the larva makes an exit hole for the adult emergence. The pupa is dark
brown and robust. Pupal period lasts for 5 to 1 2 days in summer and
1 2 to 36 days in winter. The pest completes its life cycle in 46 days in
summer and 7 1 days in winter in Indian conditions and has four to six
generations in a year.
6. Control measures. As the borer is internal feeder, preventive
measures should be employed. Removal of dead hearts and destruction
of larvae check the spread of the insect. After harvesting. the stubbles
should be removed as it minimise the pest infestation. Since the adult
moths are attracted towards light, light traps should be placed in the
fields to collect the adults. The collected adults should be killed. Kalyan
Sona is most susceptive to the pink borer followed by Sonora 64, C-28 1 ,
S-227 etc., therefore, these varieties should not be sown in areas
susceptible for attack of this pest If chemical treatment is necessary.
spray of carbaryl 0. 1% or endosulfan 0.07% thrice at an mterval of 1 5
days from a month after sowing gives protection from S . inferens.
Application of monocrotophos @ 0.25 kg a.i ./ha is also effective.
( Z-5 7)
352 J
Insect Injurious to Crops
Pests of Pulses
India is the largest producer and consumer of pulses accounting for 33%
of world area and 22% of the productivity. Pulse crops are cultivated over
an area of 24 million hectares with production of about 1 5 million tonnes.
Among the kharif pulse crops, pigeonpea (Cajanus cajan), mungbean
( Vigna mungo) and urdbean (Vigna radiata) and rabi pulse crops, chickpea
( Cicer arietinum), pea (Pisum sativum), lentil (Lens culinaris) are important
pulse crops of our country. All these crops are highly infested by hundred
of insects that reduce the yield upto 30-80% and the monetary value of
such losses have been estimated at Rs. 4000-5000 crores by the Indian
Institute of Pulses Research, Kanpur, U.P. The biology of only one insect
pest, the gram pod borer is given below.
[ I] The gram pod borer : Helicoverpa ( = Heliothis) armigera
( Lepidoptera : Noctuidae)
1. Distribution. Helicoverpa armigra is a cosmopolitan species feeding on
hundreds of the food plants. In USA, it is considered as a major pest on
cotton and sweet corn. .In India, it is recorded from most of the states on
one or other food plants (Fig. 1 7).
2. Host plants. H. armigera is a polyphagous insect and feed on
several agricultural crops distantly related taxonomically. Plants belonging
to Poaceae, Papilionaceae, Solanaceae and Malvaceae are most
preferred such as chickpeas, pigeonpeas, beans, soybeans, sunflower,
sorghum, maize, cotton, tobacco and winter cereals; vegetables including
beans and peas, capsicums, brassicas, lettuce, sweet corn, tomatoes; and
fruits such as citrus, strawberries, ginger, cape, gooseberries etc.
3. Importance. The H. armigera causes severe damage to pulses
particularly gram in north India and cotton and maize throughout the
country. Other crops mentioned above as host plant are also severely
C
B
�
A
Fig. 1 7. The grampod borer, Helzcoverpa arm1gera. (A) Larva. (B) Pupa, (C) Adult
(Z-57)
Insect Injurious to Crops
damaged
by
pigeonpea,
the
pest.
tomato,
l 353
Besides,
com,
it
millets,
following crops aJs.o:
In case of gram and pigeonpea,
also damages
etc.
the larvae chew leaves and bore into the pods and consume the seeds.
In cotton growing areas, it damage the cotton boll by boring into it and
feeding the seeds. It severely damage the com in new as well as old
world countries.
4. Appearance. The moth has light yellowish brown forewings and
grey to grey brown hindwings
which has
a
a broad dark band on the
outer third of the_ wing. Moths fly at ')l ight and are strongly attracted to
light. At rest the wings are held roofwise over the body. H. armigera
have a distinct kidney-shaped spot in the middle of the forewing, and a
pale patch in the middle of the dark band on the hindwing
.
5. Life cycle. Eggs can be laid all over the plant, but are most
abundant in the crop during flowering stage. Eggs are dome-like with a
ribbed surface.
They are about half the size of a pinhead and pearly
white in colour when
first laid; they later change to cream and then
brown. The eggs hatch in 3 to 7 days in warm weather. As many as
1500 eggs may be laid by a female over a 1 4 day period with peak
laying at about 7 days. The young caterpillars are predominantly green.
Large larvae, up to 30-40 mm long when mature, usually display striped
patterns and may vary in colour from light green to brown to black and
have distinct hairs when held up to the light. Larvae are mature after 2
to 3 weeks and pupate in the soil. The reddish-brown pupa forms in a
cell in the soil at a depth of 5 to IO cm. Pupation takes IO to 14 days
in summer but may be extended to several months in winter. The life
cycle
takes
about
5 to 7 weeks
in
summer.
Subsequent
generations
survive on successive plantings of a crop or on a succession of different
crop hosts.
6.
Control
measures.
Cultural
practices
like
post
harvesting
ploughing of the fields, rotation of crops, planting resistant cultivars, and
mechanical picking of larvae hanging down the pods sh()uld be adopted.
Provision of bird perching
myna
voraciously
marigold,
and
feeds
Tagetis erecta
larval
population
places js also effective as birds particularly
the
caterpillars.
Raising
as a trap crop for H.
on
main
marigold flower for oviposition.
crop
Use
due
was found very effective against
application
of
consumption.
At
present
make
the
to
yellow
the
higher
H. am1 igera
crop
pathogenic
fungi
flower
reduce the egg
preference
unsuitable
of
Bacillus
of a bacterial pathogen,
thuringiensis
insecticides
of
armigera
on pulses as
(Beauveria
for
human
spp.) and
viruses (Nuclear Polyhedrosis Virus) are available that infect the grubs
of
H. armigera.
Similarly,
mass-reared
Trichogramma chilonis
are also
available for its biological control particularly on cotton crop. A larval
parasitoid
agent.
Campoletis chlorideae
also
shows
potential
as
its
biocontrol
(Z-57)
354 1
Insect Injurious to Crops
Pests of Maize
Com is an important staple food and animal feed and it ranks third behind
wheat and rice. In India, the area under maize cultivation (6 million
hectares during 1 995-96) has significantly increased during the last decade,
but the national average yield of l lq/ha is significantly lower than the
world average of 29 q/ha. The high infestation level of insect pests is one
of the major constraints of this low yield. Following important insects
damaging the crop are enlisted here: stem fly (Antherigona orientalis), red
hairy caterpillar (Amsacta moorei), stem borers (Chilo - partellus, Sesamia
inferens -biology is given under wheat pests), white grub (Holotricha
consanguinea), armyworm (Mythimea separata), earworm (Helicoverpa
armigera- biology is given under pulse pests), aphids (Rhopalosiphum
maidis, Hysteroneura setariae) etc. Biology of one of them is given below.
[ I) The maize stem borer : Chilo partellus ( = C. zanellus)
(Lepidoptera : Pyralidoidea, Crambidae)
1. Distribution. The maize shoot borer Chilo partellus is widely distributed
but is considered as a major pest of corn in India, Sri Lanka, Pakistan and
Uganda (Fig. 1 8).
2. Host plants. Corn is the main host of C. partellus, but it also
damage other cereal crops such as rice, millets, sugarcane, munja, etc.
3. Importance. C. partellus is a major pest of corn and is one of
the limiting factor in the successful cultivation of this crop. It causes an
average of 55-83% grain loss and 28% forage yield loss in north India.
The newly hatched caterpillars feed the newly sprung shoots, leaves,
cobs of maize and also bore into the stem and kill the central shoot
causing dead hearts. It infests the plant usually a month after sowing till
harvest. When fully grown up plants are infested, they lose their vigour
and form weak ears.
4. Appearance. The adult moth is straw coloured, medium sized (25
mm across wingspan) and bears double rows of black spots
on the forewings. The forewings are darker than hindwings. The
terminal end of female is dilated and covered with tuft of hairs.
5. Life cycle. The female moth lays 150-300 eggs either singly or in
masses arranged in rows, and normally overlapping eacb other on the
lower surface of the leaf near the midrib and occasionally on stalk. The
eggs are scale-like, flattish oval and yellowish in colour. After a week of
incubation, eggs hatch. The newly hatched larvae which are dirty white
in colour with black head, bite their way into the stem causing dead
hearts. The midribs of the leaves are often noticed mined by the newly
hatched larvae. Most of the second instar larvae migrate to the
neighbouring plants. The larva undergoes five moults but may undergo
(Z-57)
Insect Injurious to Crops
[ 355
�
A
Fig. 1 8 . The maize stem
B
c
borer, Chilo partellus. (A) Larva, (B) Pupa, (C) Adult.
extra moults during winter. The larva becomes full-grown within 1 -4
weeks and measures 25 mm in length with four longitudinal stripes over
the body. The last instar larva constructs a silken thread and pupates
inside stem. It takes 6- 1 2 days for the adult emergence. The life cycle
completes within 5-7 weeks in summer and 12-25 weeks in winter.
6. Control measures. In order to save the crop from the borer,
infected shoots and leaves should be removed. Similarly, after harvesting,
the stubbles should be collected and burnt. Certain biocontrol agents
like its egg parasitoid Trichogramma spp. and the larval parasitoid,
Bracon chinensis be released in the infested field. If chemical treatment
becomes necessary, spray of carbaryl 0. 1 % and endosulfan 0.07% thrice
at interval of 15 days from a month after sowing gives protection. To
avoid costly schedule of pesticide application following economical
practice be adopted. At first, spray on entire field on a 10-days old
crop with insecticides such as sevin. 50 WP ( lOOg) or folithion 50 E C
( 1 75 ml) o r thiodan 35 E C ( 100 ml). It should b e followed b y two spot
applications of 1 kg dust of diptex 5% or sumithion 5% mixed in soil
or 200-500 g granules of sevin 4G or lindane 6G in the infested whorls
at weekly intervals.
Pests of Vegetables
Vegetables are the edible products of herbaceous plants and can be
grouped according to the edible part of each plant such as leaves (lettuce,
cabbage), roots (carrot, radish), tubers (potato), bulbs (onion, garlic),
flowers (broccoli, cauliflower), fruits (brinjal, tomato, pumpkin, okra,
beans) and seeds (peas). Most vegetables are valuable sources of vitamins,
minerals, and fiber and are low in fat and calories. With cereals and
legumes, they are important to a healthy diet. Several insects infest
these crops. Following are description of few insect pests of some
vegetable crops.
Insect Injurious to Crops
356 J
[ I] The red pumpkin beetle : Aulacophora indica
( = Aulacophora similis, A. testacea, Raphidopalpa foveicollis,
R. benga.lensis) (Coleoptera : Chrysomelidae)
1. Distribution. The red pumpkin beetle is widely distributed in old world
countries such as India, Sri Lanka, Myanmar, Nepal, Bhutan, Andaman,
Nicobar,
Thailand,
Cambodia,
Laos,
Vietnam,
Hainan,
China,
Taiwan,
Philippines, Ryukyu Is., Japan, Korea, Siberia, Sunda Is., Micronesia, New
Guinea, Samoa, Fiji, Peninsular Malaysia, and Borneo. In India, it is widely
distributed in
Uttar Pradesh,
Bihar,
Haryana,
Punjab, Madhya Pradesh,
West Bengal, Maharastra etc. The other species of pumpkin beetles are
A. cincta (= A . stevens1)
(grey coloured) and
A . lewisii
(blue coloured)
(Fig. 19).
2. Host
plants. The red pumkin
beetle
feeds
upon
almost
all
(l.Agenaria vulgaris), ghia torai
(Luffa cylindrica, L. aegyptica), pumpkin (Cucurbita pepo, C. maxima),
cucumber, tinda (Citrullus vulgaris), snake gourd, melon etc.
cucurbit vegetables such as bottle gourd
3. Importance. The adults and grubs both cause a great deal of
damage to plants. The beetles bite holes on the leaves and also feed on
flowers,
buds,
stems
and
even
fruits
making
them
unfit
for
human
consumption. The infestation of young plants causes stunted growth and
brings about its death.
The grubs
stem and fruits that come
in
stay in the soil,
contact
with the
soil
feed on the root,
and thus
damage
them.
4. Appearance. Adult beetles are small measuring 7.0 mm in length
and 3.7 mm in width. The elytra of red pumpkin beetle is pale orange
yellow to deep pale brown while in case of blue pumpkin beetle i� is
blue and
it is yellowish in yellow pumpkin beetle.
/
S. Life cycle. The female lays 1 50-300 eggs either singly or in groups
on
humid
soil.
The
eggs
are
brownish
or
orange
and
elongated
in
shape. The incubation period varies between 5 days in summer to 1 5
B
Fig.
19. The pwnpkin beetle, Aulacophora
c
sp. (A) Larva, (B) Pupa, (C) Adult.
Insect Injurious to Crops
[ 357
days in winter. The grubs are small, slender, elongate, creamy yellow
with brown head and legs, and mature in 1 3-25 days and pupate in the
soil. The pupal period ranges from 7- 1 7 days. In a year there may be
5-8 generations of the beetle.
6. Control measures. After harvesting, the remains of the crop
should be burnt to kill the diapausing stages of the pest. The fields
should also be deeply ploughed to expose the eggs and grubs which are
later on destroyed by -natural means. The resistant varieties should be
sown and care should also be taken in sowing time. All cucurbit plants
should be sown before November to avoid infestation. Collection and
destruction of adults and grubs prevent the insect to attain pest status.
The spray application of methyl parathion 0.05%, parathion 0.025% or
phosphamidon 0.04% is of great use in checking the pest population.
[ II] The brinjal shoot and fruit borer : Leucinodes otbonalis
(Lepidoptera : Pyraustidae)
1. Distribution. L. orbonalis is distributed throughout the country. It is also
reported from Belgium, Myanmar, Sri Lanka, China, Malaysia and
Germany (Fig. 20).
2. Host plants. The main host plant is brinjal (Solanum melongena)
but occasionally it also cause damage to tomato, potato and other
solanaceous wild plants.
3. Importance. L. orbonalis is the most serious pest of brinjal. The
caterpillar bores into the terminal tender shoots causing 'dead hearts' . It
also bores into flower buds and developing fruits causing shedding of
buds and making the fruits unfit for human consumption. It may
damage up to 70% of the crop.
4. Appearance. The adult moth is small with white wings with
triangular brownish or red markings. The size of the moth is 2.0 cm
across the expanded wings.
5. Life cycle. The female lays about 250 eggs singly on tender shoots
and developing fruits of brinjal. The eggs are flat and white in colour.
Incubation period ranges between 3-5 days. The newly emerged larva
feeds the stem, flower buds or fruits making tunnel. The infested fruits
may be marked by the presence of entry hole filled with excreta. The
larval period is 1 4-20 days during which it moults 4-5 times. The
full-grown larva is pale white with violet spots arranged in linear lines
on the body. It measures about 1 6-20 mm in length. The hairs are
sparsely distributed on warts on the body. It pupates in a grey, tough
and boat-shaped cocoon on the £tern and fruit. Pupal period lasts after
6- 1 1 days and adult moth emerges out and continues its life cycle. The
moth is active throughout the year. In a year, it passes through 1 0- 1 3
generations in a year.
Insect Injurious to Crops
358 J
c
B
�
A
Fig. 20. The brinjal fruit and shoot borer,
(C) Adult.
Leucinodes orbonalis.
(A) Larva, (B) Pupa,
6. Control measures. After harvesting, the fields should be deeply
ploughed to expose the pupae which are later on destroyed by natural
means. Removal of damaged shoot and fruits is one of the best
approach to r�duce the yield loss caused by L. orbonalis. The following
insecticides may be applied to control the pest infestation: malathion
emulsion, fenitrothion, nuvacron, profenofos, cypermethrin, carbaryl,
thuricide HP and dimethoate (Rogor-40).
[ III] The fruit fly : Bactrocera ( = Dacus) cucurbitae
(Diptera : Tephritidae)
1. Distribution. The species of Bactrocera are cosmopolitan in distribution.
In India, B. cucurbitae and B. ciliatus are most common species (Fig. 2 1).
2. Host plants. The main food plants of the pumpkin fruit fly are
bitter gourd, snake gourd, melon and tondli. In addition, it also feeds
on guava, mango, ber and other fruits.
3. Importance. B. cucurbitae is most notorious pest of bitter gourd
and melon. The maggots of the fly cause damage by boring into the
mellow fruits which at last rot and fall off the plant. In addition to the
fruits noted above, this pest also damages a number of vegetables.
4. Appearance. Adult
is
wedge-shaped
small
insect
with
reddish-brown body and black and white spots on the head. Greenish
yellow lines are �resent over the thorax. The females are bigger than
males and measure 6-7 mm in length. The outer margin of the wings
provided with brownish lines with grey spots. The last abdominal
segments modify to conical ovipositor for oviposition inside the rind of
the fruits. While sitting or ovipositing, the wings are always expanded.
5. Life cycle. The breeding of the fly begins with the rainy season.
The female makes holes in the rind of tender fruits and inserts the eggs
singly or in groups of 4 to 1 0 into them. A single female lays usually
Insect Injurious to Crops
I 359
�
A
([[Tr J 1/ '/ '1 DY
B
c
Fig. 2 1 . The melon fruit fly, Bactrocera (= Dacus) cucurbitiae. (A) Egg, (B) Larva,
(C) Pupa, D. Adult.
1 50-200 eggs in her life. The eggs are tiny, cylindrical and glossy white
in colour. The maggots that hatch out from the eggs in 2-9 days feed
on the pulp and seeds of the fruit. Larval period is 5-9 days in summer
and nearly three weeks long in winter. The full-grown maggots drop to
the ground and pupate in the soil at a depth of 2 to 15 cm. The pupal
period is 5-1 1 days after which the adult fly emerges out from the
pupal case. The total life cycle completes within 12 to 34 days
depending on the temperature. Adults hibernate during winter. They
become active in hot weather. There are 7- 1 0 generations of the fly in
a year.
6. Control measures. Clean cultivation, i .e., removal and destruction
of fallen and infested fruits daily and deep ploughing after harvesting
the crop to destroy pupae provide considerable success in preventing
the pest infestation. Application of baits having fermented palm juice or
protein hydrolysate and suitable insecticide provides good result. The
flies when they congregate and rest on the under surface of the leaves
may be controlled by spray application of cypermethrin 0.025 %. Spray
application of three to five rounds of profenofos 0.05% or fenthion
0. 1 % or carbaryl 0. 1% or malathion 0.05% at intervals of 1 5 days
commencing from flowering may be useful. Before each application the
fruits should be harvested.
[ IV] The epilachna beetles :
Epilachna dodecastigma, E. vigintioctopunctata
( Coleoptera : Coccinellidae)
1. Distribution. The adult epilachna beetles as well as its grubs cause
serious damage to cucurbit, brinjal and potato throughout the India and
other south-east Asian countries (Fig. 22).
The epilachna beetles are polyphagus mostly
2. Host plants.
feeding on vegetables such as bitter gourd, brinjal, potato, tomato, etc.
E. dodecastigma prefers cucurbit vegetables while E. vigintioctopunctata
mostly feeds on solanaceous vegetables.
Insect Injurious to Crops
360 1
AOOl'J
c
Fig. 22. The
(D) Adult.
epilachna beetle,
D
Epilachna vigintioctopunctata. (A) Eggs,
(B) Grub, (C) Pupa,
3. Importance. Both grubs and adult beetles feed by scrapping
chlorophyll from epidermal layers of leaves in a semicurcular pattern in
rows. The infested leaves tum brown which gradually dry away and fall
off resulting into complete defoliation of the plant.
4. Appearance. The adult beetles are 8 mm long and 5-6 mm wide
and are spherical, pale brown with black spots. E. dodecastigma has 6
spots on each elytron while other species such as E. vigintioctopunctata
bears 28 spots.
5. Life cycle. The epilachna beetles are active throughout May to
August in hilly areas and May to September in plains. They hibernate in
winter in the heaps of dry plants, cracks and cervices or in the soil.
The female lays about 450 eggs in clusters, each cluster contains 15-50
eggs, on the under surface of the leaves. The eggs are cigar-shaped,
bright yellowish in colour. The egg hatches within 2-7 days (2-3 days in
summer, 4-7 days in winter). The young larvae are small, flat and yellow
in colour with yellow spines or hairs on the. dorsum. After passing
through four moults during 7-2 1 days, it becomes full-grown. The
full-grown grub is about 8 mm in length and 4 mm in width. Pupation
takes place on the underside of the leaves. The pupa is yellow orange
in colour having brown white margins on the dorsum. Its anterior
portion is smooth while posterior region is spinous. Pupal period lasts
after few days in summer and 1-2 weeks in winter, after which adult
beetle emerges out. The life cycle is completed within 1 5-54 days. The
adult survives for 4 weeks to 6 months. There are 7-8 generations of the
epilachna beetle in plains and 1 -2 generations in hills.
6. Control measures. The leaves having eggs and grubs and adult
insects should be collected and destroyed in the intial stage. Spraying of
0. 1 % carbaryl or 0.02% diazinon or 0.05% malathion or dichlorvos
(DDVP) provide considerable protection from the insect. During March,
a number of parasitoids such as Pediobius foveolatus, parasitise more
Insect Injurious to Crops
than
{ 361
70% of the grubs and at this period application of insecticides
should be avoided.
[ V] Tb� potato tubermoth :
Phthorimaea (= Gnorimoschema) operculella
(Lepidoptera : Gelechaidae)
1. Distribution. The potato tuber moth is distributed throughout the world
where potato is cultivated such as India, Australia, New Zealand, China,
Iran, France, South America, North America, etc. In India, it is considered
as pest in Uttar Pradesh, Uttaranchal, Himachal Pradesh, Maharastra and
Bihar (Fig.
23).
2. Host plants. The main host of P. operculella is potato tuber both
in fields as well as in storage. In addition, it also feeds on tomato,
brinjal, tobacco and other wild plants of Solanaceae family.
3. Importance. Only
the
caterpillars
damage
the
crop.
Damage
consists of foliage injury caused by the mining between the leaf surface
and in the stems. Severe loss of the tubers also results, both in the field
and
in storage, owing to the larval tunnels which are contaminated with
excrement and permit the entrance of decay
organisms.
The caterpillars
may cause upto
70-90% loss of the tubers particularly in storage.
4. Appearance. The adult is a small grey moth with a wing expanse
of about 1 2- 1 5 mm. The wings are narrow, fringed with hairs, and
multicoloured with black and brown spots.
5. Life cycle. The potato tuber moth hibernates in winter as larvae
or pupae in the soil or in storage. The moths emerge with the coming
of warm weather and begin to
lay
eggs
potato tuber. Each female lays about
on potato leaves
or eyes of
100-200 eggs. Incubation period is
about 3-5 days. The larvae pass through 4 instars in reaching full
development. The larval period varies between 5-16 days. - The dark
brown headed larva in its full growth looks pinkish white in colour. The
larvae pupates in silken greyish cocoon within the trash fallen under the
plants
on
the
cracks
on
walls
soil.
In
storage,
it
also
pupates
on
storage
bags
and
The pupal period lasts for 7-10 days. A
complete life cycle takes about a month. Several generations, usually 8-9,
and
are produced each
floors.
season.
6. Control measures. The spring crop should be planted earlier and
growing tubers must be covered with at least 5 cm of soil. The crop
should
be
harvested
as
early
as
possible
avoiding
leaving
exposed
potatoes overnight as night is congenial to their egg laying. Infested and
discarded potatoes
should be destroyed as
they may
serve as breeding
material. Infested leaves should be collected and destroyed. If necessary,
5% malathion powder may . be dusted over infested crop @ 2.5 kg/ha.
seed potatoes should be treated with 5% malathion @ 1 00 g/q
The
362 }
Insect Injurious to Crops
c
B
Fig.
23.
4J�1*��
A
The potato tuber moth, Phthorimaea operculella.
(A) Larva,
(B) Pupa, (C) Adult.
potato. In order to preserve the potato from the infestation, the godown
should be fumigated with methyl bromide 4-8 kg/100 m3 . Potatoes
should be stored at the lower temperature particularly less than lO"C
temperature.
[ VI] The cabbage butterfly : Pieris brassicae
(Lepidoptera : Pieridae)
1. Distribution. The cabbage butterfly is distributed all over the world
where cabbage is cultivated. In India, it is found all over the country but
it is more prevalent in north India (Fig. 24).
2. Host plants. The main host plant is cabbage, but it equally
infests other brassica crops such as cauliflower, broccoli, radish, kale,
mustard and related plants, as well as lettuce in United States.
3. Importance. Only the caterpillars cause irrepairable damage to
cabbage. They feed on the leaves of cabbage and cauliflower and thus
destroy them.
4. Appearance. The cabbage butterfly is yellowish white in colour.
The apical angles of forewings
are black while rest of the wing is
yellowish white.
There are two black spots on each forewing. The
length of the butterfly is 65 mm across the wing expansion. Females are
little longer than males.
5. Life cycle. The female lays 50-80 eggs usually in cluster on the
uncfersurface of the leaves. The eggs are yellow in colour, flask-shaped
and about 1 mm long and 0.5 mm wide. Incubation period varies
between 3 days in summer and 1 7 days in winter. The young larvae are
about 2 mm in length and light yellow in colour. The body is hairy. The
larval period lasts for 1 5 days in summer to 40 days in winter.
Full-grown larva is 19 mm in length, bluish green in colour with
yellowish grey shades and possess 5 pairs of pseudolegs. It pupates on
leaves of the cabbage or branches of nearby shrubs. The pupa is
Insect Injurious to Crops
I 363
c
A
Fig. 24. The cabbage butterfly, Pieris brassicae.
(A) Larva, (B) Pupa, (C) Adult.
enclosed within a silken cocoon. The pupal period varies from 7 days in
summer to 28 days in winters. The female copulates just after
emergence and begins to lay eggs.
6. Control measures. Initially the caterpillars feed in groups and at
that stage they should be collected and destroyed. The full-grown
caterpillars are not much active and can be seen moving anywhere in
the fields, therefore, th�y can be picked by hand and killed. If
nec�ssary, spraying of 0.05% endosulfan or 0.05% carbaryl or 0.03%
diclorovos or 5 % malathion is quite satisfactory in checking their
infestation.
Pests of Oilseeds
Oilseed crops play a vital role in India's agricultural economy and are
cultivated in an area of about 1 7 million hectare ( 1 3 % of gross cropped
area). Among the oilseed crops grown in India, groundnut (Arachis
hypogaea) accounts for 45% of the total area cropped under oilseeds
followed by rapeseed mustard (Brassica spp.). Insect pests cause severe
losses to the groundnut and are one of the major constraints for low yield
of groundnut. The crop is attacked by about 1 00 species of insects
throughout the country at different stages of plant growth during different
seasons, but only few insects viz., the aphid (Aphis craccivora), thrips
(Scirtothrips schultze� Frankliniella schultzei), jassids, (Empoasca kerri), leaf
miner (Aproaerema modicella), hairy caterpillars (Amsacta spp.), tobacco
caterpillar (Spodoptera litura), white grubs (Holotrichia spp.) and termites
(Odontotermes spp.) are recognised as the important pests of groundnuts.
The total annual loss from field pests alone has been estimated about
Rs. 1 600 million. Among the brassica crops following species/cultivars are
commonly grown in India: Brassica campestris var. yellow sarsoon,
B. campestris var . toria, B. campestris var. brown sarsoon, B. napus
(gobhi sarsoon), B. juncea (rai), B. nigra (Banarasi rai), B. alba
364 J
Insect Injurious to Crops
(white mustard), B. carinata (karan rai) and Eruca sativa (taramira) .
Several insects damage the crop at its various stage reducing the yield.
Among them, mustard aphid, Lipaphis erysimi is the key pest. Two other
insects, the mustard sawfly, Athalia lugens praxima and the painted bug,
Bagrada cruciferarum also damage the crop to considerable extent.
[ I] The red hairy caterpillars
(Lepidoptera: Arctiidae)
:
Amsacta albistriga and A. moorei
1. Distribution. Both the species of red hairy caterpillars are very similar
in habit and habitat. The red hairy caterpillar A. albistriga is widely
distributed in south India and Tamil Nadu, Andhra Pradesh and Karnataka
are the most suffered states. In noi:th and central India A . m oorei is most
common. H owever, frequent mating between both species was observed in
Pollachi tract of Tamil Nadu (Fig. 25).
2. Hosts plants. The red hairy caterpillars are polyphagous insects but
particularly destructive to groundnut, mungbean and blackgram. Besides
these crops, it also feeds on sorghum, cotton, castor, finger millet, pearl
millet, ragi, maize, soybean, horsegram, clusterbean, pigeonpea, sesame, etc.
3. Importance. This is one of the worst pest of groundnut causing
much loss to the crop particularly in south India. The young larvae feed
gregariously on the under surface of the leaves by scrapping the
chlorophyll for 4-5 days on the same leaves where eggs are laid. The
skeletonised leaves can be easily detected even from the distant places.
When larvae grow they disperse and feed individually by devouring
leaves, flowers and growing points. When a large number of caterpillars
infest the crop, only the base stems of plants remain resulting in heavy
and occasionally total yield loss.
4. Appearance. The adults are medium sized moths and are about
25 mm in length. The forewings are white with brownish streaks all over
and yellowish streak along the anterior margin and the hindwings are
white with black markings. A yellow band is seen on head. Larvae are
initially ash brown in colour but when fall grown assume reddish colour
with black bands on either ends with long reddish brown hairs all over
the body.
5. Life cycle. Usually adult moths emerge two days after the onset
of heavy rains. They copulate immediately and oviposit on the same
night. A female moth lays 600-700 eggs and sometimes as high as 2300
eggs are also laid by a single female. The eggs are laid in a cluster of
30-40 usually at the underside of the leaves. Eggs are laid on the other
available host plants and even on the clods of the earth. The oviposition
lasts for 2-4 days and the incubation period lasts for 3-4 days. Emerged
larvae remain on the same leaves for 3-5 days, feed gregariously by
scraping the chlorophyll giving them papery appearance. Larval period
lnsect lnjurious to Crops
B
[ 365
c
Fig. 25. The red hairy caterpillars, (A) Larva, (B) Adult A m sacta albisrriga,
(C) A. moorei
ranges from 20-35 days. Pupation talces place in soil at a depth of 1 0-20
cm under the trees, hedges, shady corners or bunds. The pupae remain
in soil in diapause stage until the next season. There is only one
generation in a year. However, in some places at Tamil Nadu a short
cycle of insect has also been observed where the larvae of the early
emergence pupate and emerge - after short pupal period and infest the
crops.
6. Control measures. The egg masses, young larvae and adults should
be manually collected and destroyed. Restricting the dispersal of larvae
from one field to another by digging trenches across the march of the
larvae also provide good mechanical control. Trap crops like barnyard
millet (Echinochloa frumentacea), cowpea and castor should be grown as
trap crops. ULV formulation of heliotox and high volume spraying of
dichlorvos protect the crop from this insect. In nature, the parasitoids,
Exorista civiloides, Tachina fallax and three species of Cotesia found
parasitising larvae. Trichogramma evanesc-ens minutum and
manolus are potential mortality factor for the caterpillars.
Telenomus
[ II] The mustard aphid : Lipaphis erysimi
(Hemiptera : Aphididae)
1. Distribution. L. erysimi is a cosmopolitan species. In India it is more
prevalent in north Indian states such as Punjab, Haryana, Uttar Pradesh,
Himachal Pradesh, Uttaranchal, Bihar, Madhya Pradesh, Chhatisgarh,
West Bengal, Sikkim, Assam, Gujarat, Rajasthan and northern areas of
Maharastra (Fig. 26).
2. Host plants. Many genera and species of Brassica and other
species of Brassicaceae such as Raphanus and Rorippa serve as primary
366 J
Insect Injurious to Crops
Fig. 26. The mustard aphid (viviparous form), Lipaph1s erysimi. (A) First instar nymph,
(B) Third instar nymph, (C) Last instar nymph, (D) Adult winged form.
food plants for the mustard aphid. In addition, plants such as Barbarea,
Capsella, Erysimum, lberis, Lepidium, Matthiola, Nasturtium, Sinapis,
Sisymbrium, Thlaspi, etc. also occasionally serve as food plant for
mustard aphid.
3. Importance. Both nymph and adult aphids attack all aerial parts
of the mustard plants. They directly damage the plants by sucking their
nutrients which causes general devitalisation of plants. They also
indirectly affect the health .of the plant by their copious secretion of
honeydew that occlude$ the stomata! openings of the leaves and thus
hamper their normal physiological processes like photosynthesis and
respiration. Deposition of honeydew on leaf surface also allows the
growth of black mould which in tum proves detrimental to the plant
life. When attack of this pest occurs in early stages of the plant, the
leaves get discoloured, curled and withered. Plant remains stunted and
ultimately dries up. Damage is more severe when aphid attacks in
flowering and fruiting stages of the plant. The affected flowers get
discoloured and distorted and fall down, hence_ no pod is formed.
Attacked pods get curled, shrivelled and no seed formation takes place.
If seeds set, they get shrivelled and there is a drastic reduction in seed
weight, oil content and seed viability. L erysimi is also a vector of
about 1 0 non-persistent plant viruses including cabbage black ring spot
and mosaic diseases of cauliflower, radish, and turnip. Under different
agro-climatic conditions the mustard aphid damage the mustard crop
from 35 to 90 per cent, particularly in north India where mustard is the
principal oil crop.
4. Appearance.
The mustard aphid like other aphids exhibits
polymorphism. The wingless (apterae) and winged (alatae) forms may
_
Insect Injurious to Crops
[ 3{J 7
occur simultaneously in the
field. The apterae are small to
medium-sized, yellowish green, grey green, or olive green, with a white
wax bloom. In humid conditions the body is often more densely coated
with wax secreted by cornicles which are tube like structures on the
posterior portion of dorsum of the abdomen. The alatae have a dusky
green abdomen with conspicuous dark lateral sclerites, and dusky wing
veins. Sometimes they are observed in large numbers on the undersides
of leaves, which may curl and turn yellow, or in inflorescences of host
plants.
The size of apterae is 1 .4-2.4 mm long and that of alatae
1 .4-2.2 mm.
5. Life cycle. Though the pattern of life cycle of L. erysim i is
predominantly anholocyclic (continuously parthenogenetic) in India,
however, others living in temperate climates are holocyclic (sexual
generation alternates with parthenogenetic reproduction). Sexuales have
been reported from Europe, India, China and New Zealand. In India,
the mustard aphid appears in November on the rape and mustard
crops. Initially a small colony of females colonise and reproduce
parthenogenetically.
The
females
are
v1v1parous
(more
precisely
larviparous) and give birth of nymphs. The growth of the nymphs is very
fast and within 1 -2 weeks they become adult. Both winged and wingless
forms develop. The developmental period varies with food plants, for
example, on mustard and cauliflower, it takes about 1 2 days but on
radish it takes 1 1 days only. The reproductive period of both forms
varies considerably. The winged forms reproduce for 1 3- 1 7 days while
wingless forms 12-22 days on different brassica plants. A single female
of winged form gives birth of 35-40 11ymphs while wingless form
larviposits 70- 135 nymphs at the rate of 3.5 nymphs/day in her life span.
Maximum number of nymphs are laid on mustard crop. The aphid is
active throughout December to February passing through at least 16
overlapping generations. The females survived for 26-38 days on
different food plants.
The . winged forms migrate from one field to
others and spread the infestation.
6. Control measures.
The prophylactic measures should be
employed first. Cultivation of resistant varieties, applicatk>n of adequate
amount of nitrogen fertiliser, frequency of irrigation, trap cropping etc.
are some cultural methods which should be followed to minimise the
infestation of the aphids. If necessary, application of following
insecticides should be done: chinimix (5%), chlorpyriphos (0.04%),
dichlorvos (0.20%), dimethoate (0.06%), endosulfan (0.07%), M.l.P.C.
(50%), malathion (0.20%) , methyl-o-demeton (0.05%), monocrotophos
(0.08%),
phosphamidon
(0. 1 0% ) , and quinalphos (0.05% ).
Other
insecticides
like
bifenthrin,
cyfluthrin,
cypermethrin,
dicrotophos,
(Z-57)
368 J
Insect Injurious to Crops
ethiofencarb,
fenvalerate,
furadan,
imidacloprid,
methamidophos,
parathion, permethrin, pirimicarb also give satisfactory results.
[ Ill] The mustard sawfly : Athalia lugens proxima
(Hymenoptera : Tenthridinidae)
1. Distribution. Like mustard aphid, the mustard sawfly is also distributed
where the brassica crops are cultivated. It is maialy found in India,
Bangladesh, Sri Lanka, Myanmar, Indonesia, China, Great Britain, Spain,
Germany, Japan, Africa etc (Fig. 27).
2. Host plants. It is high1y host specific and feeds on only plants of
Brassicaceae family such as mustard, tori, rape, turnip, cabbage,
cauliflower, radish etc. However, it prefers mustard and turnip plant.
3. Importance. Only larval stages of the sawfly' damage the seedlings
of the brassica crops during August to November and also damage the
crops during October to March. The adults are free living and feed on
pollen and other plant sugary secretions. The young grubs feed in situ
first by making minute excavations and then very small holes in the
leaves. The grown-up larvae feed from the margin and in condition of
severe damage, the crop looks as if it has been grazed by cattle.
Sometimes, it feeds the epidermis of shoots. Due to this the plant dries
and dies. If the infested plant survived, remain stunted without fruits.
The "losses caused by this insect are about 1 5%.
4. Appearance. The adult is a small, 1 5 mm long, and is orange in
color having smoky wings with black veins. The femora and thorax are
yellow. The female possesses a saw-like ovipositor. They have two pairs of
black coloured wings. The adults are diurnal.
5. Life cycle. The female has serrated ovipositor, hence called sawfly.
With the help of its ovipositor the female places eggs inside the tissues of
the leaves, generally in the lower surface. A single female lays 30- 1 30 eggs
in her life of 6-8 days. The eggs, which are laid singly, are oval and cream
coloured and hatch in about 6-8 days. Newly hatched grubs which are light
green in colour and measure 1 .8 - 3 .0 mm in length, feed on the leaves in
groups of 4-8 during morning and evening periods. 1n day time, they hide
themselves in leaves or soil. As they attain maturity, they put on green black
colour with five lateral stripes. The larva passes through 6 larval stages in
1 6-35 days. The full-grown larva is darker in colour and measures 14 mm
in length. For pupation, the larvae descend from the plant and enter the
soil to a depth of 25-30 mm, and construct a waterproof silken parchment
like cocoon over which soil particles are adhered. Pupal period varies with
the food plants. On mustard, it is about 7-10 days on average. The adults
are very active during September to December after which its population
declines. The complete life cycle requires about 3 1-7 5 days in different
seasons. Two to three generations occur during October to March.
(Z-57)
Insect Injurious to Crops
�
A
[ 369
8
Fig. 27. The mustard sawfly, Athalia /ugens maxima. (A) Larva, (B) Pupa, (C) Adult.
6. Control measures. The crop sanitation practices should be
followed as these keep the field free from pest incidence. Timely
irrigation helps in killing the larvae through drowning. The grubs should
also be hand-picked and killed. If insecticidal treatments become
inevitable, following insecticides may be sprayed: 0.03% diazinon or
0. 1 % malathion or 0.03% dimethoate. Dusting of 2% folidol powder
also give satisfactory result.
.
[ IV] The painted bug : Bagrada cruciferarum (= B. picta)
(Hemiptera : Pentatomidae)
1. Distribution. The painted bug is distributed in India, East Africa,
Sri Lanka, Pakistan, East and West Asia, Afganistan etc. In India, it
is observed at most of the places where mustard is grown (Fig. 28) .
2. Host plants. The painted bug is polyphagous and feeds on
mustard, cabbage, cauliflower, radish and other brassica plants. It also
feeds on maize, sugarcane, bean, indigo, coffee etc.
3. Importance. The nymphs as well as adults suck the sap from
leaves, shoots and pods and adversely affect the vigour of the plant.
The growth of the plant is reduced and plants may dry. Both the
nymphs and adults excrete resinous substance that damag�s the siliqua.
It is most destructive during March in Uttar Pradesh.
4. Appearance. The adult bug is flat, small, 5-7 mm long and 3-4
mm wide and black in colour with red and yellow spots. The colour of
antennae and legs of the adult is black or smoky. The scutellum of the
bug is very large at least half as long as the abdomen. The adults are
capable to survive under acute starvation.
5. Life cycle. The bug is active during March to December. In
winter, the adults are abundant. The female lays eggs singly or m
cluster of 3-5 eggs on the leaves, petiole, stem of the plant and also in
soil below plant debris. Each female lays about 90-200 eggs. The eggs
are oval or barrel shaped and yellow or brownish in colour. Eggs tum
pink after 3 days of deposition. After 3-5 days of incubation period in
summer and 20 days in winter, the eggs hatch into tiny nymphs. The
(Z-57)
insect Injurious to Crops
3 70 1
c:f38o
0 0
M
I
A
8
c
Fig. 28. The painted bug, Bagrada cruciferarum (=B. picta). (A) Eggs, (B)
(C) Adults.
Nymph,
young nymph begins to suck the plant juice. After passing through 5
nymphal instars the nymph becomes adult. The colour of first and
second instar is bright orange and of third and fourth is red. Nymphal
period is about 1 4-2 1 days. The total life cycle takes about 2 1 (in
summer)
56 (in winter) days and up to 9 overlapping generations of
the bug may occur in a year. D uring hot summer months, bugs can be
seen congregating over several weeds from where they switch over to
the germinating brassica vegetables in July to August.
6. Control measures.
The first irrigation should be applied 3-4
weeks after sowing the crop. It will give an effective control of the
painted bug. The nymphs and adults both should be collected by
sweeping nets and killed. The stubbles of the old infested plants should
be destroyed. If insecticidal treatments become necessary, following
insecticides may be sprayed to control the bugs: 0.02% diazinon or
0. 1% malathion or 0.03% dimethoate. Dusting of 2% folidol powder
also give satisfactory result.
-
Pests of Fruit Trees
[ I) The San Jose scale : Quadraspidiotus perniciosus
(Hemiptera : Diaspididae)
1. Distribution. The San Jose scale is native to China but was introduced
from Japan into San Jose, California. Currently, the species is distributed
throughout southern Canada, United States, India, South Africa and
New Zealand (Fig. 29).
2. Host plants. San Jose scale attacks most cultivated fruits and a
large number of omamencal shrubs and trees such as pear, plum, peach,
cherry, currant and black currant. Citrus fruits are sometimes heavily
infested.
3. Importance. The species is a serious fruit tree pest throughout
its range. Both adults and nymphs suck sap from the wood and leaves
of trees, reducing vigour and subsequently crop yield. Developing fruits
(Z-57)
Insect Injurious to Crops
A
[ 371
B
Fig. 29. The San Jose scale, Quadraspidiotus perniciosus. (A) Male,
case, (C) Adult female.
(l:IJ
i11:male inside scale
are also attacked, leaving grey, mottled blemishes that reduce quality. ff
infestations of this insect are left unchecked, the population may cause
the death of trees in the orchard. Terminals characteristically die first.
Infested fruit develop a reddish purple ring surrpunding each spot
where a scale settles.
4. Appearance. Adult females are apterous. circular, about 2 mm in
diameter, and covered with waxy scales secreted by the body. Adult
males are oval and about 1 mm long. Both males and females are
brown to black and have a raised nipple at the top of the scale cover.
Nymphal scales are also oval and light but turn dark with age. Young
nymphs that have not produced scales are commonly called 'crawlers'
which are yellow and resemble mites.
5. Life cycle. Nymphs overwinter during the first instar in a state of
diapause. After moulting twice (in March and May), they emerge either
as males or females. Females are viviparous and each produces 8- 1 0
nymphs per day from late May onwards. The egg-laying period i s over
6 weeks. The average numbc>r of nymphs produced on a favourable host
plant is about 400. These tiny, yellow crawlers wander randomly until
they find a suitable place to settle. Upon settling, the tiny crawlers
insert their mouthparts into the host plant, feed, and secrete a white,
waxy material. This stage is usually referred to as the ' 'white cap' ' stage.
There are four generations. The summer generations overlap, and
crawlers are present throughout the summer and fall. Growth is
completed in 30-40 days, and 2-3 overlapping generations are produced
each season. These insects are very prolific; the progeny from one
fertile female could be well over 30,000,000 in a single season. The
crawlers make local spread. Many of these are carried from place to
place on the feet of birds and on other insects. Long distance dispersal
is largely through transportation -of infested plants by man.
6. Control measures. Several parasitic wasps as well as predatory
ladybird beetles are very important in keeping populations checked. San
Insect Injurious to Crops
372 1
Jose scale is very polyphagous and develops on more than 1 50 species
of host, especially on apple. A specific parasitoid of the San Jose scale,
Prospaltella perniciosi,
(Chilocorus orbus)
and twice-stabbed lady beetle
and another small beetle,
Cybocephalus californicus
check its population
in nature. However, pesticides used during the season can disrupt these
natural
controls,
damage
allowing
potential
the
scales
of this pest,
to
growers
increase
rapidly.
Due
should consider annual
to
the
use
of
dormant oil sprays. In heavy populations, it may be necessary to apply
an
organophosphate
dormant period.
insecticide
plus
If the dormant oil
oil
spray
sprays
during
provide
the
inadequate
delayed
control,
pesticides also are effective when applied soon after the emergence of
the scale crawlers. This usually occurs in May. Spray of narrow range
oil
plus
Diazinon
SOWP
or
chlorpyriphos
or
methidathion
gives
considerable success in controlling the pest.
[ II] The woolly apple aphid : Eriosoma lanigerum
(Hemiptera : Aphididae)
1. Distribution. The woolly apple aphid is almost cosmopolitan and
is distributed in U SA, Europe, South-East Asia, Australia and Africa
(Fig. 30).
2. Host plants.
The woolly apple aphids mainly feed on apple,
elm, mountain ash, pear and hawthorn.
3. Importance. The woolly apple aphids feed on sap from large root
knots,
underground
branches.
Primary
portions
injury,
of
trunks,
however,
is
and
caused
wounds
by
on
root
trunks
feeding,
and
which
causes stunting of growth. Infested trees often have many short fibrous
roots. Under severe infestations trees may die. The injury on elm causes
the formation of close clusters of stunted leaves or rosettes, at the tips
of the twigs. The leaves being lined with purplish masses of aphids are
covered with white powdery secretions.
4. Appearance. Adults and nymphs are red to purple and covered
with bluish white, cotton like wax filaments. Winged and wingless forms
appear during the year.
S. Life cycle. For the greater part of the year, wingless females
capable of producing y9ungs parthenogenetically occur on the apple. It
is
only
the
late
summer
or early autumn that winged
forms
appear.
Wingless females and nymphs are found on the roots, and there is a
general
trunk
but
and
autumn.
irregular and
branches,
Eggs
incomplete
upward
in
early
migration between roots and
summer,
and
downward
in
the
late
overwinter on elm bark, however, a number of wingless
individual also overwinter on the roots of apple trees. In spring, eggs
hatch and wingless females begin parthenogenetically
rapid rate
on elm trees.
reproducing
at a
Winged individuals develop in early summer
Insect Injurious to Crops
I 373
B
Fig. 30. The wooly aphid, Eriosoma lanigerum. (A) nymph, (B) Adult.
and disperse to other plant hosts like apple. Reproduction continues
throughout the summer, and in fall winged individuals appear again,
mate, and oviposit overwintering eggs.
6. Control measures. Every effort should be made to ensure that
both the roots and the aerial parts of young stock are free from woolly
a hids before planting. The release of an aphelinid parasitoid, Aphelinus
mali, has successfully suppressed populations of the woolly aphid in
practically all the areas where the wasp was released, including the Kulu
Valley in India. Here, applications of synthetic poisons are now
generally unnecessary. However, if it become necessary, methyl demeton
(0.025%) or fenitrophin (0.05%) may be sprayed to kill the crawlers.
p
[ III] The mango leaf hopper : Idiocerus atkinsoni
(Hemiptera : Jassidae)
1. Distribution. The mango leaf hopper is found in all the mango-growing
tracts of India such as Uttar Pradesh, Bihar, Punjab, Andhra Pradesh and
Maharastra. Apart of India, it is also distributed in Malaysia, Formosa,
Indonesia and other East Indies countries (Fig. 3 1 ).
2. Host plants. It is highly host specific and feeds only on mango
and hence is a monophagous species.
3. Importance. The adults as well as enormous number of nymphs
that suck the sap of flowers and young buds mainly cause the damage
of mango crop. The infested parts consequently fall prematurely. They
also excrete honeydew, which gives the affected plants an oily
appearance and serves as base for the development of moulds. Feeding
of inflorescence by the hoppers reduce the vigour of the plant resulting
in reduction of fruit setting. Complete failure of mango crop may result
during years of heavy infestation. Low mango yield in Uttar Pradesh is
largely contributed to this pest.
4. Appearance. Mango hopper is small insect measuring about 5 mm
in length with grey brown colouration. The wings are held roof-like over
Insect Injurious to Crops
374 1
B
A
Fig. 3 1 . The mango leaf hopper,
(A) Nymph, (B) Adult.
the body.
The
head
is
Idiocerus atkinsoni.
broad with prominent eyes.
The hindlegs are
thickly covered with small bristles.
5. Life cycle. The mango hopper passes the winter in adult stage on
the stems, underr.eath the bark and among the leaves, etc. In middle of
the February, they begin to feed.
In spring, the female lays eggs on
fresh leaves and inflorescence. Indeed the eggs are inserted singly in tl}e
tissue of the inflorescence or of the young leaves. Each female lays over
200 eggs. The incubation period is about 5- 7 days after which young
yellowish
green
nymphs
emerge
out
and
suck
the
sap
from
the
inflorescence and buds. The growing nymphs excrete excess of sugar as
honeydew
which
development
provide
of moulds
They mature within
moult
favourable
giving
condition
for
black appearance
the
growth
of the
infested
and
trees.
1 3- 1 5 days passing through 3 nymphal stages and
into winged adults.
The mature
nymphs
and adults both spread
over the other parts of the tree by leaping or hopping. The entii:e life
cycle takes about 1 8-21 days_. The activity of adults ceases during May
onward as they hide themselves underneath the barks and undersurface
of the leaves and on a slight· disturbance they fly in all the directions.
Usually
only
one
generation
occurs
but
some
times
the
second
generation may starts with the onset of the monsoon.
6. Control measures. Density of mango trees should be appropriate
as dense plantation favours the growth and development of the hopper.
When
trees
are
heavily
infested,
spraying
of
insecticides
such
as
(0.02%), carbaryl (0.01%), endosulfan (0.03%) and
(0.0 1%) provide protection from the pest. The severity of
phosphomidon
dimethoate
the
pest
winter
may
be
particularly
reduced
in
if the plants
morning
with
fish-oil-resin soap dissolved in water.
are
sprayed
strong
resin
in
the
previous
compound
or
Insect Injurious to Crops
[ 3 75
[ IV] The red palm weevil or coconut weevil :
Rhynchophorus ferrugineus
(Coleoptera : Curculionidae)
1. Distribution. In India, the red palm weevil is mainly distributed in
Peninsular India. It is also found in Pakistan, Sri Lanka, Bangladesh, New
Guinea, Philippines and Malaysia (Fig. 32).
2. Host plants. The main food plants of the red palm weevil are
coconut, sago, da.te and other palms.
3. Importance. On heavy infestation, the plants are killed. A few
small holes with protruding chewed fibrous material and oozing out of a
brown liquid from such holes indicate the early infestation by the pest.
In the advanced stage of attack the central shoot shows sign of wilting
and a large mass of grubs, pupae and adults of the weevil could be
seen inside the trunk at the affected portion. In the grown up trees the
crown region alone is infested.
4. Appearance. The weevil is of large size, reddish brown in colour
with 6 dark spots on the thorax and in the male the conspicuous long
snout has a tuft of hairs. The adults are diurnal and are good flier.
5. Life cycle. The female lays eggs in scooped out small cavities on
the palm that contain decaying organic matter. The eggs are also laid in
soil. A single female deposits up to 1 50-400 oval and white eggs. The
incubation period is 2-5 days. The apodous light yellowish grub with a
red head, feed on the soft tissues of the growing areas making tunnels
inside. The grubs become full-grown in 36 to about 1 00 days and
pupates in a fibrous cocoon inside the trunk itself. It emerges as an
adult after 1 2-33 days of pupation period. The adults survive for 50- 1 1 0
days. The male lives longer than females.
6. Control measures. The dying plants and already damaged plants
should be destroyed and as far as possible inflicting mechanical injuries
Fig.
32. The
red palm weevil, Rhynchophorus ferrugineus. (A) Larva, (B) Adult.
Insect Injurious to Crops
376 J
on trees should be avoided as at such places the female may oviposit.
1be infested portion of the palm tree should be scooped out and
dressed with tar. A solution of 1 % Pyrocone E (a mixture of pyrethrin
and piperonyl butaoxide in 1 : 10 ratio) or 1 % carbaryl when injected
through holes in the crown at 1 000- 1 500 ml per grown up palm trees
prevent the infestation of the plant.
[ V] The rhinoceros beetle : Oryctes rhinoceros
( Coleoptera : Scarabaeidae)
1. Distribution. The rhinoceros beetle is one of the major pest of palm
trees and distributed throughout in South-East Asian countries, Southern
China, Philippines and South Pacific Islands (Fig. 33).
2. Host plants. It infests coconut, sago, date palm, pineapple,
sugarcane, aloe, African palm, palmyrah and other palms.
3. Importance. The damage is imposed by the adults which burrow
by remaining in between leaf sheaths near the crown and thus cut
across the leaf in its folded conditions. The damaged leaves show
characteristic holes in the leaflets. Frequent infestation results in stunting
of trees and death of growing point in young plantations.
4. Appearance. The . adult beetle is stout, black or reddish blaqk,
about 5 mm long and has horn projecting dorsally from the head in
male, in female the horn is short. The adults are able to fly for
considerable distance.
5. Life cycle. The preoviposition time for the beetle is 20-60 days
after which the female lays eggs in manure pits, decaying vegetable
matter, undisturbed heaps etc. to a depth of 5 - 1 5 cm. A single female
lays 1 00- 1 40 eggs. The eggs are oval and creamy white in colour. The
incubation period lasts for 8- 1 8 days. The young grubs feed on the
decaying matter. They mature in 1 00- 1 80 days after passing through
three larval stages. The full-grown larva is stout, sluggish and white in
A
Fig.
33.
The rhinoceros beetle,
B
c
Oryctes rhinoceros. (A) Larva, (B) Adult.
Insect Injurious to Crops
[ 377
colour with a pale brown head and is usually found at a depth of 5- 1 30
cm. Pupation takes place in earthen cells at a depth of 30 to 1 00 cm
and emerge as adult in 10-25 days. The adults then fly towards the
palm trees to infest them. The total life cycle ranges from 1 00-240 days.
Adult survives for up to 290 days. Thus only one generation is possible
in a year.
6. Control measures. The grubs in their breeding places should be
killed by spray , application of carbaryl 0. 1 % solution at least once m
three months. Decaying trunk of trees in the coconut gardens should be
destroyed as they serve as breeding grounds. The beetles should be
taken out from the crown with the help of iron hooks and a mixture of
sand and carbaryl dust in equal proportion should be filled in the axils.
of innermost 2-3 leaves on the crown twice a year during pre- and post
monsoon periods. There are a number of natural enemies such as fungi
(Beauveria bassiana), predators (Platymeris laevicollis, a reduviid bug;
Santalus paralellus, a histerid beetle) which can be promoted for the
control of rhinoceros beetles.
[ VI] The citrus butterfly : Papilio demoleus
(Lepidoptera : Papilionidae)
1. Distribution. The citrus butterfly or lemon butterfly is distributed
throughout northeast Arabia, India and Sri Lanka, through most of
Southeast Asia and the Lesser Sunda Islands to Australia and part of
New Guinea (Fig. 34) .
2. Host plants. The main host plant is different varieties of citrus or
lemon. Other host plants on which it survives are curry leaf, Aegle
marmelos and Psoralea corylifolia.
3. Importance. The caterpillars of the lemon butterfly feed the
tender leaves and terminal shoots so that only midrib is left hanging.
During severe infestation, the plants do not bear fruit.
4. Appearance. The lemon butterfly is big sized butterfly measuring
about 28 mm long and 100 mm across wingspan. The wings are buff
coloured with wide black edges containing buff spots. The hindwings
also have two eyespots: one red and one blue or one black and one
yellow. Antennae are black and clubbed.
5. Life cycle. The female lays 75-250 eggs within a week. The eggs are
laid singly on the under surface of the tender leaves of the citrus. The
eggs are smooth, shiny greyish yellow in colour. The incubation period is
4-6 days. The caterpillar in its early instar stage is brownish and blackish
in colour and resembles bird dropping. The caterpillars pass through 5
larval instars. The grown up caterpillar is cylindrical, stout and green with
black between segments, orange feet, and short spines on the thorax and
ninth abdominal segment. The larval periods range from 2 weeks in
378 1
Insect Injurious to Crops
c
Fig.
34. The citrus butterfly, Papilw demoleus. (A) Larva, (B) Pupa, (C) Adult.
summer to 5-6 weeks in winter and pupal period from 1 week in summer
to 2 week in winter. The pupa is brown and attached to a stem of the
foodplant by a girdle of fine silken threads secreted by the last instar
caterpillar. A complete life cycle may take 3 weeks to 1 3 weeks. Four
overlapping generations have been observed during a year. In hill areas it
passes winter in pupal stage.
6. Control measures. The caterpillars can be hand-picked and killed.
Spray application of profenofos 0.05% or cyperrnethrin 0.025% or diazinon
0.02% or 0.05% malathion controls the pest.
Pests of Castor
The
castor-oil plant, Ricinus communis, of the famil)' Euphorbiaceae is a
native to tropical Africa. The seeds contain the castor oil of commerce. In
India and China the plant is an important crop for industrial uses as
lubricant and pharmaceutical uses as cathartic. In addition, the castor oil
is used as a plasticiser in nitrocellulose compositions, in cosmetics, and in
insulation products. It is also used in the manufacture of waterproof
lacquers and paints. The world output of castor seeds exceeded 1 .3 million
metric tonnes annually. The castor semilooper, Achaea janata severely
damages the castor crop along with some other insect pests.
[ I} The castor semilooper : Achaea janata
(Lepidoptera : Noctuidae)
1. Distribution. The castor semilooper is distributed throughout Africa,
north India, · Pakistan and other South-East Asian countries (Fig. 35).
2. Host plants. The main host plant is castor (Ricinus communis)
but it can also survived on rose, pomegrante, Tridax procumbens,
Euphorbia hirta, etc.
Insect Injurious to Crops
[ 379
B
A
Fig.
35.
The castor semilooper, A chaea janata. (A) Larva, (8) Adult.
3. Importance. The caterpillar feeds voraciously on leaves, tender
petioles, young capsules, etc. Under heavy infestation, the plants are
defoliated within a short period. It occurs during August to January.
4. Appearance. The adult moth is pale reddish brown with black
hindwings having a medially white and three large white spots on the
outer margin. It measures 60-70 mm across wing.
5. Life cycle. The female lays about 450 (300-650) eggs which are
blue green and rounded in shape. The eggs are laid singly at 1 -6 eggs
per leaf. The incubation period is 2-5 days. The larvae feed voraciously
and become full-grown in 1 1- 1 5 days. The full-grown larva has black
head, a red spot on the black loop formed due to the non-functional
first pair of prolegs and red anal tubercles. The larvae may be grey
with lateral red and brown stripes or black with lateral white stripes. It
undergoes 5 moults and pupates in the soil or among fallen leaves.
After 1 0- 14 days of pupation period, adult moth emerges out in
summer. There are 5-6 generations in a year. The winter is passed in
pupal stage.
6. Control measures. In nature, a braconid parasitoid Microplitis
ophiusae tends to regulate its population. The larvae may be
hand-picked and destroyed. If necessary, spray application of 0.07%
endosnlfan or 0.025% methyl parathion or 0.1% carbaryl gives
satisfactory result.
Important Questions
I.
2.
3.
4.
5.
6.
7.
Enumerate some major insect pests of cotton and describe the distribution, host plants,
damaged caused, life history and control measures of any one of them.
Describe the bionomics of sugarcane top borer Scirpophaga nivella or root borer
Emmalocera depresse/la. Suggest appropriate control measures for regulating their
numbers.
Describe the mode of damage of different insect pests attacking paddy crop in your
area and suggest the control measures for any one of them.
What are the crops infested by Helicoverpa armigera caterpillars? How the inetdence of
this pest in cotton or clnckpea agro-ecosystem be minimised ?
Give an account of vegetable pests. Descnbe the life cycle, mode of damage and control
measures of red pumpkin beetle or brinjal fruit borer.
Describe in detail the appearance, mode of damage, life cycle and control measures of
the mustard aphid, Lipaphis erysimi.
Write short notes on : (i) San Joi;e scale (ii) Erwsoma lanigerum, (ni) Mango leaf
hopper, (iv) Papilio demoleuse.
24
Methods of Insect Pest Management
The number of pests, and pests caused losses in crops have increased
substantially in the last 50 years, the period marked the use of external
inputs, largely the use of synthetic fertilisers and pesticides in agriculture.
This0 change is worldwide including in the USA where crop loss due to
pests has doubled in the past 40 years. In India, annual crop loss to pests
has � ncreased from 5000-6000 Crore Rupees in ?O's to approximately
38-40000 Crore Rupees by 2002.
.
The insect pest management is the application of technology, in the
context of biological knowledge, to achieve a satisfactory reduction qf
insect pest numbers or effects and to maintain the pest population
below levels that cause economic damage. It includes multiple tactics
such as the use of natural enemies, cultivation of resistant crop varieties,
and insecticides applied in a compatible manner. It also includes the
use of such tactics that help in the conservation of environmental
quality. There are many tools for insect pest management but no one
method is without drawbacks. For convenience of study, the insect pest
management may be grouped into physical, mechanical, cultural,
biological, chemical, hormonal, genetical and legal practices. Few tactics
are preventive such as physical and mechanical measures, cultural
practices and legal control that prevent the insect to attain a pest status
while others are curative such as biological, biopesticidal and chemical
control that reduce the number of insects infesting the crop or human
belongings.
·
Methods of Insect Pest Management
{ 381
Physical Control Measures
Physical controls aim to reduce pest populations by using devices which
affect them physically or alter their physical environment. It involves the
devices like barriers, excluders, or collectors and includes the ust- of heat,
light, electricity, X rays, and so on, to kill insects directly, reduce their
reproductive capacity, or to attract them to something that will kill them.
[ I] Temperature
(cold storage, sun drying, stem
heating or hot water treatment)
Both high and low temperatures have been used to destroy pest insects in
a variety of situations. Most insects become inactive at temperatures of
about 4°C or below, and �any stored products maintained at such
tem eratures are not damaged, although the insects present would not
likely be killed. The potato tuber moth Plztlzorimaea opereulella does not
damage potato kept in cold storages. High temperatures have been used
against insects that infest stored grain, coffee bean, various seeds, citrus
fruits, clothing, bedding, furniture, baled fibrous materials, bulbs, soil, and
logs. The exposure of infested stored grains and cottonseeds to sun on a
cemented floor in May-June for 4-5 hours kills flour beetles and pink
bollworms, respectively. Such sun exposure of grains also reduces the
moister content of the grain and if it is less than 8% RH, the grains escape
insect infestation. If cottonseeds are heated at 60°C it kills hibernating
larvae, if any. There are various kinds of heating machines in the market
as per requirement of the infested articles. Steam sterilisation of soil is
done to kill soil insects. Planting materials are sometimes subjected to hot
water treatment to get rid of infection of pathogens and hidden infestation
of boring insects. Whether low or high temperatures are used depends in
part on the nature of the product to be protected or disinfested.
p
[ II] Electromagnetic fields and ionising radiations
The high-frequency electric fields, particularly against stored-grain pests;
ionising radiation such as X-rays and y- rays against insects attacking
materials that would not themselves be harmed by the radiation reduce
population of certain insects. For example, y-radiation (75- 100 Gy) has
been used to irradiate mangoes being shipped abroad to kill eggs and
larvae of the fruit fly, Bactrocera tryoni. At a dose of 8 kr irradiation has
produced complete sterility in the furniture beetle, Anobium punctatum
and the powder post beetle, Lyctus brunneus. In irradiation of grains,
a dose required for insect control is not harmful to consumers. It also
does not affect the nutritional status of the grain. High frequency radio
waves generate temperature of about 80"C in grains that kill rice and
382 J
Methods of Insect Pest Management
granary weevils and flour beetles within a minute. Though the cost of
radiation disinfestation of stored grain is higher than conventional chemical
fumigation it is hoped that in near future it may find practical application
for large scale control of stored grain insects.
[ III] Accoustical devices
Accoustical or sound producing devices for frightening away of vertebrate
pests like birds, monkeys etc., are in use but in insect control its
application is limited. It is effective against wood boring insects.
Most of the physical control measures for insects require special
infrastructural facilities that are beyond the scope of adoption by
common cultivators under Indian conditions. However, use of heat
energy (sun drying) for control of insect pests is a common practice in
villages.
Mechanical Control Measures
The mechanical method is one of the most ancient methods of pest control
as this does not involve any special artifact to kill insect except the use of
manual labour. It include the use of simple manual techniques or devices
such as handpicking, hitting and crushing, jarring and shaking, and the use
of various kinds of barriers, excluders (e.g., screens), and traps. These
methods of insect control are frequently labour-intensive and laborious and
can not be applied commercially but are useful either in small scale
cultivation or at community level on the onset of infestation.
[ I] Handpicking of infested plant parts and their destruction
Handpicking of infested plant parts as well as the insect pests is effective
in controlling the insect pests. The easily detectable egg masses of rice
stem borer (Scirpophaga incertulus), groundnut red hairy caterpillars
(Amsacta spp., Spilosoma obliqua), pyrilla (Pyrilla perpusilla), grubs of
mustard sawfly (Athalia lugens maxima), caterpillars and pupae of citrus
butterfly (Pappilio demoleus) and sugarcane stem borer (Chilo
infuscatellus), all stages of epilachna beetles (Epilachna spp.) on brinjal
and cucurbits, tomato and tobacco hornworms can easily be handpicked
and killed. In tea gardens of northeast India where looper caterpillar,
Biston suppressaria. often appear in epidemic form, boys are appointed on
contract basis to collect ancl destroy the caterpillars.
[ II] Netting, bagging and dislodging of insect pests
Insects like leaf hoppers, earhead bugs, grasshoppers, red pumpkin beetles
etc. can be netted and killed (for different types of nets see chapter 2).
Bagging and killing of hoppers migrating from one field to others is very
useful device to check their population. Passing a rope across a rice crop
Methods of Insect Pest Management
{ 383
is sometimes done so as to dislodge the caseworms over the standing water
which then drained out to collect the pest.
[ III] Trenching
Trenching is a very good device for controlling locusts at nymphal stages,
i.e., hopper stages which occur in masses and are incapable of flight.
Trenches or pits of about 45 cm deep and 30 cm wide are dug at some
distance in front of marching hopper bands. The herds of hoppers are
driven to the trenches wherein they are buried alive.
[ IV] Burning
Adults and hoppers of locusts gathered on bushes or trees are killed using
flame throwers, if available, otherwise with kerosene-oil torches. The
burning of crop stubble such as rice and sugarcane is highly effective
against the root and stem borers. Burning of sugarcane leaves kill the egg
masses of the pyrilla.
[VJ Hitting and crushing
The fly swatter, the bare hand, or any of a wide variety of implements can
be useful against a few insects within a dwelllng or on one's person.
Bedbugs, cattle lice, horse flies sitting over the cattle, etc. can easily be
directly bitted and crushed to death.
[ VI] Jarring, shaking, hand beating and hooking
Jarring and shaking or hand beating of shrubs or trees, especially fruit
trees, is sometimes used to remove insects, particularly beetles. Sheets, or
buckets having kerosenised water or other material, can be used beneath
the plant being shaken to trap the insects that fall into them. The
rhinoceros beetles can be taken out from the crown of the coconut palms
with the help of iron hooks.
[ VII] Sieving and winnowing
The stored grain pests such as Tribolium, Sitophilus and Trogodenna can
easily be removed from the grains by sieving and winnowing the grains.
[ VIII] Insect barriers or mechanical excluders
Several mechanical means are employed to act as barriers to insect
movement. Sticky materials in which insects become hopelessly entangled
have been used, for example, in the form of flypaper that traps numerous
flying insects. Sticky materials have also been applied in bands about the
trunks of trees to protect them from oviposition damage caused by the
periodical cicada. Paper and tin collors are placed around small plants like
potato and tobacco to protect them from cutworms. Tar, lime and creosote
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384 1
Methods of Insect Pest Management
are used as insect barriers. Metal collars around tree trunks have also been
used for the same purpose and are effective against any nonflyil\g insects
that would otherwise attack the branches and foliage. Metal is also used in
the construction of shields around the foundation of houses and buildings
to prevent attack by subterranean termites. Screens of metal, cloth,
fiberglass, or plastic have been used to cover various openings such as
doors, windows, ventilators, etc. to allow the passage of air but exclude
most insects. Cloth netting is useful to protect sleeping persons from
mosquitoes and other biting insects.
[ IX] Insect traps
Traps are used for control, survey, and surveillance purposes. Various
types of traps (e.g., light traps, pheromone traps, bait traps, see chapter 2)
have been designed for catching insects and then killing them. Control
traps are usually used along with some attractive stimulus (e.g., light, food,
or sex pheromone) and with some means of killing the insects that enter
(e.g., a pesticide or an electrically charged grid). A light trap having 200
candle light intensity fitted 3 m high attracts the moth Amsacta albistriga in
ground nut fields. The serious pests of paddy such as Nephotettix spp. and
Orsedlia_ · oryzae, and cotton pest such as Helicoverpa armigera can be
trappeo either in light traps or pheromone traps and killed. Survey and
surveillance traps such as yellow sticky tra s and yellow pan water traps
are used to detect the presence of potential pest species like aphids and
some flies during the day. It helps in evaluating the effectiveness of any
control procedures that may have been carried out in a given area, and to
monitor levels of various economically important species. A cross-shaped
trap (20x20 cm) painted yellow mimics the plant colour and architecture.
When mustard oil (allyl-isothiocyanate), an important plant volatile that the
flies orient to, are combined, they make a very effective trap for monitoring
flight activity of this pest.
p
[ X] Provision of bird perching objects
Several birds are known to feed upon insects such as myana (Acridotherus
tristis) and serve as natural control of the insects. Therefore, provision of
perching objects like branched twigs in the fields having chickpea and
cotton crops ( @ 75 twigs/ha) cause considerable protection from the
injurious insects like Helicoverpa armigera which are devoured by
insectivorous birds.
Cultural Control Measures
Cultural practices refer to that broad set of management tactics or options
that may be manipulated by farmers to achieve crop production goals, or
(Z-5 7)
Methods of Insect Pest Management
[ 385
the manipulation of the environment to improve crop production. Cultural
control, on the other hand, is the deliberate manipulation of the cropping
system or specific crop production practices to reduce pest populations or
to avoid pest injury to crops. Cultural control, though provide control
inferior to that of pesticides, is a valuable control on the average pest
density, and therefore, is valuable in reducing the challenge that
insecticides may be called upon to meet in the future. These tactics may
include: clean cultivation, crop rotation, tilling, irrigation, use of fertilisers,
sowing and harvesting time, destruction of crop residues, weeds etc., use of
resistant crop varieties, nutrient management, etc.
[ I] Selection of seeds and cultivars
The very first event in the farming is the selection of seeds. The seeds
should be healthy. Seeds damaged by insects or other pests, if sown may
cause poor germination or poor health of seedlings. Also, seeds of resistant
crop varieties should be used for crop production. The susceptible varieties
of crops should never be cultivated. Crop cultivars resistant to major pests
and diseases have been developed in rice, wheat, maize and sorghum,
sugarcane, and to a limited extent in pulse and oilseed crops. Brown plant
hopper resistance in rice cultivars, stem borer resistance in maize, shootfly
resistance in sorghum, scale resistance in sugarcane, and limited resistance
to pod borers and diseases in chickpea and pigeonpea have helped reduce
the overall pesticide load on the food crops. At present emphasis is being
given to biotechnological approach to exploit somaclonal variation and
genetic engineering to evolve new multiple resistant crop varieties.
Sugarcane varieties Co 6907, COC 67 1 , Co 8014 are resistant for scale
insects and CoS 767, Co 1 1 58 are resistant for top borer; cotton varieties
027, F 1 14, SRT- 1 , Sujata, Digvijay are resistant for bollworms and
Aboharia, CCH-3-5 195, 27 16 SR, Khandawa-2, Mahalaxmi, F 414 are
resistant for jassids; chickpea Yarieties JG 3 1 5, ICCV 7, C 235, Pusa 209,
Pant CE I , Pant CE 2, AKG 33, BG 373 are resistant for gram pod borer;
pea varieties JP 92A, JP l 79A, P 402 are resistant for pod borer
and leafminer.
[ II] Clean cultivation
Farm hygiene often has a pest control purpose. The disposal or destruction
of crop residues removes residual pest populations, e.g. ; stalk-boring grubs
in maize, sugarcane root borers, pink bollworm larvae. The elimination of
plant debris on the soil surface, in which many pests find shelter for
hibernation such as flea beetles, whiteflies of brassica, also reduce insect
infestation. Destruction of crop residues of cotton followed by a gap before
cotton is again planted is compulsory in many countries in the world. The
infested fruits of bitter gourd, brinjal, guava, tomato etc. should be
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Methods of Insect Pest Management
removed from the fields as if they remain in the field, the adults emerging
from these infested fruits will re-infest the standing crop. Decaying trunk
of trees in the coconut gardens should be destroyed as they serve as
breeding grounds of rhinoceros beetle. The beetle also deposit eggs on the
heaps of manure so it should be kept in cover with soil. Burning of
sugarcane trash is suggested for reducing the population of pyrilla by way
of destruction of the egg masses.
( III] Destruction or provision of alternate hosts
Many pest species feed on alternate hosts that allow for populations to
build up or act as trap crops. For example, the black cutworm, Agrotis
ipsilon, is a major pest of corn seedlings. Young larvae feed on weeds then
move onto corn seedlings until they reach the fourth instar. They cause
serious damage by cutting or drilling the plants. However, if the seedlings
can reach the four-leaf stage before being infested, no significant yield
reductions occur. However, this strategy is a two- edged sword. If the
grower waits until the corn reaches the four-leaf stage before cultivating or
using herbicides to control the weeds, yield reductions occur due to weed
competition. So the best management tactic is to apply preplant herbicides
at least 14 days before planting to reduce weed populations, hence
minimising the number of ovi[1bsitional sites and early instar food sources.
Elimination of malvaceous weeds nearby cotton fields reduces the
population of cotton-stainer bug, Dysdercus spp.
[ IV] Crop rotation or maintenance of a host-free habitat
By rotating crops or maintaining host-free habitats, the normal life cycle of
an insect pest is interrupted by effectively placing the insect in a non-host
habitat. The crop rotation is one of the oldest and most widespread farm
practices often directly motivated by pest control, and it is still one of the
most effective controls of such insect species which are mono- or
oligophagous and have long generation cycles with limited dispersal
capabilities. For example, the potato tuber moth overwinter in the fields.
After they emerge from the soil, they need the host plant to reproduce.
Therefore, if potato crop is not taken for 2-3 years in the infested field and
some other crops are grown that are not serve as its host, the moth will
,
not survive and the life cycle will be disrupted. Usually rotation of cereal
crops by legume crops protects the both crops from insect infestation.
Continuous cultivation of paddy crop in the same locality or practice of
ratooning of sugarcane provide a continuous supply of food and shelter to
the pest for ceaseless breeding. This results in a quick build up of
population of pest. The crop rotation is very effective in reducing the many
soil insects such as chafers, wireworms, shoot-boring flies etc.
(Z-57)
Methods of Insect Pest Management
{ 387
[ V] Tillage operation
Tillage operation includes soil turning and residue-burying practices,
seedbed preparation, and cultivation. Some forms of tillage can reduce pest
populations indirectly by destroying wild vegetation (weeds) and volunteer
crop plants in and around crop-production habitats. By tillage practice, the
quiescent stages of insects, such as pupae, are exposed to dehydration or
to predation by birds and various stages may be mechanically destroyed, or
otherwise the larvae and pupae may be buried so deep that they cannot
emerge as adults. Overwintering populations of Helicoverpa armigera, the
com earworm, may be greatly reduced by either fall or spring ploughing
operations. Overwintering survival of the soybean stem borer is inversely
related to depth of burial of soybean crop residue following harvest.
Post-harvest ploughing also directly control the population of many insects
inhabiting the soil, e.g., post-harvest ploughing of winter paddy fields which
expose the hibernating larvae of rice yellow stem borer that hibernate in
stubbles. Similarly, the larvae of pink bollworm, Pectinophora gossypiella,
and spotted bollworms, Earias spp., inside cotton seeds are found in the
soil, if they are buried in deep soil by tilling the field, will not emerge as
adults in the coming spring. Tillage practices also indirectly benefit the
crop by better incorporation of nutrients in the soil that nourish the
seedlings and plants for healthy and balanced growth. Such plants have
intrinsic ability to tolerate some damage by pests.
[VI] Timing of planting or harvesting
Alterations in planting date and harvest date can frequently result in
escape from damaging pest infestations. 'This practice is more meaningful
if planting of crop is done on the basis of information on the population
dynamics of insects. Early sown mustard crop either escapes aphid attack
or has less degree of infestation. Brassica campestris var. toria escapes
attack of Lipaphis erysimi if sown in mid September. Safflower when sown
early escapes the attack of safflower aphid, Uroleucon carthami, particularly
at early stage of the crop. Likewise, early sowing of chickpea and pea
during 0ctober are less infested by the noxious pest Helicoverpa armigera.
The pink bollworm overwinters as last instar larvae, and the
diapause is regulated by short days (< 12 hours of light). By harvesting
early, the number of overwintering larvae is reduced to low levels. The
manipulation of time of harvesting can be done in three ways: (i) by
advancing the date of sowing, (ii) by sowing short duration varieties of
the crop, or (iii) by harvesting in proper time without any delay after
the crop is mature. It is a common experience in some cases that
potato harvesting is sometimes unnecessarily delayed. During this stage
quite some tubers may remain exposed which serve as the most
388 j
Methods of Insect Pest Management
favoured site of egg laying by tuber . moths, which are carried to the
storage where they find further favourable conditions for breeding thus
causing severe damage to tubers.
[ VII] Cultivation of trap crops
Crop monocultures are often damaged more severely by pests than is the
same crop if cultivated along with other crops. However, there are cases in
which such diversity can aggravate pest problems. It is in these situations
that trap crops can be important. Trap crop is generally used to ward off
the insects from the main crop. It prevents the insects from reaching the
main crop. Trap crop is more attractive and susceptible than the main
crop. The planting of trap crop is done in such a time that its susceptible
stage coincides with peak activity of the insect. Mustard as trap crop has
been found very useful in the management of Lipaphis erysimi
and Brevicoryne brassciae on cabbage when planted in mustard 2 :
cabbage 9 ratio.
[ VIII] Nutrient management or manuring
It is an established fact that healthy plants are less attacked by pests.
There are 20 essential plant elements which are needed for the growth and
development of the plants. Out of these, N, P and K are major nutrients.
In general, high nitrogen supply results in increased tissue softness and
water content as carbohydrates making the plant more susceptible to
attack by insects like aphids, leafhoppers, mites, thrips and leaf-miners.
Presence of higher level of P and K makes the plant less susceptible for
aphids. Thus manipulation of these major nutrients can be used to manage
the insect population under control. Higher proportion of N:P:K (80:40:30)
showed higher population of mustard aphid whereas 40:80:40 ratio reduced
aphid infestation. Similarly, high N:P:K (225:90:45) increased population of
cabbage aphid on cauliflower. Enhancement of succulent cotton growth
through nitrogen fertilisation causes severe attack of the cotton aphid
(Aphis gossypii) and the cotton bollworm (Helicoverpa armigera). The
excess of nitrogen fertiliser also renders paddy plants more vulnerable to
the infestation of brown plant hopper (Nilaparvata lugens) and rice gall
midge (Orseolia oryzae). Therefore, a balanced plant nutrients should be
provided for each and every crop which will not only decrease insect
attack but also make the plant more healthy. Use of organic manure for
crop cultivation should be encouraged as it makes the plant healthy and
attract less insect pests as compare to synthetic manures.
[ IX] Pruning of crops
Clipping off the apical part of young leaves during initiation of infestation
of rice hispa, Dicladispa armigera was found to be beneficial as this
Methods of Insect Pest Management
{ 389
operation removes the eggs as the adults prefer to insert eggs in this region
of young leaves. Pruning of the dead branches of perennial plants such as
fruit trees removes the borer larvae and pupae. The proper pruning of
infested leaves of citrus plants prevent the infestation of citrus leaf miner
(Phyllocnistis citrella), citrus red scale (Aonidiella aurantii), etc.
[ X] Plant density
Population of plant per unit area has quite a few functions with regard to
the pest infestation. Usually sedentary insects such as scale insects and
mealy bugs disperse during their first nymphal stage and plant to plant
movement is facilitated when plants are close and touch each other.
Similarly, the brown plant hopper, Nilaparvata lugens, favours dark and
humid condition and hence, colonises the basal region of paddy plants. A
dense planting of susceptible varieties provides congenial condition for its
population build up and thus causing serious damage to the crop.
Therefore, skip row planting of crops is recommended.
[ XI] Water management
The water management practice for insect control is practicable only in dry
season cultivation or in irrigated land. Water can be used directly for
suffocating insects or indirectly by changing the overall health of the plant.
Flood irrigation is frequently used to reduce populations of wireworms in
vegetables and sugarcane crops. Likewise, flooding can be used to control
white grubs in sugarcane, especially under conditions of high temperature.
Furrow irrigated potato fields tend to crack upon drying, exposing potato
tubers to ovipositing potato tubermoths. In areas where this is problem,
overhead sprinkler irrigation is recommended. High moisture content in
the soil is unfavourable for mustard sawfly (Athalia lugens proxima),
therefore, irrigation of mustard crop during January adversely affects the
population of mustard sawfly larvae.
[ XII] Strip farming and intercropping
It includes mixed intercropping, row intercropping, strip cropping, relay
cropping and passageway intercropping. Intercropping is preferred over
monoculture to avoid risk of crop failure, better utilisation of farm
resources and labour, and to protect the crop from insect pests. Intercrop
reduces the attraction of pest to the host, adversely modify the
microclimate of the pest habitat which may result in impeded dispersal,
increased emigration and reduced survival of the pest in the intercrop. It
has been shown that infestation of Aphis gossypii is less in pure crops of
green gram, black gram and sunflower as compared to main crop in
combinations with cotton. When beans are intercropped with older or
densely populated maize, fewer plants of former were infested by the
390 1
Methods of Insect Pest Management
aphid, Aphis fabae. Similarly, intercropping of groundnut with pearl millet
reduced the incidence of Aphis craccivora on main crop. It has been
suggested that in growing pulse crops in a mixed cropping system
pigeonpea (Cajanus sp.) should not be intercropped with bean (Phaseolus
sp.) rather monocot plants such as millets, sorghum or corn would be a
better intercrop as they have different pest complexes.
[ XIII] Provision of food sources for natural enemies
Many natural enemy species require food sources in the form of pollen,
nectar, or harmless arthropods that are not present in particular crop
habitats. These food requirements may be provided to support natural
enemy populations by encouraging deliberate development of certain wild
vegetation habitats near plantings of the crop. For example, natural
biological control of the grape leafhopper, Erythroneura elegantula, the
most important pest of grapes in the San Joaquin Valley of California, can
be achieved by an egg parasitoid, Anagrus epos. However, Anagrus is only
effective when vineyards are located within 5 km of streams and rivers.
A. epos does not successfully overwinter on the grape leafhopper, but does
on populations of the blackberry leafhopper, Dikrella califomica, which
survives on blackberry stands in stream and river bottoms. Vineyards
planted near blackberry stands along rivers and streams have high levels of
parasitism of E. elegantula.
Chemical Control
The chemical control is the control strategy of insects that make use of
natural or synthetic chemicals that cause directly the death, repulsion, or
attraction of insects. The chemical insecticide (henceforth insecticide) is a
chemical or a mixture of chemicals employed to kill insects and related
arthropods (ticks and mites). The term pesticide encompasses herbicides,
rodenticides, fungicides and other substances in addition to insecticides.
The insecticides may be obtained from the natural resources, the
bioinsecticides, usually extracted from plants such as neem products or by
synthesis (synthetic insecticides). Pheromones, food lures, oviposition lures,
repellents, antifeedants are other chemicals (either secreted by the insects
themselves or synthesised in laboratories) which are in use as chemical
control measures.
1. Brief History
In several parts of the world particularly in Europe and China, various
substances, such as sulphur, hellebore (a poisonous herb), and arsenic were
used as pesticides before the recorded history. Prior to the 1940s the
insecticidal value of a number of inorganic chemicals (e.g., arsenic,
mercuric chloride, carbon disulphide) and organic chemicals of botanical
Methods of Insect Pest Management
[ 391
origin (e.g., pyrethrum, nicotine) was known and put to extensive use. The
discovery of DDT by Paul Muller in Europe in 1 939 revolutionised insect
control and marked the beginning of the development and application of
synthetic organic insecticides. Since that time hundreds of compounds of
varying insecticidal value have been discovered, and thousands of new
potential toxicants are being evaluated each year by a detailed screening
process. More than 90% of currently used pesticides are of the synthetic
organic variety.
2. Descirable Attributes of Insecticides
During the early periods of development of modern synthetic insecticides
the only aim was to evolve more effective insecticides, i.e., the
consideration was unidirectional. However, in recent years due to
awareness of environmental risks of and rapid development of resistance of
insects to insecticides, development of newer insecticides becomes more
difficult. At present it is rather impossible to have an ideal insecticide that
fulfil all the desired qualities such as: (i) it should be safe to non-target
organisms but be highly efficient to kill the target insects, (ii) it should not
be phytotoxic nor should it impairs the germination of seeds, and cause
damage to flowers and fruits, (iii) it should not impart off-flavour of food
materials, (iv) it should kill the target insects very quickly, (v) it should be
persistence in toxicity, i.e., it should maintain lethal action for a longer
period, (vi) it should be quickly degradable if persistence is not required,
(vii) it should be stable during longer storage, (viii) it should be cheaper
and within the reach of poor farmers, etc. However, these attributes differ
in different situations.
3. Classification of Insecticides
Insecticides may be classified in different ways based on (i) developmental
stages killed (ovicides, larvicides, and adulticides), (ii) the primary route of
entry of the toxicants (stomach poisons, contact poisons, fumigants),
(iii) the chemical nature of the toxicants, (inorganic, organic insecticides),
(iv) mode of action (respiratory or nerve poison), etc. These categories
are by no means completely exclusive (e.g., many contact insecticides also
act quite effectively via the oral route).
Classification based on route of entry
The insecticides may be grouped according to the site of insect encounter.
The entry may be through stomach (stomach poison), cuticle (contact
poison), and spiracles (fumigants).
1. Stomach poison. The stomach poisons enter the insect body
through the gut while feeding the treated foliage or baits containing
poison or during cleaning the body parts like tarsi and antennae with
392 1
Methods of Insect Pest Management
the mouthparts that have acquired insecticides while crawling over the
treated surface. As these insecticides are fatal for the insects by acting
directly on the gut or being absorbed through the gut and then coming
in contact with the vital organs, it should not have any property that
deter feeding or cause
repulsion
to the insect desired to kill.
should be fairly stable and
will
insoluble
in
phytotoxic.
The
and
cockroaches
and
arsenicals
other
boric
crawling
water. It should
acid
insects,
It
also not be
(H3B O .J, used against
classical examples of
are the
this group of insecticide.
The
arsenicals
are
the
compounds
of
arsenic,
and
comprise
important and widely used stomach poisons for the insects. Although the
arsenicals
treated
are
ideal
surfaces
stomach
that
poison
may
be
but they
fatal
to
leave
residues
humans
and
upon
the
the
livestock.
Arsenicals include arsenites and arsenates . The arsenites are highly toxic
to insects
but
are
also
phytotoxic
and
therefore,
are used
as
poison
Paris Green
baits, for example, sodium arsenite, Paris Green. The
or
[Cu(C2H 30 2) 2 .3Cu(As0 2) 2] is a stomach poison
and was used against Colorado potato beetle in USA. The arsenates are
copper
less
acetoarsenite
toxic
applied
for insects
than
as
dust
or
as
extensively
used
to
protect
against
chewing
commercial
insects.
calcium
a
arsenites
spray.
The
arsenate
fruit
but
safer for plants
Acid lead arsenate
trees,
ornamental
LD50 value
is
the
for rat
mixture
may
be
(PbHAsO.J
and
is
plants
and
forest
825 mg/kg. The
is
tricalcium arsenate
of
[Ca 3(As0 .J 2 and acid calcium arsenate. Ca3HAsO 4 and is more toxic
than lead arsenate. It is used against cotton boll weevil on cotton and
insects pests of potatoes, tomatoes etc.
The modern organic stomach poisons, so-called systemic insecticides,
are taken up and translocated within plants and animals. Insects feeding
on
the
protected
susceptible
host
contact
the
individuals are killed.
insecticide . through
Systemics
in
the
the
plants
gut,
and
mostly
kill
piercing and sucking insects such as aphids, pyrilla, scales etc. as they
receive higher insecticide dose than chewing insects feeding on the same
plant.
In
livestock,
systemics
Hypoderma
like cattle grubs,
are often
spp.,
used
Gastrophilus
against
internal
parasites
spp.
2. Contact poisons. Contact poisons are the major group of modern
insecticides. They
usually enter the body through the cuticle
insects
walk
crawl
like
leaf or
or
over
a
treated
surface.
The
when the
insecticides
are
absorbed through the integument. If the treated surface is a food source
a
flower,
these
poisons
may
also
enter
the
gut
and
be
absorbed through it. In order to be effective, such insecticides have to
be
accumulated
by
contact insecticides
the
insects
in
sufficient
should be able to penetrate
amount.
the
Therefore,
the
integument of the
Methods of Insect Pest Management
{ 393
insects and non-degradable until it reaches the target organs within the
body. Almost all the pesticides of plant origin are contact poisons.
3. Fumigants. The site of contact of fumigants is the tracheal
system. They are thus respiratory poison. Fumigants are insecticides that
become gases at above 5"C. These insecticides are applied to enclosures
and to soil. Being highly volatile, it enters the tracheal system, circulate
and
subsequently
ethylene
are
dibromide
absorbed
(EDB),
by
body
HCN,
tissues.
phosphine,
Methyl
bromide,
dichlorvos
(DDVP),
dichloropropene and dichloropropane (DD), lindane· are the examples of
fumigants. Effective fumigants have high penetrating ability and kill all
stages of the insects in enclosures, including eggs without hampering the
germinability and viability of the seeds. Such insecticides should also not
be inflammable and corrosive.
(a) Dichloropropene
(CHCl=CHCH 2Cl)
and dichloropropane
(ClCH 2CH(Cl)CH 3). These two insecticides are often mixed together as
a soil fumigant and collectively known as D-D. Because it can damage
plants
and
sometimes
cause
off-flavouring
of potatoes,
the
mixture is
applied well in advance of planting.
(b) Paradichlorobenzene (PDB) [C6H4Cl2]. It is a white crystalline
material and vaporises very slowly to form a noninflammable gas with an
ether-like odour. It is used as a soil fumigant against peach tree borers,
and as a household fumigant and repellent for cloth-moths and carpet
beetles. It is also used to protect stuffed museum specimens from attack of
dermestid beetles. The safe limit for prolonged human exposure is 75 ppm.
(c) Naphthalene.
compound
and
is
Like
one
PDB,
of the
naphthalene
most
common
is
a
white
fumigants
crystalline
used
to
kill
household insects. They are produced as solids that emit gas at a slow
'
rate. They are used as moth balls or flakes for protection against
clothes moth. They are also used as soil fumigal)tS.
Cl
A
-O
CI
8
Fig. L A. Naphthafene crystals B. p-dichlorobenzene (PDB) crystals)
(d) Methyl bromide
(CH 3Br) . Methyl bromide is a highly volatile
insecticide widely used as a general fumigant.
It is fairly stable and
nonflammable and is very toxic to insects and mites. It penetrates well
and desorbs quickly. It has been particularly useful in the fumigation of
mills,
granaries,
and
warehouses.
Methyl
bromide
is
a
dangerous
cumulative poison to mammals, affecting the central nervous system and
causing disturbances of vision and equilibrium and, in cases of acute
394 J
Methods of Insect Pest Management
poisoning convulsion and death. The safe limit for prolonged human
exposure is 20 ppm.
(e) Carbon bisulphide (CS 2). CS 2 is a colourless liquid with
unpleasant odour. It is used to control household insects particularly
pests of stored grains, clothes and furniture. Although CS2 is highly
inflammable and explosive above 1 % in air, it has a very good
penetrating power and does not alter the grain quality and seed
germination. It also does not leave any residue on seeds. Safe limit for
prolonged human exposure is 20 ppm in air.
(fJ Carbon tetrachloride (CCl .J . Like CS2, CC14 is also a colourless
liquid. It is less toxic than HCN. It is usually mixed with CS2 to avoid
danger from fire. Carbon tetrachloride is highly toxic to mammals
producing severe kidney and liver damage. The safe limit for prolonged
exposure is 25 ppm.
(g) Chloropicrin (Cl 3CN0 2). This compound is the active ingredient
of tear gas used by the police but it is a fumigant in its own right,
being effective against insects. It is neither inflammable nor explosive
and can be used over a wide range of temperature. It is usually
employed to fumigate flour mills, grain elevators and bins.
(h) Phosphine (PH3). It is a hydrogen phosphide used to fumigate
storage grains and flour. Phosphine is highly toxic to all the
developmental stages of insects. The commercial product of this
fumigant is available in the form of tablets of aluminium phosphide and
ammonium carbonate. The latter prevents spontaneous igmt10n of
phosphine. One tablet of 3 g is sufficient enough to protect 1 00 kg of
grains. Its safe dosage for human beings is 0.05 ppm.
(i) Nicotine (C ioH 14N2). Nicotine is also used extensively as a
greenhouse fumigant, where its high degree of safety both to the plants
and to the humans made it favourite. For fumigation free nicotine (not
nicotine sulphate) is used. It is volatilised by painting or dropping the
liquid over hot steam pipes or by heating it in sallow pans.
Classification based on mode of action
The insecticides are also classified on the basis of their mode of action on
the insects which are as follows :
1. Physical poison. Some insecticides kill the insects by their
physical actions and no direct chemical or biochemical effect is caused.
These insecticides are seldom used as such but are incorporated
sometimes in the formulation. Following are some of the physical effects
that these insecticides exert :
(a) Asphyxiation. Natural oils (such as petroleum) or their emulsion
when applied on insects, block the respiratory tracts by closing the
spiracles or the passage of air such as on scale insects where it closes
Methods of Insect Pest Management
[ 395
the breathing pores. Also, the eggs of insects are killed by its
application as it closes the micropyle of the eggs through which they
respire. Because unless the oil is highly refined, it has phytotoxic effects
and can only be used during dormant stage of the plant particularly
trees.
(b) Mechanical injury. Dust of aluminium oxide cause abrassion of
the cuticle leading to desiccation of the insects to death. Boric acid, in
addition to being a stomach poison, acts as an abrasive dust in killing
insects.
2. Protoplasmic poison. Most of the inorganic insecticides and some
organic insecticides like nitrophenols, mineral oils, formaldehyde are
protoplasmic insecticides. When these insecticides are ingested cause
precipitation of the cellular proteins in the midgut epithelium leading to
death.
3. Respiratory poison. Most of the fumigants like HCN, H2S and
CO that along with air enter the respiratory system and interfere with
the cellular respiration by inhibiting the respiratory enzymes killing the
insects.
4. Nerve poison. Most of the conventional insecticides act as nerve
poisons. These affect the nervous system mostly as narcotics, axonic
poisons, or synaptic poisons.
(a) Narcotics. Many fumigants particularly those containing Cl, Br,
and F are narcotics, inducing unconsciousness in insects. These narcotics
are fat soluble and stored in fatty tissues, including nerve sheaths and
lipoproteins of the brain.
(b) A.xonic poisons. The axonic poisons act primarily by interrupting
normal axonic transmission of the nervous system. The axon of the
nerve is an elongated extension of the cell body that transmit nerve
impulses to other cells. These impulses are electrical and arise from the
flow of Na+ and K+ ions through the cell membrane, creating a
wavelike action potential (an impulse). Subsequently, the action potential
is followed by a resting potential. All the chlorinated hydrocarbons and
pyrethroids are believed to disrupt normal transmission along the axon.
Cyclodienes and pyrethroids are believed to induce changes in axonic
membrane permeability, causing repetitive discharges. Such discharges
result in convulsions, paralysis and death.
(c) Synaptic poisons. The nerve poisons essentially interfere with the
function of the enzyme acetylcholinesterase due to which the
acetylcholine synthesised in the synaptic gap of the neurons during
conduct of impulse is not hydrolysed into acetic acid and choline. As a
result, nerve continues to transmit the impulse and also produces several
coactive substances that are toxic to the insects and hinders the normal
nerve conduction causing tremor, convulsion, paralysis and finally death
396 J
of
Methods of Insect Pest Management
the
insects.
Most
of
the
modem
synthetic
organic
insecticides
(organophosphates, carbamates) are of this category. Few nerve poisons
such
as
DDT
has
different
mode
of
action,
it
does
not
acetylcholinesterase but causes nervous excitation leading to
inhibit
exhaustion
and death of the treated insects. Pyrethrins and nicotines are also nerve
poison. Nicotine and nicotine sulphate poison by mimicking acetylcholine
at
the
and
synapse;
nicotine.
inhibition
the
This
receptors
cannot
phenomenon
mode
known.
of _action
Aldrin,
effects.
in
between
symptoms
acetylcholine
similar
to
the
of acetylcholinesterase.
5. Poison of general nature.
their
distinguish
results
excitable
In
the
chlordane
Rotenone,
depression.
or
ryania,
In case of certain insecticides either
sequence
and
of
toxaphene
actions
induce
are
not
delayed
properly
neurotoxic
sabadilla and phenothiazine produce muscular
case
of
ryania,
the
membrane
of
muscle
which
alkaloid,
ryanodine,
results
in
up
to
disrupts
a
the
threefold
increase in oxygen consumption, followed by flaccid paralysis and death.
Classification based on chemical nature
The most precise method of classification of insecticides is according to
their chemical makeup. Chemically the insecticides are primarily of two
kinds:
inorganic
and
organic
insectic�des.
LD50 values of most of the
common insecticides in rats are given in the table 1 .
1. Inorganic insecticides. There are very few inorganic insecticides
used
today.
In
the
past,
[Ca 3(As0 � 2] ,
copper
contact
and
lead
arsenate
acetoarsenite
(PbHAs04),
green),
(Paris
sodium fluosilicate, cryolite and sulphur were used.
mites
poison
and
some
cockroach
and
Na 3AlF�
is
stomach
fungi.
poison
Sodium
grasshopper baits,
somewhat
effective
and
is
is
cryolite
against
arsenate
fluoride,
Sulphur is both a
applied as
fluosilicate
and
calcium
sodium
an
[sodium
a
a dust
insecticide
number
against
used
in
fluoroaluminate,
of
insects
and
relatively nontoxic to mammals. The mineral sodium aluminum fluoride
(kryocide) is mined in Greenland and is a by-product of the aluminum
industry. This insecticide is still used to control foliage feeding pests of
potatoes and grapes. Because it is applied at 1 0 kg/ha per application,
there is
concern about long-term residues. Borax (N32B407) is useful
for fly maggot control in manure pits and wounds of animals infested by
them.
2.
Organic
insecticides
insecticides.
including
There
petroleum
and
are
a
wide
vegetable
range
oils,
of
organic
botanicals
and
synthetic chemicals used to control insect pests.
(a) Naturally occurring organic insecticides.
Hydrocarbon oils such as
petroleum and mineral oils are heterogeneous mixture of saturated and
unsaturated
chains
and
cyclic
hydrocarbons.
Certain
fractions
of
this
Methods of Insect Pest Management
mixture
are
much
more
[ 39 7
useful
insecticides
than
other
fractions.
Generally, the lower the viscosity the safer it is to use with respect to
phytotoxicity as the phytotoxicity
increases
with
increase in distillation
range, because the greater the distillation range the less volatile the oil.
However, the heavier spray
oil
fractions are more effective at killing
insects than the lighter oils. These oils are highly phytotoxic
in their
natural state but when used in an emulsion, they may be safely applied
to plants.
Superior horticultural
oil or dormant oil
is a highly refined
mineral oil that kills insects and their eggs by suffocating them. Jt is
applied in late winter or very early spring to kill scales,
aphids,
other overwintering insects. It is also used in the fall to kill eggs.
Biopesticides
or
bioinsecticides
(Fig.
are
2)
derived
from
and
plants
(botanicals) or animals and are in use in modern agriculture due to their
upper hand over synthetic insecticides as usually they are not toxic to
non-target animals and are easily degradable. Use of pyrethrins, nicotine,
azadirachtin,
rotenone.
etc.
are
time
honoured
insecticides.
Some
phytoproducts act as attractants (geraniol and methyl eugenol), some as
repellents (citronella and oil of cedar), some as solvents (cottonseed oil),
and some as carriers
of insecticides
(pulverised walnut-shell). Yet, the
primary use of plant derivatives is as insect toxicants.
Nicotine
alkaloid
Nicotine
[ C 1 ofI 1 4N2,
derived
alkaloid
from
and
Black
Leaf®] .
tobacco
nicotine
C ommercially
(Nicotiana
sulphate
tabacum
have
been
nicotine
and
used
N.
is
an
rustica).
as
contact
insecticides, fumigants, and stomach poisons. It is highly toxic to a great
number of insects as a nerve poison. Nicotine sulphate is very toxic to
insects, as well as to humans. It causes severe disruption and failure of
the human
nervous
system,
is
easily
absorbed through the eyes,
should only be used as a last resort.
with
diluted
mixtures.
skin,
and is extremely fast-acting. Nicotine sulphate
and mucous membranes,
Best results have been reported
It biodegrades rapidly
with
little
residual
effect.
This material is sold as sprays because dusts are too dangerous to use.
It is used against piercing sucking insects and mites. Nicotine sulphate is
banned in India but is manufactured for export only.
Pyrethrum.
The insecticidal
activity of pyrethrum discovered around
1 800 is extracted from the flowers of Chrysanthem um coccineum,
C. cinerariaefolium and C. cameum (Family : Asteracae) . Originally
pyrethrum flowers came from
Yugoslavia and Japan,
but
Kenya
now
supplies most of them. It is made up of four compounds : pyrethrins I
and
II and cinerins
I
II.
and
The
cinerins
are more stable than the
pyrethrins. This insecticide is commonly contained in household aerosol
sprays
because
characteristics.
it
This
has
a
chemical
wide
spectrum
attacks
the
and
insects
rapid
knockdown
peripheral
nervous
system and for this reason has a rapid knockdown, however, the insects
398 J
Methods of Insect Pest Management
soon recover to full act1v1ty. Therefore, some synergists are added in the
formulation. It is available as spray and dust for use on fruit trees,
vegetables and flowers. This insecticides is readily breakdown m
presence of sunlight.
Rotenone (C23H 220 J . Rotenone 1s found in the roots of several
species of leguminous plants in the genus Derris, grown principally in
the Far East, and in the genus Lonchocarpus, found mostly in the
Amazon Basin of South America. It is probably the second-most used
botanical. Rotenone is a white to yellowish white crystal and is readily
detoxified by the action of air and light. It is a metabolic inhibitor (i.e.,
inhibits the respiratory chain, the oxidation of NADH- linked substrate)
and is a broad-spectrum contact and stomach poison that affects insect
nerve and muscle cells, causing the insects to stop feeding and die
anywhere from a few hours to a few days after ingestion. It is most
effective against leaf-eating caterpillars and beetles, can be applied as a
spray or dust. It is available in a variety of strengths as well as in
combination with pyrethrin and ryania. Crystalline rotenone is also
commercially available and is used for mothproofing. If rotenone is
eaten by humans or other mammals it is broken down by the liver with
no long term negative effects. It is extremely toxic to fishes. Indians in
South America mash the roots and allow the exudate to flow into
streams to kill fish for food.
Ryania. Ryania is a botanical extract, extract from the stem and
roots of a woody South American plant Ryania speciosa. Like nicotine,
the active ingredient in this material is an alkaloid ryanodine
(C 25H 3 50 9N). Ryania is a stomach poison that causes insects to stop
feeding soon after ingestion. It is reported to be most effective when
used in hot weather. Ryania is moderately toxic but considered to be
Uo
"-
A
(CH,),
II
CH2CH•CHCH•CH2 CH3C =CH
I
C- OCH,
II
0
B
OQ
H, so.
A. Pyrethnn I,
OCHi
c
I
N
CH,
E
D
Fig. 2. Biopesticides :
F Rotenone.
0
__/I
o -c�
F
B. Pyrethrin II, C. Cinerin I, D. Cinerin II, E Nicotine sulphate,
Methods of Insect Pest Management
[ 399
relatively hannless to humans and other mammals. Ryania has been
used most widely against caterpillars on orchard trees, particularly
against codling moth (Cydia pomonella) on apples.
Azadirachtin (C 35H440 16). Azadirachtin is the most active compound
found in neem (Azadirachta indica) plants and is highly toxic to several
insect pests such as cotton aphids, cotton bollworms, brown plant
hopper, cabbage butterfly etc. Indeed, the neem plants contain
thousands of chemical constituents.
O f special interest are the
terpenoids that are unique to neem. More than a hundred terpenoids
are known from different parts of the neem plant. Azadirachtin is one
of the terpenoid. Several different kinds of azadirachtins (A to K) have
been isolated, the most abundant of which is Azadirachtin-A. The neem
terpenoids are present in all parts of the plant, in the living tissues
especially in the seed kernels. The neem products (Neem, Nimbicide,
Achook, BioNeem, Neemix, Azatin) work on the metamorphosis of
insects. When the azadirachtin enters the body of larvae, the activity of
moulting hormone, ecdysone is suppressed and the larva fails to moult,
remains in the larval stage and ultimately dies. If the concentration of
azadirachtin is not sufficient, the larva manages to enter the pupal stage
but dies at this stage and if the concentration is still less the adult
emerging from the pupa is 100 % malformed, absolutely sterile without
any capacity for reproduction. The chitin synthesis is also inhibited. In
addition, it has property of feeding deterrence. Another way in which
azadirachtin and other terpenoids reduce pests by deterring oviposition.
Neem based insecticides can be used to manage pests on vegetables,
fruit, ornamentals, and lawns and can be found at many home garden
centers. Neem has been used with , variable results to manage aphids,
boxelder bugs, annyworms, cabbage loopers, Colorado potato beetles,
mites, corn earworms, cutworms, com borers, flea beetles, fungus gnats,
flies, grasshoppers, leafhoppers, leafminers, mites, spruce budworms, tent
caterpillars, thrips, whiteflies, and many others. Since azadirachtin is not
a stable compound, most of the neem based pesticides are manufactured
from its seed kernels. One of the most desirable properties of neem is
its low degree of toxicity, LD50 for rat is more than 5000 mg/kg; it is
considered almost nontoxic to humans and animals, and is completely
biodegradable. Neem is most effective as a foliar spray applied
periodically to new flushes of growth.
Sabadilla is a broad spectrum insecticide that comes from the seeds
of a lily, sabadilla (Schoenocaulon officinale) indigenous to Central and
South America. The insecticidal constituents are complex group of
alkaloids. It affects the nerve cells of insects causing paralysis and then
death. It is primarily used for adult insects that are hard to control with
other botanical insecticides. Although the dust is considered to be the
(Z-57)
400 1
Methods of Insect Pest Management
least toxic of all registered botanical insecticides, the active alkaloids in
its pure, extracted form are very toxic and can make a person sick if
ingested or absorbed through the skin and mucous membranes. Sabadilla
is highly toxic to honeybees and should only be used in the evening,
after they have returned to their hives. It degrades rapidly in sunlight
and air, leaving no harmful residues.
Insecticidal products isolated from animal are quite new. A
substance isolated from marine annelids, Lumbrineris heteropoda and L
brevicirra, has been found to possess insecticidal properties having
neurotoxic effects on insects. The toxin is known as neristoxin
(4-N,N-dimethylamino)- l , 2-dithiolane). Some of its allied products have
now been synthesised and marketed.
Although these biopesticides are still in use, they are extremely
expensive to produce and for this reason they are rarely used in a
commercial setting except the neem products.
(b) Synthetic organic insecticides. The synthetic organic insecticides
Chlorinated hydrocarbons, organophosphates,
are of four classes
carbamates and pyrethroids.
Chlorinated hydrocarbons are the oldest insecticides having been
the first widely used synthetic organic insecticides. All insecticides of
this group contain at least chlorine, hydrogen, and carbon. Some of the
insecticides also contain oxygen and sulphur. There is a large number of
chlorinated hydrocarbons including DDT, HCH (=BHC), methoxy�hlor
with their analogues and isomeric forms such as lindane {y-BHC). Other
insecticides of this group are chlorinated terpenes (toxaphene),
cyclodienes (aldrin, dieldrin, chlordane, isodrin, heptachlor, endrin, etc.)
and other compounds like chlordecone (kepone) and endosulfan
(thiodon) (Fig. 3). Most of these chemicals have been banned from use
because of their persistence in the environment and toxicity to nontarget
organisms.
DDT (C 14H9C15) [ l , 1 , 1-trichloro-2,2-bis(p-chlorophenyl)ethane] is also
called dichlorodiphenyltrichloroethane, hence DDT. Technical DDT is a
white to cream-coloured amorphous waxy powder. It is one of the first
synthetic organic insecticides, which is representative of the
organochlorine chemicals. Organochlorine molecules tend to be very
stable because of the placement of the chlorine ions in the molecule.
Soon after DDT was released into the market in the early 1940s, it was
used primarily to control lice,· fleas, mosquitoes, house flies etc. that
were vectoring human diseases and in agriculture. DDT acts as either a
contact or a stomach poison to insects, affecting the sensory organs and
nervous system and causing violent agitation at first, followed by
paralysis and death. Unfortunately, the overhelming effectiveness of
DDT and its exceptionally low cost contributed to overuse and,
(Z-57)
Methods of Insect Pest Managem ent
[ 401
subsequently, to its demise. The use of DDT is banned in agriculture in
most of the countries of the world including India.
Methoxychlor [ l , l, l-trichloro-2, 2-bis(p-methoxyphenyl) ethane] is an
important DDT analogue. It is a white crystalline pale buff flaky
powder. It is less toxic to mammals (1/251h to 1/501h of DDT) and is
not accumulated in fatty tissues or excreted in milk, hence is preferred
for use on animals. It is more toxic to some insects than DDT, e.g., it
has a faster knockdown of house flies than DDT.
HCH (C6H6Cl6) and Lindane are also important. HCH (1 ,2,3,4,5,6hexachlorocyclohexane) earlier called BHC (benzene hexachloride), has a
wider spectrum than DDT and is effective against aphids. It has a
musty odour and flavour and comprises 5 isomers, only y isomer was
isolated, manufactured, and sold directly as the insecticide Iindane.
Lindane is odourless and volatile and was widely used as a household
fumigant. It is generally formulated as wettable powder containing
5�25% "(-isomer or as a dust containing 0.5-2% "(-isomer, for agricultural
uses. Like DDT, it is a nerve poison to insects. Presently, HCH and
lindane have been banned for use. In India, use of lindane formulation
generating smoke for indoor use is prohibited, but is permitted for use
for control of insect pests of field crops.
Cyclodeines such as aldrin (C12H 8C16), dieldrin (C1 2H 8C160),
chlordane (C 1oH6C13), isodrin, heptachlor (C1 oH5C17), endrin, mirex,
endosulfan (C9H6C16o 3s, Thiodan ®), chlordecone (Kepone®) etc. are
developed after DDT and HCH. They are persistent chemicals, stable in
soil and relatively so in sunlight. Cyclodiens are usually formulated as
wettable powder. Therefore, many were used ifl great quantities against
such soil insects as corn rootworms, wireworms, cutworms, etc. Most of
the cyclodeines are more toxic to mammals than DDT and are more
dangerous to apply. Technical chlordane is a dark amber viscous liquid
with a cedar-like odour. Aldrin is a white crystalline solid, insoluble in
water and a tan to brown in colour. It is easily converted in plant and
animal tissues as dieldrin, hence it shows the same toxic effects as
dieldrin. Dieldrin is the epoxy of aldrin and is one of the most
persistent chemicals. It is used in the situation where long lasting
residual effect is advantageous. Endosulfan is a brownish crystalline
solid and is a mixture of two isomers. Heptachlor is a derivative of
chlordane. It is a white crystalline solid, 4-5 time more toxic to insects
than chlordane. Due to growing ineffectiveness from insecticide
resistance and problems with residue uptake in harvested produce, these
insecticides are banned in most of the countries including India.
Cyclodeines have also been eliminated for use even in termite control.
In India, use of dieldrin is restricted for locust control only.
(Z-57)
Methods of Insect Pest Management
402 J
CH,o
od�D
OCH,
c
C1)yc1
c1VC1
Cl
/O
O= S
)tJ
D
C
I
Cl2
'o
I
Cl
I
Cl
Cl
H
J
L
K
Fig. 3. Chlorinated hydrocarbons: A. DDT, B. TDE (=DDD), C. Methoxychlor, D. HCH (=BHC),
E. Chlordane, F. Heptachlor, G. Aldrin, H. Chlordecone, I. Endosulfan, J. Dieldrin, K. Endrin,
L. Toxaphene.
Toxaphene (C 1 off 1 0Cl8) is a chloroterpene and is used exclusively in
agriculture. Until a few years ago, toxaphene was the single most widely
used insecticide in agriculture particularly against grasshoppers, cotton
insects and livestock pests. It is unstable in the presence of prolonged
exposure to sunlight. Toxaphene is formulated as a 25-40% wettable
powder, as an emulsive concentrate, as a kerosene solution, and as a
dust. Though it is not highly toxic to birds and mammals as they easily
metabolised it, but is highly toxic to fishes. Like other countries,
toxaphene is also banned for use in India.
Organophosphate (OP) insecticides are derived from phosphoric
acid and are some of the most toxic insecticides. The OP insecticides
are the most romantic group of insecticides as they show both systemic
and non-systemic action. Some systemic OP insecticides are dimethoate,
disulfoton, dicrotophos, oxydemetonmethyl while others are non-systemic
insecticides. Unlike chlorinated hydrocarbons, OPs are unstable in the
presence of sunlight and quickly break down into nontoxic compounds.
Because of this property, and development of resistance by insects
against chlorinated hydrocarbons, OPs have replaced the latter in many
control programmes. In fact, OPs are perhaps the most widely used
group of insecticides today. These compounds are characterised as
having different alcohols attached to their phosphorus atoms, and the
various phosphoric acids produced are termed esters. These esters have
different combinations of oxygen, carbon, sulphur, and nitrogen, and
accordingly these compounds may be divided into 3 groups, viz.,
aliphatic, phenyl and heterocyclic derivatives.
(Z-57)
•
Methods of Insect Pest Management
[ 403
Aliphatic derivatives are compound with straight carbon chains
(Fig. 4) such as TEPP (tetraethylpyrophosphate) which was used for fly
control in dairy barns. TEPP (Bladan ®) is a colourless, hydroscopic
liquid, miscible in water but rapidly hydrolysed to nontoxic components.
It is extremely toxic. Malathion (C 1 oH 1 90 6PS2), most effective aliphatic
derivative of all OPs has been used for all types of agricultural insect
pests and household insects. It has also been used for head, body and
crab louse problems. It �s formulated as dusts or sprays. The malathion
is one of the safest of all insecticides. Malathion kills insects by contact
or vapour action and also is a stomach poison. Pure malathion is a
colourless liquid while technical grade is brown with a garlic odour.
Because of its low toxicity, it is useful for household application. Other
aliphatic derivatives are plant systemics such as Schradan (OMPA),
dimethoate (C5H 12N0 3PS 2, Cygon®), disulfoton (Di-Syston®), demeton
Dimecron®),
(C 1 oH 1 90 5NCIP,
(Systox®),
phosphamidon
oX:ydemetonmethyl
(C6H 1 50 4PS 2, Meta-Systox R®),
monocrotophos
(C 7H 1 40 5NP), dicrotophos (C 8H 160 5Bidrin ® ), trichlorfon (C4H 8CI304P,
Dylox®), acephate (C4H 10N0 3PS, Orthene®), phorate (C7H 1 p 2PS 3 ,
Thimes ®) etc. These insecticides are applied in soil and are taken up
by the plants and translocated to stem and foliage. They are highly
effective
against
piercing
sucking
insects.
Schradan
(octamethylpyrophosphoramidate), the first OP compound to be studied
as systemic insecticide, is a colourless, odourless liquid that is miscible
with water and most organic solvents. It is safe enough to use. In India,
0
0
II
0
0
II
3l2
fII -0-P-N(CH
II
(CH3)2N
(Cff3)2 N
(C2Hs 0)2 P- 0 - P (OC2Hs )2
'N(C H 3 )2
A
s
I
II
0
II
(CH30)2 P-0 - C - CHC- N(CH3)z
F
II
C
o
?i
(CH30)2 P-S-CH2CH2- S- C2Hs
D
CH3
(CH3 0)2 P - S - CH- C - OC2 H�
9
0
II
II
II
CH2 - C - OC2H�
B
(CH30)z P - S - CH2C-NH- CH3
0
0
�
E
0
OH
11
I
(CH o) 2P - CHCCJ3
3
G
S
11
(C2HsO)i P - S - CH2CH2-S-C2Hs
H
Fig. 4. Aliphatic oranophasphates. A. TEPP, B. Schradan, C. Malathion, D. Dimethoate,
E. Oxydemetonmethyl, F Dicrotophos, G. Trichlorfon, H. Disulfoton, I. Acephate, 1. Phorate.
404 1
Methods of Insect Pest Management
monocrotophos is restricted for use in vegetables. Dimethoate also . acts
as contact poison against mites. Trichlorfon is a selective OP and is
comparatively unharmful to the natural enemies of the insects. Acephate
is more recent insecticide widel
used in agriculture particularly for
management of vegetable pests. It is a foliar spray insecticide of
moderate persistence with systemic activity of about 1 5 days. It is useful
against aphids, leafmiriers, caterpillars sawflies and thrips. Phorate is an
older systemic OP compound that is economical and effective against
corn rootworms. In plant tissue, phorate is rapidly oxidised. The
resultant oxidative sulphoxide and sulfone metabolites are responsible for
the
systemic
toxicity
of the
compound.
Dichlorovos
(DD VP,
®
C4H 7Cl20 4P, Vapona ) is a colourless to amber liquid and is a very
volatile insecticide which gives rapid knockdown and kill of house flies.
It has been widely used in dry or liquid bait or in resin strands for fly
control and is also used on livestock to control flies.
The phenyl derivatives differ basically from aliphatics in having a
phenyl ring, which has one of the hydrogen displaced by a phosphorus
moeity and others displaced by CH3 , Cl, CN, N02, or S (Fig. 5). These
OP compounds are more stable than aliphatic OPs. These OP
compounds include parathion (methyl- and ethyl parathion), stirophos
(Gardona ®), famphur (Warbex®), fenthion (C 1
150 3PS 2, Baytex®),
profenophos
(C 1 1H 1 5BrC10 3PS,
Curacron®),
sulprophos
(Bolstar®),
fonophos (Dyfonate®), isofenophos (C 15H 24N0 4PS), etc. Parathion is
most widely used phenyl OP compound. Its ethyl form (ethyl parathion,
y
oH
�
(CH30)1 -o-c
A
(CH3o)
J
-o
0'
_
D
II
i
'
CHCl c
B
so2N(CH3)2 C2Hso -. _ 0
C3H1S "
£>
Cl
Cl
Cl
0'
� D'
...9 0
'
scH3
_
E
C 2H so ..
P-0
C3H1S ...-
C2HsO - - s
C2 Hs
Cl
F
Br
H
Fig. 5 Phenyl organophosphates. A Ethyl parathion, B. Methyl paratluon, C. Stirofos, D. Famphur,
E. Fenthion, F. Profenofos, G. Sulprofos, H. Fonofos.
Methods of Insect Pest Management
{ 405
now banned in India) was used against aphids. It is very toxic to
humans and was replaced by other form of parathion, the methyl
parathion which has a broad range of toxicity to many insects. Methyl
parathion is a white crystalline compound, however, the technical
product is an amber liquid. It is less stable than parathion and is too
toxic to domestic use. Its use, in India, is permitted only on those crops
where honey bees are not acting as pollinator. Stirophos is less toxic
and is used for livestock parasites. Famphur and fenthion are used as
animal systemics against cattle grubs but should not be used on lactating
dairy cattle. These two insecticides are simply poured over the animal
body and are absorbed through the skin. Profenophos and suprophos
have been used against field crops while fonophos is used against soil
insects in both field and vegetable crops.
The heterocyclic derivatives OPs (fig. 6), like phenyl OPs, have ring
structures but differ in having one or more carbon atoms displaced by
0, N, or S. Also, structural rings in this group may have 3, 5, or 6
atoms. These compounds are most stable and long-lasting of the OPs.
This group of OPs includes diazinon (C1 2H 2 1Np3PS), azinphosmethyl
(Guthion ®), chlorpyriphos (C9H 1 1C13N03PS, Dursban®, Lorsban®),
methidathion (Supracide®), and phosmet (lmidan®).1 Diazinon is one of
the most common heterocyclic OP which is moderately safe and hence
is recommended for household and garden sprays. It ·is formulated as a
25% wettable powder and 25% emulsive concentrate. Like diazinon,
azinphosmethyl is an older insecticide of this group which has been
extensively used on cotton against insect and mite pests and is
formulated as 15% wettable powder and 15% emulsive concentrate.
Chlorpyriphos is one of the most useful insecticides. It is a stomach
and contact poison with a long residual life in the soil and a short one
on foliage. Though it is moderately toxic to animals, it is relatively safe
to apply. Chlorpyriphos is effective against cockroaches, termites and
8
A
s
c
s
o< '-yoCH1
\
f/
II
(CH30)2P- S-CHi- N - N
D
Fig. 6. Heterocyclic organophosphates. A. Diazmon, B. Azinophosrnethyl, C. Chlorpyrifos, D. Methi­
dathion, E. Phosrnet.
406 1
Methods of Insect Pest Management
other household insect pests and can be applied on pets. Its other
formulations are also used on field crop against insect pests.
Methidathion and phosmet have uses on field, forage, fruits, and nut
crops against a variety of insect and mite pests.
Carbamates. The carbamates (Fig. 7) are broad spectrum anticho­
linesterase insecticides that have had wide application in agriculture.
They were developed in the early 1950s and are very similar in
environmental persistence and mode of action to that of the
organophosphates. The carbamates tend to break down rapidly once
applied, leaving no harmful residues. However, if these chemicals are
incorporated into the soil where they are not exposed to light and the
soil pH is low, they may persist for 1-2 years. A distinct limitation of
carbamates in pest management is their toxicity to pollinating and
parasitic Hymenoptera. These insecticides are produced from carbamic
acid and have an -OCON- group in the molecules. Carbamates are
rapidly detoxified and eliminated from animal tissues and thus are not
accumulative in fats or excreted in milk. The carbamates are divided
into three groups as : heterocyclic, phenyle and oxime carbamates.
The
heterocyclic carbamates
includes
Isolane
(C 1 ofl 1 7N30 2,
l-isopropyle-3-methyl-5-pyrazolyl
N,N-dimethyl
carbamate),
dimetan
(5,5-dimethyldihydroresorchinyl N,N-dimethyl carbamate), etc. Isolane is
a contact and systemic while dimetan is systemic poison and are
especially effective against aphids and flies.
The phenyle carbamates are esters of N-methylcarbamic acid. It
includes
carbaryl
(1 -naphthyl
N-methylcarbamate),
carbofuran
(2,3-dihydro-2-2-dimethyl-7 benzofuranyl methyl carbamate), proxpur
((2-isopropoxy-phenyl N-methyl carbamate), carbosulfan ((2,3-dihydro-2,
2-dimethyl benzofuran-7-yl(dibutylarninothio)-methyl carbamate).
Carbaryl (C 12H 1 1N0 2) (Sevin®), a naphthylcarbamate, is the oldest
of the effective carbamates. It has low toxicity to humans and, therefore,
g:��
o�
o-'Jc-NH- cu,
(X)
--?
CH i
0-C-NH-CHi
0
II
B
CH3
0
CH3- S-CCH=N-O-C-NH-CH1
CH3
I
II
I
D
cu,OoJ-N1o-c••
CHi
c
0<>1NH-<:H,
OCH(CH3'1
E
Fig. 7. Carbamates. A. Carbary!, B. Carbofuran, C. Trimethacarb, D. Aldicarb, E. Propoxur.
·
Methods of Insect Pest Management
[ 407
is a common insecticide for use against household insects. It is widely
used
on
fruit trees,
vegetables
and cotton insects
but
should
not be
sprayed on the crops at the flowering stage as it also kills the pollinator
insects.
Carbofuran (C 1 2H 15N03) (Furadan®) is widely used as a soil
insecticide for suppression of nematodes, corn rootworms, and other soil
insect pests. It is a plant systemic insecticide. Its drawback is that it is
highly toxic to human beings. Continuous use of carbofuran in the same
field
results
degrade the
an
increase
in
the
population
of
microorganism
compound rapidly after application reducing
its
that
effect on
the target pests.
Propoxur (C1 1H 15N03, Baygon®) is used against cockroaches, sand
flies, chinch bugs and adult mosquitoes. It is particularly effective against
species
in
restaurants
and
homes
that
have
become
resistant
to
organophosphate insecticides.
The
oxime carbamates are the oximes
aldehydes
and
ketones.
2(methylthiopropionaldehyde
It
of
includes
aliphatic
and
aldicarb
0-(methylcarbamoyl)
cyclic
(2-methyl-
oxime),
methomyl
(S-methyl thiomethyl carbamoyl thioacetamidate), thiodicarb etc.
Aldicarb (C7H 14N203S, Temik®) is a systemic insecticide, nematicide
and acaricide. It is highly toxic and formulated as
10%
granules. Aldicarb
is used to control the insect pests of potato.
Methomyl (C5H 1oN203S) is effective against sucking insects
caterpillars such as cabbage looper and diamond back moth.
Pyrethroids. The
insecticides
tend
to
but are
be
side-chains.
more
pyrethroid
synthetic
stable
Although
the
insecticides
analogues
because
of
pyrethroids
(Fig.
of the
8)
natural
the
lower
affect
the
are
not
new
pyrethrins
reactivity
peripheral
and
of
and
the
nervous
system, causing a quick knockdown, the primary target seems to be the
ganglia of the central nervous system. These chemicals are now widely
used throughout the world. Pyrethroids are the fastest developing group
of
modern
insecticides
and
are
replacing
many
older
insecticides
because · of their great effectiveness and safety of application.
insecticides are highly
toxic
to
insects
These
at very low rates and unlike
natural pyrethrum, there is less recovery of poisoned insects. The added
advantage
of use
of these
insecticides
than organophosphates and carbamates.
is
their lower application
The pyrethroids
cost
are categorised
into
generations
of development.
Allethrin
belongs
to
the
first
generation, and several compounds including_ resmethrin belong to the
second generation. Fenvalerate (Pydrin®) and permethrin (Ambush®,
Pounce ®) introduced in 1972 and 1973 belongs to the third generation
carbamates.
These
insecticides
are
applied
on
many
crops
such
as
Methods of Insect Pest Management
408 1
C0 H�CHJ
A
ooCH-OC
CH=CCh
N
C
O I CH3CH3
0ACH=CBr2
ooCH-OC
O I CN
Cl
OO�H-0-�-�H-NH-0"CF3
CH3
O I
II
D
E
�
�
�
�
�
F
II
�
CN
CH
CH3
-
Fig. 8. Pyrethroids. A. Allethrin, B. Fenvalerate, C. Pennethrin, D. Cypennethrin,
F. Fluvalinate.
E. Deltamethrin,
cotton, com, soybeans etc. where they are effective against several above
ground insect pests. Fourth generation carbamates are even more potent
insecticides than the earlier ones. Application rates for these may be
reduced to only one-tenth those of third generation pyrethroids for
similar effectiveness. Some of the recent, truly exciting insecticides in
this category are cypermethrin (Ammo®), flucythrinate (Payoff'®),
fluvalinate (Mavrik®, Spur®), deltamethrin (Decis®).
Allethrin (C 1 gH 2603) is cheap to produce and is effective against
house flies and mosquitoes. It is a clear brownish viscous liquid and has
contact, stomach and respiratory action and brings about quick
knockdown of flies and mosquitoes. It is moderately toxic to animals.
Resmethrin is an aromatic substitute of allethrin and is the most
effective insecticide. Pure isomers of permethrin is an odourless and
Methods of Insect Pest Management
colouless
crystalline
solid
but
{ 409
the
technical
grade
is
a
pale
brown
viscous liquid. It is a broad spectrum insecticide, used against a variety
of pests on nut,
and
cereal
termite
fruit,
crops.
control.
cockroaches.
It
It
is
also
Permethrin
vegetable, cotton,
used
in
controls
is
ornamental, mushroom,
greenhouses,
animal
available
in
home
gardens,
ectoparasites,
dusts,
biting
potato,
and
for
flies,
and
emulsifiable concentrates,
smokes, ULV and wettable powder formulations. Permethrin may persist
in fatty tissues. It does not block, or inhibit, cholinesterase enzymes. Soil
microorganisms play a large role in
the
degradation of permethrin
in
the soil. The pure isomers of cypermethrin are colourless crystals and
the technical material is a viscous yellow-brown semi-solid. Cypermethrin
is
a
broad
neurotoxin
spectrum,
non-cumulative
insecticide,
with good contact and stomach action.
and
a
It is
fast-acting
of moderately
high toxicity to mammals and readily metabolised with immediate loss of
activity.
Cypermethrin is not a plant systemic, it is readily degraded on
soil or plants
but has
good
residual
active
against
activity
on
inert
surfaces.
It
is
formulated as emulsifiable concentrates and wettable powder of various
concentrations.
particularly
Hemiptera;
It
leaf
is
and
fruit
eating
cattle ectoparasites,
sheep
a
scab,
is not recommended for household use.
Although
the
insecticide
wide
groups
range
Lepidoptera,
lice
of insect
Coleoptera
and ked.
mentioned
above
pests,
and
Cypermethrin
include
vast
majority of the insecticides used, there are few insecticides which are
notable
for
dinitrophenols,
specific
purposes
organosulphurs
and
such
as
formamidines,
thiocyanates,
organotins.
Formamidines. These insecticides are used against those insects that
have
developed
resistance
for
organophosphates
and
carbamates.
They
are quite effective against eggs and young caterpillars. Chlordimeform
(Galecron ®,
Fundal®,
formamidine.
Their
Fig.
9)
application
cotton and is strictly regulated.
e1
Q
A
is
is
one
of
restricted
the
for
most
nonfood
0
CH3
' N=CH- � -CH=N
CH
a
�
CH a
Fig. 9. Fonnarnidines. A. Chlordimefonn,
crops
CH
CH3
•·cH - NlcH,n
widely
B
used
like
O
a
' CH3
�
B. Amitraz.
Thiocyanates. Butaxyethoxyethyl thiocyanate (Lethane 384®, Fig. 10)
thiocyanoacetate
(Thanite®)
are
the
thiocyanate
and isobomyl
insecticides having creosotelike
odour.
Thanite
is
a
20%
solution
of
41 0 1
Methods of Insect Pest Management
CH3 ?, CN
�O-CCH2-S
v
8
A
Fig.
10. Thiocyanates. A. Lethane, B. Thanite.
thiocyanoacetate in deodourised kerosene. They are relatively safe for
use around the home. They give very quick knockdown of flying insects.
Table 1. Acute oral toxicity CI.Dso for rats, mg/kg body weight) of certain selected
insecticides).
Insecticides
Acephate
Aldicarb
Aldrin/dieldrin
Allethrin
Approcarb
Azadirachtin
Azinophosmethyl
Calcium arsenate
Carbary!
Carbofuran
Chlordane
Chlorpyrifos
Cypermethrin
DDT
DDVP
Demeton
Diazinon
Dicrotophos
Dieldrin
Dimethoate
DNOC
Endosulfan
Endrin
Fenthion
Fonofos
Formothion
HCH (= BHC)
m g/Kg
870-945
1-30
55-60
680-920
175
> 5000
18
298
500-750
5
225-590
97-276
300
250
56-80
2-12
150-220
21
46
155-500
26-65
1 10
18
178-310
17
375-535
600-1250
Insecticides
Heptachlor
I solane
Lead arsenate
Leptophos
Lethane
Lindane
Malathion
MethoX}{:hlor
Methyl parathion
Monocrotophos
Nicotine sulphate
Oxydemetonmethyl
Parathion
Permethrin
Phorate
Phosalone
Phosphamidon
Propoxnr
Pyrethrum
Roten one
Ryania
Sabadilla
Sehrad an
TEPP
Thanite
Toxaphene
Trichlofon
mg/kg
90
55
100
90
90
125
900-5800
6000
14
21
50-60
65-75
3.6-13
. 43()..4()()()
1-5
100-180
28
95-104
1500-1800
132
750-1200
4000
5-55
1-2
3000
69
630
Methods of Insect Pest Management
[ 4 JI
·Dinitrophenols. These insecticides have a broad range of tox1c1ty
and have been developed as herbicides, fungicides, and insecticides.
They are considerably toxic to humans. These include 4,6-dintro-o-cresol
(DNOC) and dinoseb. DNOC (Fig. 1 1 ) is formulated as a 20-33% water
paste of the sodium salt and as a 40% wettable powder for application
as a dormant ovicide, herbicide, fungicide, and blosson-thinning agent.
Today, DNOC (Syntox®) is used mainly for killing all plants in an area.
Dinoseb is used as a dormant spray against insects and mites on fruit
trees.
N�
A
Fig.
o�
H
CH3CH1�H
B
CH3
1 1 . Dintrophenols. A. Dinitrocresol, B. Dinoseb.
4. Nomenclature of Insecticides
Insecticides have three names : trade, common and chemical name. The
trade name is the name listed on the label and is determined by the
company marketing the material. The common name is the name accepted
internationally by most of the industry to use instead of the chemical name.
The chemical name provides a chemical description. For example,
Furadan ® is a trade name of a carbamate insecticide whose common
name is carbofuran. The chemical name is 2,3-dihydro-2,2-dimethyl-7benzofuranyl methylcarbamate which is extremely cumbersome to use.
5. Toxicity of the Insecticides
The toxicity of an insecticide is established by exposing test animals
(insects and vertebrates) to a range of doses and determining the number
killed at each dose. By plotting the number killed against the range of
doses using log-probit paper, it is possible to extrapolate the dose that kills
50% of the test animals. When the exact amount of insecticide being
applied per body weight (mg of toxicant per mg body weight of the insect) ;
is known, the lethal dose which kills 50% of the population can be
determined (LD5o). If the insects have been dipped.. in different
concentrations or fed foliage dipped in different concentrations, then the
lethal concentration is established (e.g., LC5o). Once these values are
calculated for different insects and for vertebrates, it is possible to
compare these values to other insecticides.
·
1
Methods of Insect Pest Management
412 1
6. Chemicals Used with Insecticides
To obtain desired results, sometime two or more chemicals are added with
the insecticides. Some of these chemicals are called synergists if they
increase the toxicity of the insecticide directly. Others, in general called
adjuvants, are added to improve adhesion, mixing, surface tension, or
smell, or serve to carry the insecticide.
1. Synergists. Some chemicals have the property of greatly
increasing toxicity of certain insecticides. When the increased toxicity is
markedly greater than the sum of the two used separately, it is called a
synergistic action and such chemicals are known as synergists. Most of
these synergists have been used with pyrethrum or pyrethroid
insecticides. They act by preventing the hydroxylation of these
insecticides by the mixed function oxidase system. They are often added
to insecticides in a ratio of 8 : 1 to 10: 1 (synergist : insecticide). The
synergists are also added to increase the effectiveness of chlorinated
hydrocarbons,
organophosphates,
carbamates
and
other
kinds
of
insecticides. Indeed, most aerosols with pyrethrum and some of the
pyrethroids for household use today are enhanced with a synergist.
Some of the most common synergists are piperonyl butaxide, sulphoxide,
MGK 264®, sesamin and sesamolin.
A
B
Fig. 12. Some synergists. A. Piperonyl butaxide, B. MGK 264. .
Piperonyl butoxide SE (PBO). I t is an emulsifiable synergist for use
·
in combination with insecticides especially synthetic pyrethroids to
overcome resistance that pests develop with constant use of insecticides.
Insects are amazingly adaptable and possess an enzyme system called
the mixed-function oxidases (MFO's) that give them the ability to
detoxify and become resistance to many insecticides, especially synthetic
pyrethroids. Continual application of pesticides start the build-up of
resistance and thus the efficiency of the spray diminishes. PBO inhibits
the action of MFO ' s and restores the killing power of the insecticide,
which results in less expensive and/or more effective pest control. Actual
results show that PBO either decreased cost of pest control since
smaller does are needed, or provided better control of insects where
cases of maximum dosages of pesticides without PBO had failed.
Methods of Insect Pest Management
[ 413
Therefore, PBO will prolong the usefulness of insecticides by
overcoming MFO resistance, improving control means, thus providing
cost savings as well as environmental benefits.
2. Solvents. Most of the organic insecticides are insoluble in water.
Such insecticides must be dissolved in some solvents before they can be
used as spray concentrates or aerosols, The selection of the solvent
depends on its solvency, phytotoxicity, animal toxicity, combustibility,
odour, and cost. Some examples of solvents used to dissolve insecticidal
compounds include carbon tetrachloride, kerosene and xylene.
3. Diluents. Diluents are the chemicals used to dilute the
concentrated insecticides. Such chemicals also serve as carriers and are
necessary to obtain proper coverage of treated surfaces. The liquid
diluents of insecticides are usually water or refined oils. When water is
used, it is necessary to add wetting and dispersing agents for proper
suspension of the insecticide. When oil solutions are used with water
emulsifying agents are added.
Solid diluents are used to formulate
insecticide dusts or granules. Common solid diluents include organic
flours (e.g., soybean flour) and minerals (bentonite clay, talc, volcanic
ash, etc.).
4. Surfactants. The surfactant is a chemical that helps or enhances
the surface-modifying properties of a pesticide formulation. Inclusion of
surfactant in the formulation of insecticides improves its emulsifying,
wetting, and spreading properties. Usually, liquid insecticides, oils, and
insecticides in water-insoluble solvents are formulated and applied as
water emulsions. Emulsions are suspensions of microscopic droplets of
one liquid in another. Effective suspensions are prepared by adding
detergent-like materials to the insecticide formulation. An emulsifying
agent is generally a long-chained hydrocarbon in which one end of the
chain is lipophilic and the other end is hydrophilic. In most instances,
when the insecticide and emulsifier are added to water, the oil carrier
disperses immediately and uniformly, giving a milky appearance.
5. Stickers. The stickers are sometimes added in the insecticide
formulation to retain their active ingredients on a surface longer than
otherwise possible. It includes casein, gelatin, and vegetable oils. Latex
is added in carbaryl to extend the residues.
6. Deodorants. Deodorants are materials which are added to
insecticides to mask their unpleasant smells. Many insecticides like the
thiocyanate, pyrethrum, and several organophosphates have strong odours
that may be offensive. This is particularly unacceptable when formulated for
domestic use. Therefore, several materials such as cedar wood oil, pine oil,
or flower scents are added to insecticide concentrates to disguise odour.
414 l
Methods of Insect Pest Management
7. Formulation of Insecticides
The residual activity of an insecticide and its utility as a management tool
can be altered according to how it is formulated. An insecticide, as it
appears on the market, is composed of a toxicant or active ingredient
(poison) and one or more inert materials (nonactive, nonpoisonous). The
mixture of active and inert ingredients for killing insects is called an
insecticide formulation. These inert materials may function as solvents,
diluents, surfactants, or stickers. The insecticides are formulated so that it
is possible to obtain uniform coverage. There are many kinds of
formulations available in the market, including liquids and solids, and a few
are preparlo!d for release of the active ingredient -over a period of time.
Only the most widely used formulations are mentioned here.
1. Liquid formulations. Liquid formulations usually are sold in
small cans and bottles, medium-sized containers, or large drums. If
mixing is required, these formulations are the most convenient to use.
(a) Emulsifrable concentrates (EC). It has been estimated that more
than 75% of all pesticides are applied as sprays. Since most of the
insecticides are soluble in organic solvents which are not miscible in
water, an emulsifying agent is added. Such a formulation is called as
emulsifiable concentrates. With this kind of formulation, the emulsifier
breaks up the insecticide into microscopic droplets, producing a milky
liquid. By diluting in water such formulation is ready for use as spray
on the crop. ECs often contain 200 to 2,000 grams of active ingredient
(a.i.) per Iiter.
(b) Solutions (S). Solutions are liquid concentrates used directly or
require diluting before spray application. When they are to be used
directly there concentration is very low, usually containing approximately
200 g a.i./liter. In such formulation, the solvent is highly refined oils.
They are applied by using a convenient atomising sprayer. Such
solutions are used mainly as household sprays, mothproofers, livestock
sprays, and space sprays in barns. In contrast, the high concentrates
usually contain 2000 g a.i./liter. If dilution is required, oil is usually the
diluent. A special kind of high-concentrate solution is the ultra low
volume concentrate (ULV) which are applied without dilution with
specialised sprayer to produce an extremely fine spray. It requires 20
time less insecticide than conventional high-volume sprays.
(c) Flowables (F or L). Some insecticides in their raw form cannot
be easily dissolved in an organic solvent or in water. These insecticides
are finely ground in oil (oil-based flowable), water (water-based
flowable), or with no lubricant (dry flowable). The particles are ground
to about 4 microns in either water or oil, then a suspending agent is
added. In the case of the water-based material, an anti-freeze agent is
[ 4 15
Methods of Insect Pest Management
added. The final product has the consistency and drying properties of a
latex paint. Insecticides formulated in this manner tend to provide better
residual
activity
constantly
than
agitated
other
to
spray
prevent
formulations.
the
insecticide
Flowables
from
must
be
out
of
coming
suspension and settling to the bottom of the spray· tank.
(d) Aerosols (A). Most of the household insecticides· are formulated
as aerosol. In this formulation, the insecticides are dissolved in v9latile
petroleum
solvents.
The
solution
then
is
pressurised
in
a
can
by
a
propellant gas like carbon dioxide or fluorocarbons. When sprayed, the
solution is atomised and quickly evaporates, leaving microsized droplets
(0. 1 -50 µm)
suspended
total-release
containers.
in
air.
Aerosols
Although
easy
are
to
sold
use,
in
push-button
aerosols
have
a
or
low
concentration of active ingredient and, therefore, are expensive.
(e) Liquified gas (LC or F).
pressure
tum
into
a
liquid.
Several fumigants
These
are
stored
in
when placed under
metal
bottles
under
pressure and are released into structures like grain bins or into the soil
by
injection.
Other
insecticidal
compounds
remain
liquid
at
normal
atmospheric pressure but tum into a gas after they are applied. They
are not stored under pressure and vaporise after they are placed in soil
or in enclosures.
2. Dry formulations. Dry formulations are usually sold in paper cans
or bags, which may be lined with plastic. Some are used directly from
the container, but others require a diluent.
(a) Dusts (D).
They
are
prepared
powder which
is
Dusts
by
are
the
grinding
simple
the
formulations
insecti1.tdal
diluted with organic
of
compound
flour or finely
insecticides.
into
a fine
ground mineral.
The concentration of dusts are usually 1 - 10 g a.i./ 100 g. Dusts are often
easy to use in small areas because they can be shaken directly on a
surface from the container or blown into cracks and crevices. with an
applicator.
However,
dusts
are
the
least
effective,
least
economical
insecticide formulations for outdoor use. This is because of wind-caused
drift and poor rate of deposit on foliage and other surfaces. Also, the
dusts are the most toxic formulation to honey bees and parasitic wasps.
These
characteristics
·
make
dusts
rather
poor
pest
management
formulations for outdoor use.
(b) Granules (G).
liquid
insecticide
particles
may
be
Granular
to
coarse
formed
formulations
particles
from
of
corncobs,
are
a
prepared
porous
walnut
by
applying
material.
shells,
clay,
These
or other
materials. The insecticide is absorbed into the granule and/or coats the
outside. The amount of active ingredient in granular formulations ranges
from
5-20%.
formulation
is
Because
much
of
safer
the
size
of
(not inhaled)
the
to
granular
apply
than
particle,
dusts
ECs. These insecticide-laced granules are usually applied in
or
this
even
soil. When
(Z-57)
4 16 1
Methods of Insect Pest Management
the granules become wet, the insecticide is slowly released into the soil
and directly kill the soil-inhabiting insects. The insecticide may also be
absorbed by the plant' s roots and translocated to the foliar parts of the
plant where it is consumed by insect pests. When dropped over plants,
granules accumulate in leaf whorls which is useful against such insects
as corn borer larvae, which feed at the whorl before boring into the
plant.
(c) Wettable powders (WP). Wettable powders look like dusts while
in the container but are formulated to be mixed with water and sprayed
on surfaces. A surfactant added to the dust allows wetting during the
mixing processes. A particle suspension results when water is mixed.
WPs are much more concentrated than dusts, containing 15 to 95%
active ingredient. A frequent stirring is needed to keep the insecticide
in suspension. WPs usually cause less phytotoxicity than ECs, but they
are more abrasive to spray pumps and nozzles. WPs should never be
used without dilution.
(d) Poisonous baits (B). This type of formulation combines an
insect-edible or other attractive substance with the insecticide to improve
effectiveness of treatment. Dried and pulverised fruit and other materials
are often used to draw insects to a spot where they ingest or simply
crawl across the insecticide. Baits can be used in buildings or outside
for agricultural pests. Usually, active ingredient concentrations are very
low in baits, on the order of 5 per cent or less.
(e) Slow-release formulations (SR). To avoid environmental risks,
unstable insecticides have been evolved such as organophosphates and
pyrethroids. However, it has caused other problems such as short-term
effectiveness and the increased expense of several applications.
Therefore, the ways to extend the life of organophosphates and other
chemicals
become
necessary.
The
formulations
of
slow-release
insecticides achieved this goal. Shell No-Pest Strip® is such a
formulation in which the volatile organophosphate, dichlorvos, was
embedded into strips of polychlorovinyl resin. The resin slows the rate
of volatilisation of the insecticide, allowing it to kill most flying and
some crawling insects in the vicinity. In another slow-release formulation
the insecticide is incorporated in a permeable covering. This process
forms microscopic spheres or microcapsules. When it is applied it
release the insecticide at a reduced but effective rate. It extends the life
of an insecticide two to four times that of an emulsifiable concentrate.
A commonly used example of this formulation is Penneap M®, a
microel)capsulated form of methyl parathion.
The abbreviations of the insecticide formulations given against their
names are printed on the label of their containers.
(Z-57)
Methods of Insect Pest Management
[ 41 7
8. Other Chemicals Used in Insect Control
There are several other chemical compounds both natural and synthetics
have been found potential in regulating the pest population by several
ways. These substances include repellents, attractants, antimetabolites,
feeding deterrents, hormones, and insect growth regulators.
1. Synthetic repellents. Repellents are substances that are mildly
toxic or nontoxic to pests, but prevent damage by causing the pests to
make oriented movements away from the source. Since very early times,
smoke from wood fires or agricultural waste fire is being used to keep
away biting and annoying insects in villages and sub-urban areas. Most
of the earlier repellent substances were quite odorous and perhaps
somewhat repellent to humans as well as insects. However, many of the
more modem, synthetic repellents have little, if any, disagreeable odour.
The characteristics of an ideal insect repellent are: it should be
nontoxic, nonirritating, and nonallergenic to humans and domestic
animals; inoffensive in odour; harmless to fabrics; persistent; effective
against a broad spectrum of pest species; cheap and non-damaging to
plastics, painted surfaces, and the like. The application of repellents
afford individual protection from insects without the necessity for
expensive and time-consuming population eradication; they do not
damage or kill beneficial animals or plants; and the ones that are
available for use are nontoxic to humans. On the other hand, repellents
are at best a temporary measure (a few hours at most) and tend to
evaporate from or rub off skin or clothing due to perspiration and the
like; they commonly have an oily feel and may have a somewhat
disagreeable odour; they must be applied in comparatively large doses
(in the range of 20-40 mg/cm2 of skin; and they may damage certain
plastics or painted surfaces.
So far, repellents have been primarily used for the protection of
man and animals against attacks from blood-sucking or otherwise
annoying insects. There are three general groups of repellents: those
used against crawling insects, feeding of insects and egg-laying of
insects.
(a) Repellents used against crawling insects. It usually consists of a
repellent barrier interposed between an insect and whatever material
happens to be attractive to it. For example, creosote has been used as
a barrier against the migration of chinch bugs, Blissus leucopterus, and
trichlorobenzene and other repellent insecticidal chemicals to protect
buildings from termite invasion. Creosotes are derived from coal and
wood tar and have been used extensively for the protection of wood
against termites, powderpost beetles, and rot organisms.
(Z-57)
418 1
Methods of Insect Pest Management
(b) Repellents used against the feeding of insects. Several chemicals
have been found that are reasonably effective in repelling insects from
feeding. Washes containing bordeaux mixture, lime, and other materials
are used to repel leafhoppers and some chewing insects, and inert dusts
such as ashes have been useful on cucurbits to repel pumpkin beetles.
An ideal repellent for plant protection would be one that would
somehow block the natural attractants to which pest species respond.
Diethyltoluamide, considered to be one of the best general repellents yet
discovered, dimethyl phthalate, ethyl hexanediol, dimethyl carbate, and
powdered sulphur are examples of repellent chemicals that have been
applied to human skin to get rid of mosquitoes or clothings to repel
cloth moths.
(c) Repellents used against egg-laying of insects. The pine-tar oil and
diphenylamine are used to repel the screwworm flies from laying eggs
about wounds of animals.
There are several compounds both natural and synthetic those
provide repellency against certain insects. These are :
(i) Essential oils. Oil of citronella (extracted from Andropogon
nardus and contains geraniol, citronellol, citronella}, borueol and
terpenes; mosquito repellent), Eucalyptus, lemon leaves, peppermint,
lavender, cedar wood oil, etc. Persons concerned about exposure to deet
(see below) use essential oils. Generally the essential oils are considered
safe to use in low dosage but overall their effectiveness is limited to less
than a hour.
(ii) Deet (N,N-diethyl-m-toluamide). Deet is by far the most
commonly used insect repellent worldwide. This is because it is the
most effective repellent against mosquitoes, ticks and other biting
insects. Deet was selected by the USDA and the US Military as the
safest and most economical against mosquitoes. The LD50 to rat is 2000
mg/kg of body weight.
(iii) MGK-326 (Di-n-propyl isocinchomeronate). It is the most
effective insect repellent against flies, gnats, and similar annoying insects.
It is far more effective than deet against these insects and it only needs
to be present in small quantities. The LD50 to rat is 6230 mg/kg of
body weight.
(iv) Paradichlorobenzene and naphthalene. These two compounds are
most commonly used as mothballs to repel cloth moths.
(v) Mixuters of repellents. Since various repellent compounds exhibit
wide differences in their activity against various insects, the use of
mixtures are recommended. A mixture containing dimethyl phthalate,
2-ethyl- 1 ,3-hexanediol, and dimethyl carbate in the proportion of 4:3 : 1
incorporated into various cream and lotions are applied to skin to repel
mosquitoes
and flies.
A repellent containing
benzyl benzoate,
(Z-57)
Methods of Insect Pest Management
{ 41 9
n-butylacetanilide,
and
2-butyl-2-ethyl-1,3-propanediol in
equal
proportion are used to impregnate clothing against mosquitoes, fleas,
ticks and chiggers. Smokes, smudges, and burning of pyrethroids (e.g.,
allethrins) are useful repellent measures for outdoors. Chemicals that
have been used as repellents against pests of livestock include low
concentrations of pyrethrums, butoxypolypropylene glycol, and dibutyl
succinate. Bordeaux mixture is often considered as the first synthetic
chemical repellent for chewing insects and leaf hoppers. It is made at
different strengths for different purposes. It is prepared by mixing
copper sulphate, hydrated lime and water.
2. Attractants. Chemicals that elicit · oriented movements by insects
towards their source are called attractants. Many of these chemicals
attract insects by olfactory stimulation. Such odoriferous chemicals that
serve as messenger in the biology of insects are known as
semiochemicals.
Interspecific
semiochemicals,
also
called
as
allelochemics, communicate between the individuals of the different
species.
The allelochemics
are
subdivided
into
allomones
and
kairomones. The allomones favour the producer/emitter and are mostly
defensive chemicals, producing negative responses in insects. They
include repellents, oviposition and feeding deterrents, and toxicants.
Conversely, kairomones favour the receiver and are advantageous to an
insect, promoting host finding, oviposition, and feeding. They include
attractants, arrestants, excitants, and stimulants.
The intraspecific
semiochemicals,
are pheromones
that communicate
between the
individuals of the same species. The allelochemics are emanated from
various sources. The allelochemics that elicit a behavioural response by
the insect pest or its natural enemies can be used in a number of
different ways in insect control.
(a) Kairomones. Kairomones
are ovipositional
attractants or
lures and are present in the host material or produced by
microorganisms associated with it that directs the insect pests toward
suitable sites for feeding or ovipositing. Thus, food lures principally act
as olfact9ry stimulant. Methyl eugenol is strongly attractive to males of
the fruit fly, Bactrocera (=Dacus) dorsalis luring them from about 800
m downwind, and this compound has been effectively used in poison
baits and traps. Similarly, Anisyl acetone is strongly attractive to the
male
melon
fly
Bactrocera
cucurbitae,
and
siglure
(sec-butyle-6-methyl-3-cyclohexene- l carboxylate)
and
trimedlure
(t-butyl-4-chloro-2-methyl cyclohexane carboxylate) serve as food lure for
the Mediterranean fruit fly, Ceratitis capitata. A mixture of geraniol and
eugenol has been used to detect infestations of Japanese beetle, Popillia
japonica.
420 1
Metho'ds of Insect Pest Management
Kairomones emanated by insect pests attract their natural eneffiles
and play a role in natural enemy augmentation and are being tested
that attract natural enemies and/or stimulate them to become more
efficient. For example, kairomones such as tricosane, isolated from the
scales of corn earworm moths have been shown to stimulate searching
behaviour of its egg parasitoid Trichogramma. When these kairomones
were applied in the fields, the foraging behaviour of the parasitoid is
enhanced following increased rate of egg parasitisation. Presently, such
kairomones are used also to enhance and conserve the natural enemies
already present in the agroecosystems as an IPM approach.
(b) Sex pherormones. The sex pheromones are used by insects to
locate a mate and have been most widely used in pest management
programmes. Sex pheromones have been identified for a wide range of
insect pests. The chemical composition and release rate of pheromones,
and trap design and its placement within the field are important
parameters that determine the effectiveness of the traps. Presently, in
insect pest management, the sex pheromones are used in three different
ways: (i) in sampling and detection, (ii) to attract and kill, and (iii) to
disrupt mating.
(i) Use of pheromones in sampling and detection of pest insects. It is
one of the oldest practical applications in pest management. Presently
the sex pheromones are employed to monitor insect activities.
Pheromone traps are used frequently to gain information about pests for
making tactical decisions. The first insect caught can serve as the
beginning point for the accumulation of degree days, or catches over a
period of time are useful in predicting population peaks or egg hatching
times. Such predictions are useful in deciding if insecticides are
necessary and, if so, when they should be applied. Some of the most
extensive uses of pheromone traps for making pest management
decisions have occurred in apple orchards. Here, pheromone traps (5- 10
traps/ha) are placed to sample insect pests and pesticides are applied
according to trap data, natural enemies, and weather information. It
reduces up to 50% pesticide inputs. Pheromone traps are used regularly
to monitor codling moth (Cydia pomonella) in deciduous fruits, pink
bollworm (Pectinophora gossypiella) and bollworm (Helicoverpa armigera)
in cotton, black cutworm (Agrotis ipsilon) in corn; and California red
scale (Aonidiella aurantii) in citrus.
(ii) Use of pheromones in attract-and-kill programmes of pest insects.
In this method, theoretically, when sex pheromones are used, a large
proportions of one sex of pest insect are attracted and killed which
reduces their mating success, and thereby, their numbers decrease in the
next generation. The traditional approach to attract-and-kill has been to
use mass traps (100 traps/ha) coated with sticky material, but recent
Methods of Insect Pest Management
[ 421
advances have allowed slow-release formulations of small particles
(dispenser) that gradually release both a pheromone and an insecticide.
Mass trapping, or trap out, has been used experimentally in fruit tree,
field, and forest crops, as well as with stored product and household
pests.
(iii) Use of pheromones to disrupt mating of pest insects. In this
approach attempts are done to impregnate the air with sex pheromone.
Theoretically, if this is carried out, insects entering the area could not
locate mates em1ttmg natural pheromones because the synthetic
pheromone impregnate the whole environment. This would seemingly
cause a reduction of reproductive rates and achieve crop protection
without the use of insecticides. This basic idea was one of the earliest
suggestions for the use of pheromones in pest management. The first
preliminary field test demonstrating the potential of this approach was
conducted in 1967 with the use of synthetic pheromone looplure of
cabbage looper, Trichoplusia ni. Tiny dispensers, as mentioned earlier,
have been developed, and these hold greatest promise for future
development. In most of the cases, success was achieved when the
population levels of insects are low. Therefore, air permeation may find
a use only during certain parts of the growing season. For example, sex
pheromone of pink bollworm, gossyplure ( 10-propyl-trans-5,9-tridecadienl
acetate; trade name: Disrupt PBW) is recommended for use against the
pink bollworm early in the season when its populations are low and
cotton plants are small. For adequate management, early-season pink
bollworm suppression with pheromones is usually supplemented by later
applications of conventional insecticides. Synthetic pheromones of
Helicoverpa armigera, Earias vitella, E. insulana, P. gossypiella and
Spodoptera litura are commercially available in India.
Pheromones are highly specific having no biological effect on
non-target animals including natural enemies of the insect pest. Unlike
insecticides, there are no problem related with residues, health hazards
and development of resistance.
3. Antifeedants or feeding deterrents. Feeding deterrents or
antifeeding chemicals are those chemicals which inhibit feeding of pests
on a treated material, without necessarily killing or repelling them.
Antifeeding
compounds
such
as
chlorinated
triphenyl
methanes,
triarylphosphines, triphenyl phosphonium salts, have been used for
several years in the mothproofing of fabric. However, the use of these
compounds in the protection of crops is a fairly new idea. Triazines like
4-dimethyltriazeno- acetanilide is not toxic to plants upto 8 kg/ha. It
prevents feeding by caterpillars, beetles and cockroaches. The carbamate
arprocarb is a systemic antifeedant against boll weevil (Anthonomus
grandis) at rates of 40- 100 ppm. Certain plant products, like extracts of
422 1
Methods of Insect Pest Management
bark or seed kernels of neem that contain several alkaloids, terpenes
etc. inhibit feeding of treated foliage by a number of chewing insects.
The use of feeding deterrents is still in the experimental stage. They
offer considerable specificity because they would affect only the insects
that feed on treated plants and would spare the parasitoids and
predators of these pest species. They are also low in mammalian
toxicity.
4. Antimetabolites. Antimetabolites are the chemicals that resemble
essential nutrients of insects and interfere with its metabolism. For
example, amethopterin is a folic acid analogue which interferes with the
formation of vitamin folic acid in insects. The antimetabolites are low in
mammalian toxicity and are thus safe to use (e.g., for insect-proofing
fabrics). They may be effective against insects that have access only to
treated food; however, they have limited value against polyphagous
insects.
5. Insect hormones or their analogues. The juvenile hormone and
ecdysone (moulting hormone)
secreted
from corpora allata and
prothoracic glands of the insects, respectively regulate their development
and metamorphosis (see chapter 1 2). In recent years, advances in
chemical technology have allowed the discovery, identification, and
synthesis of several chemicals that mimic the function of juvenile
hormone or ecdysone. Such chemicals are known as insect growth
regulators (IGRs). These chemicals potentially provide new means of
insect control. The mode of action of these compounds is to cause
premature death from abnormal moulting or metamorphosis. IGRs are
also known as biorationals, or third-generation insecticides, to reflect
their environmental safety. Ecdysone, juvenile hormone, and their
analogues have been shown to disrupt the development of an insect if
applied at appropriate times and in appropriate doses. For example,
cyasterone, a substance related to ecdysone, when injected into a
diapausing Cynthia (moth) pupa in a very low dose (0.2 µg), stimulates
termination of diapause and formation of a normal moth. However, if a
high dose (10 µg) is injected, the developmental events are accelerated
and their sequence disrupted leading to death. The juvenile hormone
and its analogue (JHA) (e.g., methoprene) can be applied with lethal
effects, either by preventing the transformation of the pupa into an
adult or by inhibiting the development of eggs. Again, as with ecdysone,
several species of plants have been shown to produce compounds that
mimic juvenile hormone activity. Understanding the chemistry and
physiological effects of IGRs may ultimately provide the key to the
synthesis of insecticides with extreme specificity.
Due to the expense of production, inability to penetrate the cuticle,
and wide range of activity, it is unlikely that ecdysones will be used
Methods of Insect Pest Management
[ 4 23
commercially in the future. However, three juvenile hormone analogues
:r;nethoprene,
kinoprene, hydroprene and a new
class
of IGRs,
diflubenzuron are registered for actual use.
(a) Methoprene. Methoprene (Trade name : Altosid® is an IGR
with good activity against many flies, mosquiotoes, beetles, moths and
bugs. Altosid is a food additive for cattle, through which it passes in
dung and affect the developing maggots. Other uses of methoprene
include the following: beetles and moths in stored tobacco (Kabat® , flea
larvae indoors (Precor®), leaf-miners in vegetable (Minex® IGR).
(b) Kinoprene. Kinoprene (Trade name: Enstar® is a strong, highly
selective JHA effective against bugs. It affects all stages, including the
eggs of whiteflies and mealybugs. Since it is an unstable compound, it is
1,1sed only in greenhouse plants.
(c) Hydroprene. Hydroprene (Trade name: Gencor®) is related with
methoprene and used indoors against cockroaches. It makes the nymph
to develop into a sterile adult. Hydroprene offers a new solution for
insect pest species that have developed resistance to conventional
insecticides.
(d) Dijlubenzuron. Diflubenzuron (C14H 9ClF2Np 2, Trade name :
Dimilin®) is more stable compound than JHAs. It acts on larvae of
most insects by inhibiting chitin synthesis and thus affects the integrity
of the insect exoskeleton. Most of then larvae die from ruptures of the
new
malformed
cuticle
or
from
starvation.
Diflubenzuron
is
recommended to control gypsy moth (Lymantria dispar) in forests and
boll weevil (Anthonomus grandis) and stainer (Dysdercus spp.) in cotton.
In addition, it is also effective against insect pests of forests, woody
ornamentals, fruits, vegetables, mushrooms, cotton, soybean, and citrus.
It is also very useful against flies, midges, and mosquitoes.
(d) Azatin, Neemazad and Neemix. These formulations consist the
extract of neem seed. The active ingredient is azadirachtin. It is an IGR
working through contact or ingestion.
9. Benefits and Risks of Chemical Control
The insecticides seem to be indispensable in maintaining high levels of
health, nutrition and quality surroundings. In agriculture, these are regular
component as their application has played an important role in the
development of modem agriculture. Its use has not only enormously
increased the yield of agricultural products but also controlled or at least
reduced several vector-borne diseases of humans and livestock. At present
more than 400 active compounds having insecticide properties with
thousands of formulations and uses have already been registered with the
US Environmental Protection Agency (EPA). As a measure of insect
control, they are usually very effective and generally act within a short
Methods of Insect Pest Management
424 J
period of time. They are effective when applied against large pest
population, and are also readily available for the users whenever needed.
However, the world has seen the environmental risks posed by these
chemicals. Its application can be very hazardous, and direct contact with
a highly toxic insecticide can cause severe illness and even death.
Because of this, careful reading of the instructions on the labels on
insecticide containers and constant concerns with safety during their
application (avoidance of spillage on clothing or skin, or inhalation of
sprays or dusts; not smoking or eating when working with toxicants) are
so important. Other important considerations are the storage of
insecticides in well-labelled containers out of reach of children and the
proper disposal of empty insecticide containers. Major concern is also
the presence of residues in food products, plant and animal, that have
been treated with insecticides at some point in their production and the
natural hazards by disrupting the intricate balance of ecosystems.
Environmental pollution with insecticides has become a matter of great
concern. Highly residual insecticides can pass well beyond their intended
targets and may reduce populations of beneficial insects and wildlife.
DDT in particular has been banned in this regard. Another major
problem associated with the use of insecticides is the development of
insecticide resistance in strains of several pest species. There are more
than 500 species of insects that have developed resistance to
insecticides. In addition, unfortunately, natural enemies of insect pests
are more susceptible to insecticides than the insect pests and are easily
eliminated from the agroecosystem.
·
B iological Control
In spite of pouring 400 million tonnes of pesticides, and even with
continual development of new and presumably better synthetic insecticides
combined with their greatly expanded usage, it has been observed that the
chemicals are not controlling pests in general. Natural hazards,
development of insect pest resistance, pest resurgence, outbreak of the
secondary pests, reduction in species diversity, alteration of decomposition
of organic material and nutrient cycling and objectionable pesticide
residues clearly show a need of change in control tactics in order to reduce
our <;iependency on pesticides and to achieve control of pests in an
economically satisfactory manner.
As mentioned earlier, ecological changes are one of the major cause
of pest outbreaks (see chapter 1 7). Various agrotechnical practices, e.g,
monoculture, selection of high yielding plant cultivars, application of
agrochemicals such as insecticides and fertilisers, etc., created conditions
favourable to certain insect species and has thus induced many folds
increases m their population. Actually, these factors disrupt the
Methods of Insect Pest Management
[ 425
interaction between phytophage insects and their natural enemies which
are the essential ecological processes that contribute to the regulation of
insect population. Whenever, this interaction is disrupted, the population
of the phytophage insects increased tremendously and they attain pest
status because they become free from the constraints imposed by the
entomophages. The current revival of interest in biological control is
also driven by a change from pest control approaches that aim to
maximise productivity and to approaches that emphasise efficiency and
the long-term sustainability of agro-ecosystems. The biological control of
pests tends to be a long-lasting, and often can be implemented at little
direct cost to producers and consumers.
The biological control may be defined as ' 'the action of the
parasitoids, parasites (e.g., nematodes), predators and pathogens (viruses,
bacteria, fungi, protozoans) in maintaining another organism' s density at
a lower average than would occur in their absence", or "the utilisation
of natural enemies to reduce the damage caused by noxious organisms
to tolerable levels" . Besides the use of natural enemies, there are
several other methods of biological nature which have been applied with
success in pest control. These methods are based on host-plant (or
animal) resistance to pest insects; autocidal methods involving the
release of sterile males; and genetic control of pests.
There are several advantages in using biological control agents. If a
biological control agent is acclimated to the target area and pesticides
are judiciously used, these agents should become a permanent fixture.
Unlike pesticides, biological control agents are safe to use and do not
pose any threat to the environment. The development of biological
control agents is considerably less expensive than the development of an
insecticide.
Though, Erasmus Darwin first observed, as early as in 1 800, an
ichneumon fly killing eggs of cabbage butterfly caterpillar, the biological
control has been the focus of considerable research by pest control experts
only for more than 1 00 years. In 1 888 vedalia lady beetle, Rodolia
cardinalis was imported in California from Australia against cottony­
cushion scale, Icerya purchasi on citrus. After, the success of Rodolia
cordinalis, the export and import of several natural enemies from one part
of the world to another had taken place. The middle history of
development of biological control, thus begins with 1 888 and lasts in 1 962.
After the publication of Carson's Silent Spring in 1 962, a general awareness
was developed regarding the chemical control of insect pests that induces
not only the development of resistance in the pest insects but also peril the
environment affecting the human beings directly or indirectly. Thereafter,
the present era of biological control begins that brought about a new hope
for us as a new tactic of insect control.
426 1
Methods of Insect Pest Management
About 1000 species of parasitoids and predators have been
introduced against almost 200 insect pests. Attempts at biological control
have been made practically all over the world with varying success. The
highest proportion of complete success of all biological control success
among the major geographical region of the world is given in following
table.
Table 2. A comparison of the number of complete, intermediate and partial biological
control successes of introduced natural enemies in m ajor geographical regions.
Biological
control
success
1. North America, 2. Central & South America, 3. Mediterranean Basin & Europe,
4. Asia, 5. Australia and New Zealand, 6. Africa, 7. West Indies, 8. Pacific Islands
6
7
29
8
18
14
9
16
19
4
5
21
39
4
38
19
26
13
Intermediate
51
10
11
6
Partial
43
11
14
8
51
27
62
Total
I
132
2
5
3
Complete
40
8
46
57
46
149.___J
[ I] Organisms utilised in biological control
The organisms which are utilised in biological control include parasitoids,
predators, parasites, microbes (fungi, bacteria and viruses).
1. Parasitoids and Predators. The major parasitoids used in
biological control belong to the order Hymenoptera (e.g., ichneumonid,
braconid, chalcid, eulophid, trichogramraatid wasps) and Diptera (e.g.,
tachinid flies). More than two thirds of the cases of successful biological
control have involved the use of Hymenoptera. Among the predatory
insects that have been used in biological control are various Coleoptera
(e.g., ladybird beetles), Neuroptera (e.g., lacewings), Diptera (e.g., hover
flies and robber flies), and Hymenoptera (e.g., certain ants).
(a) Approaches of biological control. There are four approaches of
biological control utilising parasitoids and predators: conservation,
augmentation, inundation, and introduction. The conservation of natural
enemies consists of protection of host refugia having dormant stages of
the natural enemies either from pesticides or burning etc. Providing
alternative food source to their hosts may also conserve the natural
enemies. In augmentation mass-reared bioagents are released to increase
their existing populations in the field. The foundation is the process-- by
which mass-reared bioagents are released to overkill the pest population
within the first generation of release. It is more feasible in greenhouses.
In introduction, exotic bioagents are mass-reared and released with
hopes of controlling pest species. It has been by far the most successful
of the four methods. The introduction of exotic natural enemies is more
appropriate in two situations: (i) when there are "unoccupied :iiches in
the life system of the pest, which could be filled by an introduced
species," and (ii) when "a certain niche is occupied by an organism that
Methods of Insect Pest Management
{ 427
is inherently inefficient as a regulator and that might be displaced by a
more efficient exotic regulator' ' . Both situations exist particularly when a
given pest species has been accidentally introduced from another area.
Development of a
(b) Development of a biological control programme.
biological
control
programme
takes
p lace
in
the
following
way
:
(i) definition of taxonomic status of the target pest, (ii) collection and
evaluation
of
literature
on
pest
and
natural
enemies
if
app�opriate
natural enemy is available, development of application programme; if not
step
(iii),
enemies:
(iii)
selection
Potential
of
exploration
biological
control
area
agents
and
are
collection
collected
of
from
natural
an
area
climatically and geographically and photoperiodically similar to the area
where they are to be released,
(iv)
study of biology and evaluation of
natural enemies: Efforts are made to study the biology of the biological
control agent in its home environment to identify any special needs that
may prevent it from being a successful agent,
natural
of
enemy(ies)
for
natural
enemies.
potentially
effective
introduction,
Evaluation
natural
has
(v) selection of the best
importation,
and
three
objectives:
enemy,
to
main
measure
further
the
evaluation
to
identify
modalities
of
effectiveness, and to predict field performance, (vi) development of mass
rearing,
introduction,
and
guidance
programme ,
(vii)
introduction
of
natural enemy in the field (viii) final evaluation of the effectiveness of
natural enemy in the field including cost : benefit analysis; if results are
insufficient, go back to step (iii).
(c) Criteria of parasitoidslpredators for use in biological control.
It is
difficult to find out a natural enemy with all the desirable attributes that
make it qualify as the most suitable one for the utilisation in biological
control of insect pests. However, following are some desirable attributes
of an ideal natural enemy:
(i) Synchrony with pest population.
Populations
of
a
predator
or
p arasitoid must be in synchrony with pest populations.
(ii) High host/prey searching efficiency.
natural
enemy
decides
its
effectiveness
The searching behavior of a
in
regulating
the
population
dynamics of a pest insect. The parasitoids/predators utilise several cues
(e.g., physical, mechanical, chemical etc.) associated with hosts/preys and
their
habitats.
effi ciency
(pigeonpea
and
parasitoid
able to
Binodoxys indicus
locate the host habitat
has
and
high
the
searching
host
proper
Aphis gossypii) even at lower host densities. Similarly,
beetle, Coccinella septempunctata, is able to feed at much
aphid,
the ladybird
lower
The
is
aphid
densities
in
various
habitats
and
keeps
the
aphid
populations below economically damaging levels.
(iii) Density response.
The
searching
and
feeding
natural enemies may change as pest population densities
behaviour
vary,
of
so each
428 1
Methods of Insect Pest Management
individual agent kills more pests as pest density increases, a
density-dependent response. This behaviour is also known as functional
response. The functional response is of three types: Type l represents a
rather specialised situation and is not common in insects. In this type,
there is no change in the host/prey utilisation. The type 2 response is
probably the most common type in insects involving four essential
components: rate of successful search, time for exposure to predator
and prey, handling time and hunger. In this response, the rate of
host/prey utilisation (parasitisation/ consumption) initially increases with
increase of host/prey density and later on it saturates, i.e., no change in
host/prey utilisation is observed beyond a certain host/prey density. The
type 3 response is a sigmoid response, and in addition to the above
mentioned components of the type 2 response, there is one more
component that describes it, learning of host/prey utilisation. By
understanding how a predator or parasitoid responds to changes in host
density and having population estimates of natural enemies, the pest
management practitioner can be m a better position to make
management decisions.
(iv) Survival during
adverse
conditions.
The
ability
of
overwinter/oversummer is another factor that greatly influences the success
of a natural enemy in regulating a pest population. If survivorship is low, an
agent will generally be ineffective during its first generation. The number of
generations predators and parasites go through can also affect their success
as biological control agents. A multivoltine (more than one generation per
year) agent is more likely to be in synchrony with its host and have a
greater potential for increasing its numbers within a season than a
univoltine agent (one generation per year). These information are made
available by constructing the age-specific life-tables of the natural enemies.
(v) Host/prey specificity. A specialist natural enemy (high host/prey
specific, i.e., mono- or oligophagous) tends to impose higher levels of
mortality than a generalist one (polyphagous) because a specialist is
dependent on a single host as a food source and/or oviposition site.
Because of this dependency, a specialist is more likely to reduce its host to
population levels below that which can support its own population. For this
reason, the population density of a specialist fluctuates more than that of a
generalist. The generalist is not as likely to impose high levels of mortality
on a pest species; however, a generalist is more likely to remain in high
numbers in the pest habitat by feeding on or parasitising other hosts.
(vi) Ability of host discrimination. Particularly, the parasitoids must be
able to discriminate between a parasitised and healthy hosts. This ability
prevents the wastage of biotic potential of the parasitoids, as the eggs laid
Methods of Insect Pest Management
[ 429
in already parasitised hosts (a phenomenon of superparasitism/multiple
parasitism) do not develop. In superparasitism, an individual parasitoid
deposits more eggs into/onto the hosts that can develop in the that host
while in multiple parasitism, more than one species of parasitoids deposit
eggs into/onto the host. Superparasitisation of a host by a parasitoid does
not always means that the parasitoid is unable to discriminate the hosts.
The parasitoids may superparasitise a host if the host density is low and the
parasitoid density is high and the movement of the parasitoids is limited.
(vii) Potential rate of increase. It is one of the important parameter
of a natural enemy. 'The bioagents having shorter development period
and high fecundity undergo several generations per year and can
overtake its host/prey very quickly when their population begins to
increase. It usually happens following severe winters and other adverse
weather conditions.
(viii) Mass culturability. Mass rearing of the parasitoids/predators in
the laboratory on alternative host complexes (insect as well as plant
host) is a challenge for biological control workers because it is a highly
technical job beset with numerous problems. The techniques for mass
rearing have been standardised for merely 1 5-20 species of the
parasitoids in the world. It is thus evident that probing studies and
more concerted efforts are needed in this direction. U nless natural
enemies are made as readily available as chemical pesticides, biological
control by augmentations is likely to be treated as a subject of mere
academic ihterest with no practical role whatsoever.
(d) Enhancement of biological activities ofparasitoidslpredators. Several
factors influence the biotic potential of the parasitoids at all trophic
levels. It includes the provision of supplementary resources such as
alternative hosts (both food plant resources as well as insect hosts),
adult
food
sources,
agricultural
practices,
climatic
variations,
infochemicals etc.
(i) Manipulation of habitat. Agroecosystems are amongst the most
difficult of environments in which to induce the efficient operation of
biological control agents. This is because they usually lack adequate
resources for the effective performance of the natural enemies and many
of the cultural practices used in annual cropping are damaging to
natural enemies. The diversity within the agroecosystem may be
increased
by
introducing
multiple
cropping,
intercropping,
strip
harvesting, and selective retention of weeds within the crop or
conservation of wild plants at field margins. The plants serve as a
reservoir of the alternative host species, and flowering plants are
important sources for food as the adult parasitoids feed on pollen,
nectar and other sugary plant secretions. Therefore, intercropping of
such plants or tropical application of honeydews not only attract the
Methods of Insect Pest Management
430 J
parasitoids
but
also
increase
their
retention
time
which
is
directly
related with the rate of parasitism. In cases where the natural enemies
enter
hibernation
preserve
them
in
by
Tetrastichus pyrillae
sugarcane
leaf
the
same
manipulation
and
crop
area,
care
of cultural
Epiricania melanoleuca,
Pyrilla perpusilla may
hopper,
should
practices.
As
major
be
be
taken
in
case
parasitoids
preserved
by
to
of
of
not
destroying the leaves after harvesting of the crop where the parasitoids
overwinter.
Genetic manipulation will
(ii) Genetic improvements of the parasitoids.
remain a controversial tactic in biological control until we can quantify the
likelihood
of
achieving
successful
laboratory
selection
responses
and
document the fitness and efficacy of the selected parasitoids under field
conditions.
Strain
C alifornia,
was
may
be
azinphos-methyl,
also
of
a
Trioxys pallidus,
developed
in
respectively.
enhanced
USA
walnut
which
is
aphid
resistant
to
parasitoid
guthion
in
and
The biological potential of the parasitoids
either
by
selective
hybridisation
mutagenesis, recombinant DNA technology etc.
or
through
(iii) Manipulation of behaviour of adult parasitoids by semiochemicals.
The potential of kairomones has been established in the manipulation of
the
behaviour
of
parasitoids
for
pest
management.
increased the rate of parasitism by three ways:
(i)
The
kairomones
by stimulating the
female parasitoids (ii) by retaining the female parasitoids on the treated
host patch, and (iii) by improving the egg distribution among the hosts.
By application of kairomones in the fields infested with aphid pests at
low
density
level,
the female
parasitoids
can
be
retained
for
longer
period on the treated plants. The retention and activation increases the
chance
for
mortality.
host
Also,
contact
and
results
the parasitoid
can
be
in
an
increased
attracted
towards
site by applying the sex pheromones, e.g., female
the
sex
pheromones
of the
aphid host
and
fields by putting traps containing such lures.
(iv) Artificial food supplement.
predators
honeydew
feed
(e.g.,
on
pollen,
nectar
The
and
adult
excreted by aphids) in nature.
the
Praon volucre
may
other
extent
be
of
infestation
responds
attracted
parasitoids
plant
host
in
and
secretions,
the
few
and
It has been found that
artificial spray of honeydew or sugar solution induces early appearance
of parasitoids
other
crops.
and
predators
of aphids
(e) Feasibility of biological control
factors
determine
mentioned
below:
the
effectiveness
(i) Tolerable limit of injury.
If
of
and
bollworms
Several physical
biological
of cotton
and
control
and
biological
agents
as
little or no injury is tolerable for a
crop then the adoption of biological control is unsuitable. The damage
by the pest on the marketable parts of the crop, such as fruits or fresh
Methods of Insect Pest Management
[ 431
vegetables, is an important factor deserving consideration. If such
damage is caused during the vital stage of crop growth, viz., the
reproductive stage, then biological control is not feasible. However, the
biological control is feasible in certain agroecosystmes such as cotton,
sugarcane, wheat, citrus, oil-crops etc.
(ii) Cost-benefit ratio. The measures that are expensive, e.g., the use
of insecticides in certain situations such as in cotton ecosystem, may be
prohibitive. Once the biological control agents of pests of a given crop
ecosystem are established, they maintain the balance of pest population
in the nature to the desirable level (below the economic threshold level)
reducing the cost of their re-employment.
(iii) Crop duration. A successful inoculative biological , control of
insect pests tends to be a natural phenomenon in due course of time,
the perennial crops appear to be more amenable to biological control.
The perennial crops provide more stable environment as they are less
disturbed by cultural practices that are very common in short duration
crops. Such practices are harmful to the well being of the natural
enemies. However, biological control may be successfully achieved in
annual or short duration crops by repeated release of the natural
enemies (the augmentative biological control).
(iv) Origin of the pest. Selection of biological control agents sometimes
depends on the origin of the pest, whether it is exotic or indigenous. For
exotic pests, the bioagents should be selected from their homelands and for
indigenous pests the bioagents can be searched in the same locality as such
bioagents are more physiologically adaptive. However, exploitation of both
indigenous and exotic bioagents can play beneficial roles on both types of
the pests.
(v) Pest complex and desired level of control. When the crop is
attacked by a single pest species and certain amount of damage is
tolerable, biological control is more feasible than other methods.
However, such a situation seldom exists as multiple pests usually
damage crops. In such a situation, the introduction and colonisation of
the bioagents is far from being effective against pest complex. The
potential pest in such a situation may build up to a alarming proportion
and in this condition, particularly if the damage tolerance is minimal,
biological control is not feasible. However, in such a situation
introduction of multiple bioagents may provide better results.
(vi) Availability of selective insecticides. The entomologists have
always opposed indiscriminate use of synthetic insecticides-- in the field.
Only those insecticides should be applied that cause least harm to the
natural enemies of other insects simultaneously present on the crop as it
reduces the chance of resurgence of the pests. However, unfortunately,
there are very few selective insecticides in the market. The time of
(Z-57)
432 1
Methods of Insect Pest Management
insecticide application should also be decided after careful study of the
biology of natural enemy complex of the crop.
(vii) Availability of insecticide resistant bioagents. There are situations
when use of insecticides become inevitable and in these situations the
insecticide resistant strains of natural enemies appears to be a fair
proposition. Usually adult predators such as ladybird beetles are able to
resist the insecticides, but the parasitoids are more susceptible to the
insecticides than the pests. Genetic modification of existing population
of natural enemies is a difficult job and laboratory selections have been
successful only in few cases. Certain strains of an aphid parasitoid,
Trioxys pallidus have been developed in USA which are resistant against
certain insecticides frequently used to kill aphids.
2. Parasitic nematodes. Nematode parasites of insects have been
known since the 1 7th century, but it was only in the 1 930s, that serious
consideration was given to using a nematode to control an insect. In
1 929, a nematode Neoaplectana ( = Steinernema) glaseri was 0bserved
infecting grubs of the Japanese beetle, Popillia japonica.
(a) Classification and biology. The species of rhabdtid (Rhabditida :
Nematoda)
Steinernema
feltiae,
S.
scapterisci,
Neosteinernema
longicurvicauda (Steinemematidae), and Heterorhabditis bacteriophora
(Heterorhabditidae) are most important species of entomopathogenic
nematodes. All members of the Order Rhabditida are bacteriophagous,
and many of them have phoretic assoc1at10ns with insects. The
bacterium
carried
by
Steinemematidae
is
usually
Xenorhabdus
nematophilus, and that carried by Heterorhabditidae is a species of
Photorhabdus.
(b) Pathogenicity and life cycle.
The infective juvenile enters the
insect host through the mouth, anus, spiracles, or by direct penetration
through the cuticle. If the mode of entry is by mouth or anus, the
nematode penetrates the gut wall to reach the haemocoel, and if by
spiracles, it penetrates the tracheal wall. When the infective juvenile
reaches the haemocoel of a host, it releases the bacteria, which multiply
rapidly in the haemolymph. Usually the insect dies within 24-72 hours.
Even though the bacterium is primarily responsible for the mortality of
most insect hosts, the nematode also produces a toxin that is lethal to
the insect. The infective juvenile becomes a feeding third-stage juvenile,
feeds on the bacteria and their metabolic byproducts, and moults to the
fourth stage and then to males and females of the first generation. After
mating, the females lay eggs that hatch as first-stage juveniles that moult
successively to second-, third-, and fourth-stage juveniles and then to
males and females of the second generation. The adults mate and the
eggs produced by these second-generation females hatch as first-stage
juveniles that moult to the second stage. The late second-stage juveniles
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Methods of Insect Pest Management
[ -433
cease feeding, incorporate a pellet of bacteria in the bacterial chamber,
and moult to the third stage (infective juvenile), retaining the cuticle of
the second stage as a sheath (this stage is called 'dauer'), and leave the
cadaver in search of new hosts. In some hosts, the second generation is
omitted and the eggs that are laid by first-generation adult females
develop into infective juveniles. The cycle from entry of infective
juveniles into a host from emergence of infective juveniles from a host
is temperature-dependent and varies somewhat for different species and
strains. However, it takes about 7- 10 days at 25° C in Galleria
mellonella. Differences for the Heterorhabditidae are that all juveniles of
the first generation become hermaphrodites. In the second generation,
males, females, and hermaphrodites develop.
(c) Dispersal of juveniles. The juveniles of steinemematids and
heterorhabditids disperse vertically and horizontally, both actively and
passively. Passively, they may be dispersed by rain, wind, soil, humans, or
insects. Active dispersal may be measured in centimeters, while passive
dispersal by insects may be measured in kilometers.
(d) Survival of Juveniles. The infective juveniles do not feed but can
live for weeks on stored reserves as active juveniles, and for months by
entering a near-anhydrobiotic state. This is almost certainly the most
important survival strategy for the nematode. The length of time that
juveniles survive in the soil in the absence of a host depends upon such
factors as temperature, humidity, natural enemies, and soil type.
Generally, survival is measured in weeks to months, and is better in a
sandy soil or sandy-loam soil at low moisture and with temperatures
from about 1 5-25° C than in clay soils and lower or higher
temperatures. The Heterorhabditidae do not survive as well as do
Steinemematidae.
(e) Natural enemies. Populations of entomopathogenic nematodes m
the soil are reduced by bacteria, fungi, mites, predatory nematodes,
tardigrades, and other soil organisms. Survival is better in sterilised soil
than in nonsterilised soil. Mites appear to be especially voracious
nematode-feeders.
(j) Insects controlled. Some of the insects controlled are armyworms,
carpenter worms, cat fleas, crown borers, cutworms, filth flies, flea
beetles, German cockroaches, leaf miners, mole crickets, phorid flies,
plume moths, root weevils, sclarid flies, stem borers, webworms, and
white grubs.
(g) Advantages of entomopathogenic nematodes. Entomopathogenic
nematodes have certain advantages over chemicals as control agents.
Nematodes are non-polluting and thus environmentally safe and
acceptable, although some countries do not allow the release of
non-indigenous
species.
Infective juveniles can be applied with
(Z-57)
434 l
Methods of Insect Pest Management
conventional equipment, and they are compatible with most pesticides.
They find their hosts either actively or passively, and in cryptic habitats
and sometimes in soil, they have proven superior to chemicals in
controlling the target insect. The nematodes usually reproduce in the
insect host and thus provide new infective juveniles to search for
additional host insects. The effective host range of a given species or
strain is usually rather narrow, thus they do not cause indiscriminate
mortality.
(h) Rearing. Steinernematid and heterorhabditid nematodes can be
reared in vivo in insect hosts or they can be mass produced in vitro on
solid medium or in liquid medium. For solid medium culture, a
substrate such as beef or pork kidney or liver, or chicken waste may be
used. The substrate usually is made into a paste that is coated onto a
porous substrate such as sponge. The medium is sterilised, inoculated
with the bacterium, and nematodes are added 24 hours later. Infective
juveniles are harvested after about
15
days. This method is
labour-intensive and is particularly well suited for situations where
labour is plentiful, and for the so-called cottage industry. Production in
liquid medium can be done in small containers or in fermentation tanks.
(i) Commercial products. Most of the nematode-based products
currently available are formulations of various strains of Steinemema
carpocapsae such as ORTHO BioSafe, BioVector, and Exhibit in the
USA; Sanoplant in Switzerland; BodenNiitzlinge in Germany; and Helix
in Canada. O ther species of Steinemema commercially available are S.
feltiae as Magnet in the USA, and as Nemasys and Stealth in the UK,
S. riobravis as Vector MC and S. scapterisci as Proactant Ss in the
USA. Heterorhabditis bacteriophora is available as Otinem in the USA
and H. megidis as Nemasys in the UK.
(j) Future role. The future of nematode-based products for insect
control is excellent. The technology used currently for producing,
formulating, packaging, storing, and shipping nematode products was
developed during the past 15 years. Future improvements may well
make today's technology obsolete. More efficient methods of production,
formulation, etc. will lower the cost of nematode products and make
them more competitive economically. Even though a total of 22 species
of the two genera of entomopathogenic nematodes have been described,
only six have been commercialised: S. carpocapsae, S. feltiae, S. riobravis,
S. scapteriscl, H. bacteriophora, and H. megidis. These species, and
perhaps some currently described, will add to the arsenal of nematode
weapons aimed at pest insects.
3. Microbial agents (baculoviruses, bacteria, fungi and protozoans).
Although humans have long been aware of the natural control of insect
populations by microbes, the first record of the idea of using them for
(Z-57)
Methods of Insect Pest Management
[ 4 35
insect control was in the eighteenth century. One of the major benefits
of these agents is that they are generally very host specific. This means
that these agents can be used in a pest management programme where
parasitoids, parasites and predators are being used without killing these
natural enemies. Most commercially available agents have been isolated
from insects collected in the field. However, biotechnology has provided
new laboratory engineered organisms that in most cases are an
improvement over field strains. The microorganisms that infect insects,
include viruses, bacteria, fungi, and protozoans which can reach
epidemic levels under natural conditions, causing high mortality to insect
pest populations. The use of microbial agents largely depends on
commercial mass production of the pathogens and its formulation in
such a way that it can be applied with conventional spray equipment
and can reduce the pest population.
Environmental conditions greatly influence the effectiveness of
microbial agents. Bacteria and fungi generally lose their virulence below
1 8° C, and many viruses do not replicate rapidly unless temperatures are
between 2 1 ° C and 29° C. Many viruses and bacteria are quickly killed
when exposed to sunlight. Composition of the soil can be critical;
for example, fungi seem to survive best in soils high in organic matter.
Soil pH or the pH of the foliage is important. Acid conditions are
unfavorable for Bacillus papillae spores (milky disease of the Japanese
beetle), while alkaline conditions can destroy the polyhedral structure of
polyhedrosis viruses.
Microorganisms attack the insect hosts in several ways. All
organisms enter the insect hosts by being ingested or through damage to
the insect' s integument. Fungi often enter the insect's body through the
tracheae. Baculoviruses, rickettsiae, and protozoans can be passed from
adult females to their eggs (transovarial transmission). When an
organism enters through ingestion, it is best to apply the organism at a
time of day when the insect is most actively feeding.
(a) Baculoviruses. The family Baculoviridae includes the nuclear
polyhedrosis viruses (NPV) and granulosis viruses (GV). These are
double-stranded DNA viruses (dsDNA) with rod-shaped nucleocapsids.
The infectious virus particles or virions are occluded in protein bodies
called polyhedra (NPV) or granules (GV). NPV polyhedra are larger
than the virions (usually 1 - 1 5 µm) and may contain many virions.
Infection with baculoviruses occurs when a susceptible host eats the
polyhedra or granules, which are dissolved in the basic digestive gut
juices. The virions are- -released when the protein matrices dissolve. The
virions enter the nuclei of midgut cells and eventually infect many of
the tissues and organs in the insect, primarily the fat body, epidermis,
and blood cells. Infection with baculovirus was historically called ' 'wilting
436 1
Methods of Insect Pest Management
disease" because the tissues of the host liquefy and infection of the
epidermis causes the host to appear to melt, releasing virus particles
into the environment. Baculoviruses are considered to be the most
beneficial of the insect viruses to man, because of their utility in insect
control, their specificity to the arthropods, and their more recent use in
fundamental biological studies using molecular techniques. Nevertheless,
they also cause diseases in beneficial insects and, therefore, use in the
environment as biological control agents requires an understanding of
host range and the mechanisms that control host specificity.
(i) Polyhedrosis viruses. NPVs are largely restricted to insects and
most species are relatively host specific. They are known to infect over
500 species of insects, and are best known from the Lepidoptera. The
NPV from Autographica califomica (AcMNPV) is the one of the most
intensively studied species. The infectious virus particles or virions of
NPVs can be enveloped singly (SNPV type) or in_ g:roups (MNPV type)
and are occluded in protein bodies called polyhedra. NPV polyhedra
may contain few to many virions. After ingestion by the host and
reproduction in the midgut cells, other tissues and organs in the insect
become infected, primarily the fat body, epidermis, and blood cells.
Insect larvae infected with NPV usually die from 5 to 12 days after
infection depending on viral dose, temperature, and the larval instar at
the time of infection. Just before dying, larvae often crawl to the tops
of plants or any other available structure contained in the fluid mass of
the disintegrating larvae and fall into feeding zones (leaves, leaf litter)
where they can be ingested by other conspecific larvae. NPV epizootics
are very impressive and, although they are important as naturally
occurring mortality factors for many insect species, they often occur
after the pest insect has exceeded the economic injury level.
Most of the research on virulence involves inserting genes that
produce toxic substances into the polyhedral gene. For example, genes
for insect specific toxins, inserted into the polyhedral gene locus are
expressed at the time that the polyhedral gene would have been
expressed. The toxins kill the insect at an earlier stage than occurs in a
normal infection. One NPV that has been formulated for insect control
is the MNPV of the gypsy moth, Lymantria dispar, or LdMNPV as
Gypcheck. In India, NPVs are available against Helicoverpa armigera,
Spodoptera litura and Spilosoma obliqua, castor semilooper (Achaea
janata), and red hairy caterpillars (Amsacta albistriga). NPV for control
of H. armigera on pulses like chickpea and pigeonpea has been
successfully demonstrated in many parts of the country.
In India, a number of companies, Agricultural Universities and state
departments of agriculture produce NPVs of H. armigera, S. litura and
A . albistriga and supply commercially to the farmers for pest control. The
Methods of Insect Pest Management
{ 437
product Biovirus marketed in India is a wettable powder formulation of
9
H. amz igero NPV containing 7 x 10 PIB/g having a storage stability for 2
years at 40° C. It is applied at 300-500 g/ha, 2-3 times at 10- 1 5 day interval.
(ii) Granulosis viruses. Granulosis viruses (GV) are closely related to
NPVs and are similar in structure and pathogenesis. The major
difference between these two groups is that the virions are singly
occluded into small occlusion bodies called gi:anules. Like the NPVs,
reproduction begins in the nuclei of host cells. Tissues GVs have only
been recorded from Lepidoptera. There are three major genetic types of
GV. Type 1 GV described from the cabbage looper, Trichoplusia ni,
only infects the midgut cells and subsequently the fat body cells.
Because it does not infect the tracheal matrix or epidermis, larvae may
live longer than NPV-infected insects. Type 2 GVs, first isolated from
the codling moth, Cydia pomonella, parallels NPV infections. Type 3,
known only from the western grapeleaf skeletoniser, Harrisina brillians,
infects only the midgut tissues. Several GVs have been formulated, as
microbial insecticides, e.g., GVs from the codling moth, A . albistriga,
sugarcane borers, S. litura, H. armigera, Chilo spp., and Achaea janata.
Like NPV, these viruses are produced in vivo because of difficulties
producing them in cell culture. In vivo production costs and narrow host
spectrum limits their attractiveness to industry.
(b) Bacteria. There are more than 90 species of bacteria that attack
insects but the species of Bacillus are only commercially available.
Members of this genus are spore-forming bacteria whose spores can
remain viable in the soil for years. The two most important species that
occur in India are B. popilliae and B. thuringiensis. Recently, the red
pigmented bacterium, Serratia marcescens, a non-spore-forming type, is
being used against a number of lepidopteran pests. The bacterium,
Coccobacillus acridiorum has been used against grasshoppers in part
of Africa:
B. popilliae is the causative agent of ' 'milky disease' ' in soil
inhabiting larvae of the Japanese beetle, Popillia japonica. After
ingestion, the milky disease spores germinate within the gut of
susceptible larvae. The vegetative bacteria induce localised infections in
the midgut epithelium, followed by massive bacteremia. Though, this
bacterium may be mass produced in vitro, is presently mass produced in
vivo by injecting intrahaemocoelically into larvae or adult P. japonica so
that the viable spores can be used in the field . It is effective against
the white grubs Holoytrichia consanguinea, H. serrata, and Leucopholis
lepidophora.
B. thuringiensis (Bt) occurs naturally in the soil and on plants.
Different vaneties of this bacterium produce a crystal protein that is
toxic to specific groups of insects. Bt has been available in North
4 38 J
Methods of Insect Pest Management
America as a commercial microbial insecticide since 1960s and is sold
under various trade names. These products have an excellent safety
record and can be used on crops until close to the day of harvest. Bt
can be applied using conventional spray equipment but, because the
bacteria must be eaten to be effective, good spray coverage is essential.
(i) Habitat (Crops). Bt can be used against numerous moth and
butterfly larvae and some beetle and fly larvae attacking several crops
including vegetables, cotton, tobacco, tree crops, forest crops, and
landscaping.
(ii) Formulations of Bt variety. There are several subspecies
(=varieties) of B. thuringiensis based on the serotype of flagellar
antigens, and these subspecies tend to be host specific. B. thuringiensis
kurstaki are available for the control of many caterpillar pests including
imported cabbageworm, cabbage looper, homworms, European com
borer, cutworms, some armyworms, diamondback moth, spruce budworm,
bagworms, tent caterpillars, gypsy moth caterpillars and other forest
caterpillars, and Indian meal moth larvae in stored grain. Less well
controlled are com earworm, codling moth, peach tree and squash vine
borers. B. thuringiensis tenebrionis and variety san diego are registe�ed
for use against larvae of Colorado potato beetle and elm leaf beetle
adults and larvae. B. thuringiensis israelensis, Serotype H- 1 4, Strain 164
i s marketed for use against black flies and mosquitoes, fungus gnats,
although unless used on a community-wide basis, it is probably more
effective to eliminate standing water and control weeds at the edges of
ponds. B. thuringiensis aizawai is used to control wax moth larvae in bee
hives and various caterpillars. It is important for control of diamondback
moth caterpillar which has developed resistance to B. thuringiensis
kurstaki in some areas.
The oil formulation is more effective than formulated as a wettable
powder or aqueous flowable. The gene that controls the production of
the delta-endotoxin of B. t. san diego (kills the Colorado potato beetle)
has been inserted into the bacterium Pseudomonas fluorescens and after
the fermentation has been completed the broth is chemically treated and
heated to kill the bacteria. During this process, the protein toxin
becomes encapsulated by the cell wall. This encapsulation process
appears to protect the protein against rapid degradation in the field,
making it more persistent. In another case, the B. t. san diego toxin gene
and the B.t. kurstaki toxin gene have been placed within the same
bacterial cell to produce both toxins. This product then can be used for
controlling both of these pests.
( iii) Mode of action. The toxic crystal Bt protein in commercial
formulations is only effective when eaten by insects with a specific
(usually alkaline) gut pH and the specific gut membrane structures as
Methods of Insect Pest Management
[ 439
these are required to bind the toxin. When ingested by a susceptible
insect, the protein toxin damages the gut lining, leading to gut paralysis.
Affected insects stop feeding and die from the combined effects of
starvation and tissue damage. Bt spores do not usually spread to other
insects or cause disease outbreaks on their own as occurs with many
pathogens.
(iv) Symptoms. Larvae affected by Bt become inactive, stop feeding,
and may regurgitate or . have watery excrement. The head capsule may
appear to be overly large for the body size. The larva becomes flaccid and
dies, usually within days or a week. The body contents turn brownish-black
as they decompose. Other bacteria may turn the host body red or yellow.
(v) Relative effectiveness. Some naturally occurring bacteria can cause
epizootics, especially if the pest population is under stress from lack of
food, overcrowding, or cold weather. These epizootics are not as
common as those caused by other naturally occurring pathogens.
Commercial formulations of Bt, however, are widely used. Greenhouses,
tree and field crops, waterways and thousands of acres of forests are
sprayed annually with commercial Bt products.
Successful use of these Bt formulations requires application to the
correct target species at a susceptible stage of development, in the right
concentration, at the correct temperature (warm enough for the insects
to be actively feeding), and before the insect pests bore into the crop
plant or fruit where they are protected. Young larvae are usually most
susceptible. Caterpillar growth may be retarded even if less than a lethal
dose is eaten. Determining when most of the pest population is at a
susceptible stage is key to optimizing the use of this microbial
insecticide.
(vi) Limitations of Bt application. Not all caterpillar pests are
equally susceptible to Bt. Some populations of diamondback moth, a
major worldwide pest of cole crops, have evolved resistance to the B.
thuringiensis kurstaki toxins. One important limitation is that Bt acts
strictly as a stomach poison. In order to be effective, the insect must
eat it. Because of this, insects that tunnel into plants are not well
controlled, even though they may be susceptible to the Bt toxin. For
example, codling moth, the worm in a wormy apple, can be killed by Bt
in the laboratory. In field situations, however, the larva avoids it and is
not killed since it burrows into the unprotected interior of the fruit.
Similarly, com earworm is a susceptible species, but not well controlled
by Bt since it rapidly tunnels into the com ear tip. Bt also shows great
differences in effectiveness with varying ages of most insects. Young
stages often are quite susceptible, whereas older instars may not be
easily controlled with Bt applications used at field rates. The short
persistence of Bt can be a limitation. Developing insects often stop
440 J
Methods of Insect Pest Management
feeding for 24 hours or more during periods when they moult. If the Bt
is applied at this time, it will largely be broken down before insects
resume feeding. Rain or overhead irrigation can also reduce
effectiveness by washing Bt from crop foliage. Also, insects that have
not yet hatched will not be controlled so timing becomes more critical.
Application during times when ultraviolet light is less intense (e.g. late
in the day) can imptove persistence. Finally, the very selectivity of Bt
can sometimes be a limitation. More broad spectrum insecticides are
effective against several insects.
On the other hand, separate
formulations of Bt must be used against leaf feeding beetles versus leaf
feeding caterpillars. No Bt products kill aphids, plant bugs, or some of
the other garden pests. However, the conservation of natural enemies
often more than makes up for this latter limitation. Com earworm,
squash vine borer larvae, and codling moth larvae are susceptible, but
field control is difficult because they rapidly bore into and are protected
by plant tissue. Bt is effective against European com borer if it is
applied just as the larvae are hatching. Some formulations, such as those
involving the genetic engineering of the Bt toxin, aim to overcome these
problems.
(vii) Transgenic crops using Bt. In addition to the expanded use of
Bt as a microbial pesticide, it is increasingly used as a primary source
of toxin to produce transgenic crops resistant to insects. At present
several transgenic crops expressing B. thuringiensis endotoxin genes are
grown in USA, Australia, Mexico, China viz., B t-com against European
com borer, Bt-potato against Colorado potato beetle, and Bt-cotton
against tobacco budworm (Heliothis virescens) and bollworm (Helicoverpa
armiger<i), and Bt-tomato against caterpillars. In India, Monsanto's
Bt-cotton is likely to be the first transgenic crop to be commercialised
within a year or two as it is already undergoing field trials after being
approved by the Department of Biotechnology (DBT), Government of
India.
(viii) Advantages of Bt for insect control. The primary advantage of
Bt products is their safety resulting from their selectivity. Each strain of
Bt is capable of affecting only a specific group of insects, for example,
caterpillars. Non-susceptible species are not affected. This includes
desirable species such as wildlife, pets and beneficial insects. Because Bt
does not directly affect natural enemies of insects (e.g. ladybird beetles,
parasitic wasps) as do many other insecticides, it conserves and
integrates with natural controls. Bt also is considered non-toxic to
humans. Most common formulations of Bt are registered on essentially
all food crops and do not even require an interval lapse between
application and harvest.
Methods of Insect Pest Management
. [ 441
(c) Fungi. Approximately 750 species of fungi have been reported from
insects, many of which offer great potential in insect pest management.
Beauveria bassiana, an entomofungus was the first microorganism which
was recognised as an causal agent of muscardine disease of the silkworm,
Bombyx mori. The fungi usually infect the insects by direct penetration of
their body wall. Although, the fungi are potent bioagent against several
insects their commercial production for use in agricultural systems . has not
progressed as rapidly as has the development of bacteria and viruses. One
of the major limiting factor in using fungal pathogens is that many species
are easily destroyed by fungicides used to control plant pathogens. Other
environmental factors affect the initial levels of infection and secondary
spread. In India, three species of fungi are commercially available for
insect control : Trichoderma viridae, T. harizianum and Gliocladium sp.
Following table displays the entomopathogenic fungi under development as
myco-insecticides.
Table 3. Entomopathogenic fungi
Pathogen
Aschersonia
Beauveria bassiana
as
myco-insecticides.
Target
Production
Country
Whiteflies, scale insects
Liquid
fermentation
Russia
Colorado _Jll)_tato beetle
Li9..uid
fermentation
Russia
Castor semilooper,
white _&!libs
Semi-solid
India
China
Com borer
Semi-solid
Culicinomyces
clavo�rus
Conidiobolus obsccurus
Mosquitoes
Liquid
fermentation
Australia
Aphids
Liquid
fermentation
UK, USA,
France
Entomophthora grylli
Grasshoppers
In vivo liquid
fermentation
Australia
�nle bl!£
Semi-solid
Brazil
Metarhiz1um anisopliae
P)'Tilla, rhinoceros
beetle
Semi-solid
India
Mosquito
Li9..uid
fermentation
USA
Verticillium lecanii
Green house aphids
Liquid
fermentation
UK
Most of the fungi that frequently infect aphids, beetles, flies and
leafhoppers
belong
to
Entomophthorales.
Several
environmental
conditions affect the spread of Entomophthorales and other fungi.
Epizootics generally are positively correlated with leaf wetness either
from rain, 1mgation, or heavy dew, e.g., spotted alfalfa aphid,
Therioaphis trifolii maculata populations are destroyed each year by
epizootics of the fungus Erynia radicans. Most fungal species can form
and discharge conidia at 5° C and then germinate. However, it may take
442 1
Methods of Insect Pest Management
over 1 6 hours for these events to occur, whereas it happens within a
few hours at around 20° C.
Another important factor in the primary spread of fungal pathogens
is the level of initial inoculum in the field at the beginning of the
season. Secondary spread is by movement of infected larvae and adults
coming into contact with noninfected hosts and by airborne dispersal of
infective conidia.
Several soil-borne fungi commonly infect soil-insects. Beauveria spp.
and Metarhizium anisopliae in particular infect Coleoptera. The soil
habitat generally affords a relatively stable environment for these fungi.
However, in many soils the upper few centimeters of soil reach
temperatures well above 50° C, a temperature lethal to the vegetative
stages of most insect pathogenic fungi. Applications of B. bassiana have
been applied to potato foliage to control the Colorado potato beetle
with little success. However, by using the same formulations and
incorporating the material into the soil where larvae come in contact
with the spores prior to pupation, there has been reasonable success.
Genetic Control
Genetic control involves manipulation of the mechanisms of heredity. An
outstanding example of genetic control is the sterile-male method,
sterilising natural population by chemosterilants, and other genetic tactics.
[ I] Release of sterilised males
Use of sterile-male techniques against the insect was conceived in 1 937 by
Knipling, E.F. of USDA. According to this theory, if a sufficient number
of the matings in a given population in the field resulted in no offspring,
then over a period of generations the population would decrease. Thus if
sexually sterilised males are introduced into, or induced within a wild
population each generation and if the matings of these sterilised individuals
exceed normal matings, the population will decline. If the number of sterile
individuals is kept constant (by additional releases) for each generation, the
ratio between sterile and normal matings will increase rapidly and the rate
of population decline will increase correspondingly. A drammatic success
has been achieved in the eradication of the screwworm fly, Cochliomyia
hominivorax, a parasite of cattle in the southeastern USA.
During 1 958- 1 959, up to 50 million sterile flies of both sexes were
produced (by gamma irradiation at a dose of 2500 r) each week and
more than 2 billion were released over an 1 8-month period. The area
involved 85,000 square miles, including Florida and part of Georgia and
Alabama. In this programme, more than 40 tonnes of ground meat were
required each week to rear the flies, and 20 aircraft were used to
release them. This campaign resulted in complete eradication of
Methods of Insect Pest Management
[ 443
screwworm populations from Florida. Since that time there have been
sporadic outbreaks traceable to the movement of infested animals into
the territory, but the screwworm has not been a problem since 1 959.
Similarly, in 1963 on the island of Rota, the control of melon fruit fly,
Bactrocera cucurbitae was successfully achieved by using this technique.
In Switzerland the field cockchafer, Melolontha vulgaris, a serious white
grub pest of root vegetables, was eradicated by release of only sterilised
males.
Since a vast number of sterilised males are needed in release practice
to ensure successful mating with wild females, the sterile-male method is
practical only against insects that occur in relatively small populations as
adults. Further, the sterilised males should have the vigour to compete with
wild individuals for a mate. Finally, the species involved must easily be mass
reared. The sterile-male method is impractical against insects that are very
prolific and widespread or against insects that appear in large numbers
sporadically and unpredictably (e.g., floodwater mosquitoes) as large
numbers of reared individuals would have to be maintained at all times.
[ II] Sterilising insects in the natural population by chemosterilants
In addition to radiation, there are about 300 chemicals (chemosterilants)
that induce sterility in insects when ingested. Chemicals produce sterility
primarily by causing insects to fail to produce sperm or ova, causing the
death of sperm or ova after they have been produced, or producing genetic
defects in spermatozoa that prevent zygote development. The larvae may
fail to pupate or the pupal d�velopment may be incomplete. The
chemosterilants are .also useful in the situations when release of sterile
males is not feasible, e.g., against insects that occur in very large numbers
and are difficult or impossible to rear in mass. Most research on this
approach has been directed towards the house fly Musca domestica and
several mosquitoes in isolated areas. As the chemosterilants must be
ingested to be effective, they are usually applied with baits. The
chemosterilants that are effective by contact might be used in association
with luring stimuli such as light and sex attractants. Another possibility is
the application of a chemosterilant to breeding places. However, since all
the promising chemosterilants are strongly carcinogenic or mutagenic
agents, they present a serious hazard to other animals, including humans
and hence, their use in pest management cannot be recommended and only
the future developments could change the acceptability of the method.
There are four major groups of chemicals that induce sterilisation in
insects., viz., alkylating agents, phosphorus amides, tnazmes, and
antimetabolites. Presently, the alkylating agents
(Bisulfan, TEPA,
meta-TEPA, thio-TEPA, methio-TEPA, aphoxide, apholate) represent
the largest class of chemosterilants, and their effects are similar to those
444 J
Methods of Insect Pest Management
of X-ray and g amma rays. These agents cause multiple dominant lethal
mutations or severely injured genetic material in the sperm or egg. The
alkylating agents are unstable in the environment and degrade rapidly.
Possible contamination of food and water and even small residues,
however, makes crop application unfeasible. Safe applications are
possible only under laboratory conditions, where these materials may be
used to sterilise insects in sterile-release programmes.
[ III] Other genetic tactics
The manipulation of insect genetics for suppression includes any kind of
artificial manipulation of insect gene composition to reduce population
numbers. It is genetics used in self-destruction of insect populations. These
genetic tactics for autocidal control are tentative at present; they have not
been applied. However, several genetic processes are understood, and their
use has been suggested. Fundamentally, the proposal is to alter genetic
processes in such a way as to make insects less fecund, less vigorous, or
altogether sterile; the effect would be to decrease population fitness. The
objective would be suppression or complete eradication of the species in a
large area.
Recently, genetic manipulation has been carried out to improve the
fitness of insect natural enemies. Much of the emphasis here is m the
development of beneficial insects (and other arthropods) that are
resistant to pesticides.
The genetic plasticity is one of the several properties that make
insects responsive to autocide through genetic manipulations. Most
species maintain many forms throughout their geographical range, each
representing a different genotype; potentially, these are critical resources
m
selection and breeding programmes. Additionally, insects have
relatively short life cycles and high reproductive potentials, which
enhances breeding programmes and genetic experimentation. Moreover,
technologies developed for rearing large numbers of some species may
lead to practical implementation of progr ammes in the field. The genetic
manipulations include conditional lethal mutations, inherited sterility,
hybrid sterility, cytoplasmic incompatibility, chromosomal rearrangements,
and meiotic drive mechanisms.
(a) Conditional lethal mutations. In this process the insects are bred
that are less fit than normal for certain kinds of environmental
conditions. The approach relies on specific alleles of genes that
determine fitness of individuals for factors like temperature tolerance.
These alleles result in inherited traits that do not allow survival in all
conditions encountered by the insect. Theoretically, male insects with a
dominant, homozygous, cold-sensitivity trait could be reared and released
into the environment. There, they would mate with wild females, which,
Methods of Insect Pest Management
[ 445
in tum, would produce progeny with the trait. The progeny then would
be killed by normally low winter temperatures.
(b) Inherited sterility. It has been suggested as a method to
substantially increase sterility ratios in populations, as compared with the
conventional sterile-insect release technique. Particularly, it has been
recommended for moths because these insects usually require very high
dosages of irradiation to achieve high levels of dominant lethality, and
such high levels impair competitiveness of the released insects. In
theory, if a 9: 1 ratio of sterile to fertile males were established by
releases in a conventional programme, this would result in a 90%
reduction of fertile crosses. If on the other hand, the sterility produced
by the release was not expressed until the F 1 generation, it would be
enhanced by 9%. This is because a 9: 1 original release would create a
9 : 1 ratio in both males and females of the F 1 generation and thus,
genetic death can be increased from 90% to as much as 99% by
delaying it until the next generation.
(c) Hybrid sterility. Hybrid sterility has been observed in laboratory
studies with closely related species. Similar to horse-by-donkey crosses
that result in sterile mules, this idea has been suggested as a method of
obtaining sterile insects for release. Particularly notable example was
hybridisation between the tobacco budworm, Heliothis virescens, and a
related species, H. subjlexa. These species cross quite readily in the
laboratory, producing partially sterile F1 progeny. Most important in this
phenomenon is that the hybrid males are sterile when they mate with
normal H. virescens and H. subflexa females. Although technical
problems exist, this approach has been suggested for use in sterile-insect
releases against populations of the tobacco budworm.
(d) Cytoplasmic incompatibility. It occurs when individuals from
different populations are crossed and reproductive potentials are
reduced. The reduction is derived because of incompatibility factors in
the cytoplasm causing sterility in individual eggs. Here, sterility results
when a sperm enters an egg and stimulates meiosis, but it does not fuse
with the egg pronucleus to form a zygote. The principle of cytoplasmic
incompatibility has been extensively investigated with a mosquito, Cu/ex
fatigans. Unfortunately, sorting out large numbers of male mosquitoes
from a population for release presents many technical problems.
Research into "sex-killing systems" as accomplished with house flies, has
offered a possible solution to this problem.
(e) Chromosomal rearrangements. This technique selects and bree.ds
insects with certain genetic defects or chromosome translocations or
rearrangements for
use
in
release
programmes.
1 he
genetically
rearranged males mate with wild females and produce partiaJ:y sterile
progeny. The technique has also been suggested as a means to transport
446 1
Methods of Insect Pest Management
economically advantageous genes to pest populations; advantageous
genes would confer traits in the pests that are helpful to humans. Some
desirable traits might include insecticide susceptibility, avoidance of a
crop plant, and inability to tolerate normal temperature extremes.
if) Meiotic drive mechanisms. Meiotic drive refers to the unequal
recovery of homologous chromosomes during meiosis. Here, a
conditionally lethal gene introduced into the population increases in
frequency when population numbers decline.
An example of the
mechanism can be made with the XY system of sex determination
where XX pairing produces females, and XY produces males. If a
Y-linked mutation is induced, only Y sperm would be produced, rather
than a 1 : 1 ratio of X: Y sperm. When mutant males, carrying only Y
sperm, mate with normal females, only sons are produced. A distortion
of the male-to-female sex ratio would result, favouring males, and the
population would decline. Such a population may eventually become
extinct. At present its applicability in insect control is doubtful.
(g) Replacement by innocuous forms. In this phenomenon, the harmful
insects is replaced by the forms that are not harmful. In this instance,
genetically altered replacements could be mass-reared and released with
the expressed purpose of lowering pest status through changes in
species characteristics. One of the most appropriate subjects of this
potential approach are non-vectoring strains of insects that normally
vector pathogenic microorganisms of humans. Recently, a strain of the
mosquito, Anopheles gambiae, has been discovered that is immune to
Plasmodium species, malaria-causing pathogens. After ingesting infected
blood, this mosquito strain is capable of encapsulating the infectious
stage of the microorganism, which causes its death. Therefore, this strain
is unable to transmit the infection. Production of a fully immune strain
has been achieved by selective breeding, encouraging further research on
potential releases in replacement campaigns. If natural populations could
be diluted or completely replaced by breeding with the released immune
strain, it is conceivable that malaria incidence could be reduced
significantly. However, annoyance from biting mosquitoes would remain
the same.
Although replacement by innocuous forms has not been
accomplished at the present time, the approach holds promise for
alleviating certain types of pest problems.
Legal or Regulatory Control
Insect migration from one place to other is a frequent phenomenon. The
natural barriers are the only preventive measure of insect dispersion from
one geographic area to others. In past, there were no restrictions on the
transport of plants and animals from one country to another as the danger
involved in it was not realised. Advances in transportation technology over
the past century and encouraged trade, resulted introduction of new pests.
Methods of Insect Pest Management
[ 447
Large proportions of the major pests m a modern agro-ecosystem are
exotic in origin. The exotic pests cause more damage to the human
commodities than the indigeoun ones. Introduction of San Jose scale,
woolly aphid and cottony cushion scale into India; gypsy moth and
European corn-borer into North America; and the insect pests of
Eucalyptus into New Zealand and South Africa are some notable examples
of invasion.
Legal control
involves the enactment and enforcement
of
quarantines. Quarantines are designed to prevent the entry of potential
pest species, to confine them to as small an area as practicable once
introduced, or to prevent them from being exported to other countries.
Even if quarantine measures only retard the spread of a given species,
the money saved may very well justify the cost. It should be borne in
mind that because a given species is not a major pest in its native land
does not mean that it will not be one in another region where few or
none of its natural enemies exist.
Apparently the first regulatory control legislation was passed in
Germany in 1 873. It was intended to prohibit the entry into that country of
any materials that might harbour the grape phylloxera, Phylloxera vitifoliae,
from America. In the United States, although there was earlier regulatory
legislation both at the state and national levels, the first major and effective
legislation was passed in 1 905 . The U.S . Plant Quarantine Act of 1 9 1 2
supplemented and extended earlier legislation and enforces laws to protect
the agriculture from insect pests and plant diseases by regulating
importation and interstate movement of potential carrier materials. In India
the Government passed an act (Act 1 1 of 1 9 1 4) entitled "Destructive
Insects and Pests Act 1 9 1 4 " to prevent the introduction into India of any
pest destructive to crops. This was later supplemented by a more
comprehensive statute in 1 9 17.
The legislative measures in force now in different countries can be
grouped in to 5 classes :
[ I] Legislation to prevent the
introduction of foreign pests
(International quarantine)
To prevent the entry of foreign pests, practically all countries have
restrictions on the import of plant or plant material. The enforcement of
these quarantine measures is supported by legal enactments, called
quarantine laws. The imported plants and plant materials have to be
thoroughly examined at the port of entry for the presence of foreign
insects. Government of India passed the act ' 'Destructive Insects and Pests
Act of 1 9 1 4" and developed facilities for plant quarantine inspection at the
seaports of Mumbai, Kolkata, Cochin, Chennai, Tuticorin, Rameshwaram,
Bhavnagar, and Vishakhapatanam; and at the airports of Amritsar,
(Z-57)
448 1
Methods of Insect Pest Management
Mumbai, Kolkata, Chennai, Tiruchirapalli and New Delhi. The land
frontiers are Attari-Wagah border, Amritsar district and Bongaon, Gade
Road, Kalimpong and Sukhiapokri in West Bengal. The imported plants
and plant materials have to be thoroughly examined at the port of entry for
the presence of foreign insects. The importation of consignments of
plants/plant products from foreign countries has to be done only through
any of these, ports. The consignments should be accompanied by
certificates issued by the officers of the Department of Agriculture of the
exporting country as to their freedom from pests and disease; these
certificates are called phytosanitary certificates. At the port of entry these
consignments are inspected and if necessary, fumigated to kill the pests
carried by them. In India, the Central Directorate of Plant Protection and
Quarantine was established in 1946 and from 1949 the Directorate has
established quarantine stations in a number of sea and airports and land
frontiers and is in-charge of these activities.
[ II] Legislation to prevent the s pread of
already established pests
(Domestic quarantine)
The potato tuber moth is not known in India during the beginning of 20th
century. It is assumed that it entered India during early 1900 and got
established to become a sore problem of potato storage. The • 'Destructive
Insects arid Pests Act of 19 14" had also impowered the state governments
to enact such laws as will enable it to prevent the spread of dangerous
pests within its jurisdiction. Such steps were taken at Madras (=Chennai)
when the fluted scale, Icerya purchasi, was located to be limited in the
Nilgiris and Kodaikanal areas. Under the ' 'Madras Agricultural Pests and
Diseases Act of 1 9 1 9" the state government established quarantine stations
at Mettupalayam and Gualpur for Nilgiris and Shenbagnur and Top
Stations for Kodaikanal in 1943. None of the plants listed as alternative
hosts was permitted to be transported from the notified areas without
inspection and disinfestation. Similarly, the potato tubers from Darjeeling
hill areas of West Bengal are not allowed to be brought down to the plains
for seed purposes in order to prevent wart disease.
[ Ill] Legislation to enforce the suppression
of pests in limited areas
Domestic quarantine is also a measure to contain spread of established or
introduced pests. In addition to this the State Governments may declare
some pests as notified ones and legislative measures may be taken up to
make a adoption of control measures of such pest obligatory for the
growers. "Madras Agricultural Pests and Disease Act of 1 9 19" is the first
legislative measure for such purpose in India. This act contained provisions
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Methods of Insect Pest Management
{ 449
like : (i) the government may declare a pest as notified one and can limit
the areas where it is considered notified, (ii) to growers will have to
compulsorily adopt prescribed control measure without any time lag,
(iii) the State Government will employ staff to supervise the protection
measures, (iv) if the grower does not adopt the control measure, the staff
will themselves carry out and arrange the control operation, (v) the staff
will arrange recovery of the money spent for control measures from the
growers through land revenue, and (vi) the grower is liable to be
prosecuted under the provisions of the Act if he refuses to or obstructs the
operation of control mea<;ures. The State Pest Act has been in force in
respect of cotton bollworms (E. vitella, E. insulana and Pectinophom
gossypiella) and stem weevils (Pempherulus affinis) in 1 9 1 3, red hairy
caterpillar (Amsacta albistrig� in 1930 and the coffee stem borer
(Xylotrechus quadripes) in 1946. The Act enforced for the coffee stem
borer is still in force in parts of Salem, Madurai, Coimbatore and the
th
Nilgiris. All infested coffee plants are to be removed and destroyed by 15
December every year and the stems and branches of the bushes are to be
swabbed with lindane emulsion before the emergence of the weevils during
April-May and October-December so as to prevent them from egg laying.
[ IV] Legislation to prevent the adulteration
and misbranding of pesticides.
The Insecticides Act, 1968 (No. 46 of 1968) has been enforced in 1968 by
the Government of India to regulate the import, manufacture, sale,
transport, distribution and use of insecticides with a view to prevent risk to
human beings and animals. It Government of India constituted the Central
Insecticides Board to advise the Central and State Governments on
technical matters arising out of the administration of this Act. The
Insecticides Rule of 197 1 framed under the Insecticides Act, 1968 (46 of
1968) came into force in 197 1 . It specify that the firms engaged in the
manufacture of insecticides should register themselves stating the name and
address of the manufacturer, the brand and trade name of the insecticide,
the active ingredient and other constituents of the products to be
'
manufactured and its net contents in an unit pack which should also carry
in it detailed directions for use including the antidote against the
insecticide in case of poisomng. The container should carry a "Poison"
label with skull and cross bones and a warning or caution statement. As
large number of synthetic insecticides are being manufactured, there is
every possibility of the farmers being supplied with poor quality material
and to avoid such malpractices the products have to be standardised
through the Indian Standards Institute. Such standardiseq products carry
ISI mark and are expected to conform to the level of active ingredient of
the chemical indicated in the label.
(Z-57)
450 1
Methods of Insect Pest Management
[V] Legislation regarding the insect and insect residue
contamination in foodstuffs
The occurrence of insects or their parts and the secondary products due
to infestation pose health hazard and also reduce their acceptability by
consumers. Therefore, several countries have fixed standards of such
contamination. Prevention of Food Adulteration Act of 1955 of
Government of India has been declared to set standards of permissible
limit of insects contamination and also visualised provisions to fix tolerance
limit of pesticides in foodstuffs. It enables the authorities in declaring
grains or food stuffs unsuitable for consumption and can issue orders for
their destruction. This not only restricts health hazard but also prevents
pest infestations.
Integrated Pest Management (IPM)
Bartlett ( 1956) coined the word integrated control to denote the blending
of biological control agents with intervention of chemical control. But,
today it covers wider sense and the concept is called as integrated pest
management which involves all eco-friendly practices of pest management
such as physical, cultural, legal, biological, genetical, chemical that could
reduce the insect population below the economic threshold level.
[ I] The IPM concept
By definition, IPM is the development of a set of management tactics or
practices (physical, mechanical, cultural, biological, chemical, genetical
etc.) that maintain pest populations at economically low level with as little
disturbance to the ecosystem particularly the beneficial insects and natural
enemies as may be absolutely necessary. Pesticidal intervention is minimal
or a last device and other measures are given due consideration. IPM is
dynamic and takes into account a holistic approach, i.e., interactions
between all biological agents associated with plant and the appropriate mix
of control technologies. It is essentially non-prescriptive, which means that
the farmers must understand it and practise it intelligently. The IPM
practices vary with plants, therefore, we need to develop IPM strategies
against pests of each crop in every habitat.
The IPM should serve as a "technology basket" or "menu of
technologies" from which intelligent choice can be made by the farmers.
The IPM is of primary importance as it brings economic and
environmental benefits. The IPM has been accepted as the cardinal
principle for plant protection in India. Both central and state
governments are considering it as the high priority agenda nation wide.
This is being done for the last decade or so.
(Z-57)
Methods of Insect Pest Management
[ 45 1
[ II) Critical issues of IPM
Since long back IPM has been considered as a concept that is not in
practice because of several constraints. It need following considerations :
1. Use of fertilisers. Fertilisers have become indispensable for higher
yields but the very fertiliser is the cause of incidence of pests and
diseases in most of the cases. This is more so in the soils where the
organic matters are less. Nitrogen fertilisers are known to enhance the
pest incidence. Not on!y this, excessive use of NPK over time depleted
the contents of other nutrients in the soil including even the essential
elements which make the basic frame of the plants. Therefore, increase
of nitrogen from other means seems essential. Adequate supply of good
compost, scientifically prepared, not only supplies wholesome nutrients
to the plants but also provide needed vigour and strength to resist pest
and disease and overcome drought or excess rain. Therefore, integrated
nutritional management (INM) should precede IPM .
2. Plant resistance against pests. Tolerance of the crop to pest and
disease depends on several morphological and physiological characters of
the plant, derived from the nutritional status of the soil. Plant nutrients
such as S, Mg, Ca, N, Fe, Zn, Mo, etc. in soil contribute to inducing
resistance or tolerance in plants which are being depleted over time due
to application of NPK. This has made the plant more susceptible to the
pests. Therefore, the knowledge of each crop's nutritional requirement is
prerequisite for IPM application.
3. Fore-warning of the pest incidence and cropping calendar.
Applications of control measures are always thought when the pest has
already overpowered. One must understand the predisposing conditions
for an insect or others to become pest. We know very little about the
incidence of most of the pest species. For example, the pigeonpea pod
borer, Helicoverpa armigera does not bother much to the early sown .
crop in June compared to the late July sowing under south Indian
conditions. In north India, early varieties of rapeseed mustard escape
attack of mustard aphid, Lipaphis erysimi. Similar information is needed
for other crops. For every crop there is a season and reason for its
success or failure. For most of the crops, cropping calendar of
optimum, late and early sowings to avoid certain pests and diseases are
not known.
4. Plant population. Very little attention is paid to the optimum
plant density. Gap filling is not done in many cases. Farmers will not
have separate nursery sown on the same day as the main field sown for
emergency gap filling. Even 10- 15% of less plant population causes that
much of losses.
452 J
Methods of Insect Pest Management
5. Creation of ecological niches for natural enemies. Provision of
natural niches for the natural enemies of insect pests minimise the use
of synthetic pesticides (see biological control).
[ III] The procedure of integrated control
Integrated control is often possible even on a small local scale, it is likely
to be most effective over a larger area such as rice-, sugarcane-, cotton-,
rapeseed mustard-agroecosystems.
1. Establishing economic injury level [EIL]. Presence of insects in
the crop field does not mean that it needs chemical control. Low pest
infestation sometime is beneficial to yield by stimulating plant growth or
by allowing fewer fruits to develop greater size. For example, many
leaves of soybean use up assimilate by respiration without contributing
greatly by photosynthesis. Defoliation by hand or by insects of a
proportion of the leaves on such plants can lead to an increase in yield
as the plants give optimum yield at a particular ratio of leaf area to
ground area, known as the leaf area index. However, as the number of
insects increases, it causes decrease in yield. Therefore, it is the
important decision of how far a particular pest population can be
allowed to grow before the application of insecticide. Economic damage
was defined as the amount of injury which will justify the cost of
control measures. It should be kept in mind that there is a difference
between injury and damage. Injury is the effect of pest activities on host
physiology that is usually deleterious. Damage is a measurable loss of
host utility, most often including yield quantity, quality, or aesthetics.
Therefore, injury is centred on the pest and its activities, and damage is
centred on the crop and its response to injury. The economic damage
begins to occur when money required for suppressing insect injury is
equal to the potential monetary loss from a pest population. This
beginning point of economic damage is termed gain threshold which can
be expressed as follows: Gain threshold = Management cost
(Rs./ha)/market value (Rs./kg) = kg/ha. For example, if management
costs for application of an insecticide are Rs. 1 00.00/ha and harvested
crop is marketed for Rs. 1 0.00/kg, the gain threshold would be 1 0 kg/ha.
In other words, at least 10 kg/ha would need to be saved with an
insecticide application . for the act1V1ty to be profitable. The
economic-injury level [EIL] is defined as the lowest number of potential
pests that will cause economic damage, or the minimum number of
pests that would reduce yield equal to the gain threshold. Although EIL
is expressed as number of pests/unit area, it is really a level of injury.
For example, consider the previous example of 10 kg/ha gain threshold
for pest management of a crop. If 1 insect/plant causes 1 kg/ha loss,
then the EIL for the pest is 10 insects/plant. In this example, 10
Methods of Insect Pest Management
[ 453
insects/plant potentially could consume enough plant tissue to reduce the
yields by
I 0 kg/ha. Therefore, such an insect population is considered
economic, and the management activities are justified.
2. Establishing economic threshold level [ETL]. The ETL is the
best known term and
most widely
used
index in making pest control
decision. It indicates the number (density) of the potential pest at which
management action should be taken. For this reason, it is also called as
the
action
threshold
le':'el.
Although
expressed
in
insect
numbers,
the
ETL is a really time parameter. Just as with EILs, ETLs also can be
e.g.,
expressed in insect equivalents,
nymphs/adults per leaf.
the ETL for sugarcane pyrilla is 5
3. Assessing potential natural enemy activity. The crop ecosystem
be sampled to determine whether natural mortality agents are
must
present
in
chemical
sufficient
control.
numbers
The
to
sampling
be
worth
also
conserving
determines
how
with
selective
frequently
the
economic threshold is being exceeded.
4. Implementation of cultural practices. There are several cultural
practices by which the number of insect pests can be kept below the
ETL.
Judicious
early/late
use
sowing
of
of
synthetic
the
crop,
fertilisers,
adequate
use
of
organic
irngation,
manure,
disposition
of
agricultural wastes, use of resistant cultivars, strip cropping, polyculture,
etc.
are several
cultivation
as
cultural practices
these
methods
are
that
not
must
only
be implemented in crop
low
costing
but
also
are
eco-friendly.
5. Application of biological control methods or augmentation of
environmental resistance.
The
environmental
constraints
(natural
enemies, diseases etc.) resist the growth of a pest population in nature.
The purpose of augmentation may be to provide natural enemy action
where their number is insufficient or to establish a new equilibrium pest
population
at
an
artificially
low
level.
Where
natural
enemies
have
disappeared because of lack of a vital alternative host, replacing a single
plant species may be all that is
plant resistance to the pest
will
necessary. The introduction of partial
slow down the rate of pest increase,
and the existing natural enemies may well then regulate the pest to a
lower equilibrium level. In augmenting the resistance of the environment,
cultural controls are also worth considering. Any measure which makes
conditions more suitable for natural enemies,
such as the provision of
refugiae or adult food such as nectar sources, are particularly valuable,
as are any measures such as destruction of crop residues,
break
the
life-cycle
of
the
pest
in
the
region
so
that
which may
numbers
in
subsequent generations are dependent on immigration from outside.
6. Application of selective pesticides. The biological control potential
established or already present in the environment then needs protection or
454 1
Methods of Insect Pest Management
conservation from the pesticidal sprays that are necessary whenever pest
populations reach the economic threshold. It would obviously be ideal if we
could use chemicals which were inherently selective. Moreover, perhaps
too much emphasis has been placed in the past on selectivity between a
pest and its natural enemies. There are many pest problems (e.g.,
low-density pests such as disease vectors like whiteflies or aphids) where
the pest virtually has to be eliminated. This can usually be achieved with a
pesticide. The use of pesticide kills the natural enemies of this pest and/or
other pests of the same crop. Elimination of the pests also cause death of
the natural enemies or their emigration because of the disappearance of
their host/prey. In such cases, integrated management involves pesticide
selectivity between the pest in question and the natural enemies of other
potential pests of the same crop, so that insecticide control of the key pest
does not lead to an upsurge of other pest problems. There are several
pesticides that cause less harm to the natural enemies, therefore, the
farmers should be educated to apply a specific pesticide for a specific pest.
For example, an aphidicide pirimicarb, a systemic and fumigant carbamate
which affects aphids and flies, but not ladybirds or aphid parasitoids;
another widely used insecticide with some selectivity is the organochlorine
endosulfan which seems fairly safe to parasitic wasps, used in biological
control; in Nigeria the organophosphate methomyl gave good control of a
pod borer of cowpeas without affecting its major parasitoids.
In addition, the formulation of spray, reduced dose rate, time of
application, application in space are other sources by which selectivity
of pesticide application can be made.
Important Questions
1.
Outline several practices applied in the insect pest management.
2.
3.
4.
5.
Why carbamates are considered better insecticides than chlonnated hydrocarbons?
6.
Describe properties of some pesticides of plant origin.
Write an essay on organophosphate insecticides.
What do you mean by insecticide formulations?
Describe the various types
of
formulations
What is biological control ? Describe in brn:f the organism used in btologtcal control.
7. How do you mtegrate the biological control with chemical control of insects in !PM ?
8.
What is the future of biological control in India?
9. Write an essay on the legal control of insect pests.
10.
11.
What do you understand by mtegrated pest management ? What are the steps in its
implementation ?
Write short notes on : (i) Fumigants, (ii) Btopesllcides, ( iti) Baculoviruses as bioagent,
(tv) B acteria as an insect bioagent, (v) Cnteria of a better bioagent, (vi) Feasibility of
btocontrol in India, (vii) Genetic control.
25
Beneficial Insects : Apiculture,
Sericulture and Lac Culture
I
BENEFICIAL INSECTS
]
Out of about 10,00,000 species of insects found all over the world, only less
than 10,000 insect species are pests and cause serious problems to us.
Majorities of the insect fauna either do not hann us at all or have
beneficial attributes. The insects not only provide us honey as food, silk as
clothing, but they are also beneficial to us by several other ways. Few
insects are so beneficial to us that our existence would be miserable in
their absence such as honey bees.
Insects as Suppliers of Useful Products
There are a number of insect species that render their services for
mankind by producing a variety of products that are used by human beings
and are also easily available commercially. These products are either used
as food (products of honey bees: honey, propolis, royal jelly, bee pollen),
of commerce (silk, shellac, cochineal dye, tannic acid), in medicine (insect
venom, cantharadine) etc.
[ I] Honey
Honey is a highly nutnt1ve liquid food prepared from flower nectar by
several species of honey bees. The honey bee, Apis mellifera, is the major
producer of honey and has been domesticated and is maintained in
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Apiculture, Sericulture and Lac Culture
artificial hive containers throughout the world. Apis cerana indica is
another species used by beekeepers in south India. The bees collect nectar
from the flowers and hold it in their honey stomach till their return to the
hive. In the crop it is predigested and mixed with invertase in the salivary
secretion which splits the sucrose of nectar into dextrose and levulose. The
fluid is regurgitated . into honey cells. After concentration, the fluid
becomes viscous. Its colour varies with source of nectar. The honey
prepared by the nectar gathered from mustard flowers is yellow. Water
(20%) and several sugars (30-40% levulose, 30-35% dextrose, 2-5%
sucrose, traces of maltose, 1 - 1 2% dextrines) are the main constituents of
honey. It also consists a number of other substance which are present in
small amounts such as fatty acids (acetic, butyric, citric, formic, lactic,
malic, succinic acid), amino acids, enzymes (invertase, diastase) , vitamins
(A, B complex, C), and minerals (Fe, Cu, Mn, Mg, Na, K, Ca, Si, P). The
major source of honey has been the rockbees, Apis dorsata and Apis
laboriosa. Till recently about two-thirds of the honey in the market used to
be from these wild bee colonies. Even today rockbee honey constitutes a
significant portion of the commercial honey. Tribal populations in several
parts of the country harvest honey and beeswax from rockbee and other
wild bee colonies. In some regions like the Kutch area in Gujarat, honey
is collected also from wild Apis fl.area colonies.
Maintenance of honey bees for the purpose of harvesting honey,
wax, and other products is called apiculture, or beekeeping. Honey has
been sought out and collected by humans for thousands of years and
has found use mainly as a food material because of its nutritive value.
Honey is used as a carrier in many Aurvedic and Unani medicines. It is
by itself used as laxative and a blood purifier. Being alkaline (pH>7 .0),
it does not produce acidosis. In western countries, honey is used to
prepare meads, beverages similar to wines. Honey is also widely used in
the preparation of baked goods, candies, chewing gum, beauty lotions
and ice cream. Production of honey in India is now about 1 1 ,000 metric
tonnes (Rs. 1 1 million) which is very low as compared to USA (US$
300 million)
[ II] Propolis
Honey bees gather the sap or resin, from tree bark and leaves and
combine it with nectar, their own enzymes and create propolis. They then
use this substance to seal their hives, protecting it from outside
contaminants. Propolis is comprised of 50-70% resins and balsams, 30-50%
wax, 5 - 1 0% bee pollen and 1 0% essential oils. Except for vitamin K,
propolis has all the known vitamins. Of the fourteen minerals required by
the body, propolis contains them all with the exception of sulfur. It
contains 500 times more bioflavonoids (vitamin P) than is found in oranges.
Apiculture, Sericulture and Lac Culture
[ 457
Propolis is an excellent natural antibiotic and immune system booster. Its
antibiotics create one of the most sterile environments in the animal
kingdom. Because of this, the bees use propolis at the entrance to the hive
to sterilise themselves as they come and go. It also contains a number of
unidentified compounds which work together synergistically to create a
perfectly balanced, nutritive substance. It offers antibacterial, antiseptic,
antiviral,
antibiotic,
antifungal,
anti-inflammatory,
and
antioxidant
properties. It is useful in allergies, bruises, burns, cancer, herpes zoster,
fatigue, sore throats, nasal congestion, respiratory ailments, acne, skin
disorders, respiratory infections, flu, colds, cough, ulcers, wounds, etc. It is
commercially available in form of tincture, throat spray, chewing gums and
soothing creams.
[ III] Bee pollen
Bee pollen refers to pollen which is collected from flowering plants, and
stored by honey bees in their hives. The bees only select and collect
pollens that are rich in amino acids. Bee pollen is almost complete food,
containing nearly every nutrient required to sustain life. It is richer in
protein than any animal source, yet its fat content is very low. It has been
used for energy, endurance, a free radical scavanger, weight control,
longevity, asthama, etc.
[ IV] Royal jelly
The royal jelly which is considered as the crown jewel of the beehive, is an
incredibly rich in nutrients. It is a creamy, opalescent and white liquid
synthesised by the workers exclusively for queen bee and extends her
longevity from 6 weeks to 5 years. Royal jelly contains an abundance of
nutrients, including minerals, B-complex vitamins, protein, amino acids,
collagen, essential fatty acids, etc. The composition of royal -jelly is so
complex that it has been extremely difficult for scientists to completely
breakdown its components. It is marketed in capsule, softgel, and soothing
cream.
[ V] Beeswax
Beeswax is a yellowish white solid waxy material (a mixture of cerotic acid
and myricyle palmitate) secreted by specialised epidermal glands of the
abdominal sternum of worker bees and is used to construct beehive. It is
obtained from old combs, cappings collected after honey extraction, and
comb affected by wax moth. Almost all the commercial beeswax available
in the market is obtained from the wild hives of Apis dorsata. Beeswax was
used by people as early as the sixth century for a variety of purposes and
was probably the major wax material of ancient times. The beeswax is
widely used in many cosmetics (e.g., beauty lotions, creams, lipsticks),
458 J
Apiculture, Sericulture and Lac Culture
ointments, saving creams, floor waxes, nearly smokeless church candles,
various pharmaceuticals, some polishes, dental wax, wax museum figures,
electrical and lithographing products and several other manufactured
materials. One of the major uses of beeswax is in the preparation of comb
foundation, which is affixed to the frames of a commercial beehive. This
foundation serves to induce the bees to construct honeycomb in the frames,
which in turn makes the hive much easier to manage.
[ VI] Silk
Silk is produced by the labial glands of several species of silkworms
(caterpillars of silkmoth) to construct cocoon (see Fig. 2A of chapter 1 2).
The thread of silk consists of about 75% of a tough elastic protein, fibroin
(inner layer) and remaining 25% of a gelatinous protein, sericin (outer
layer). Approximately 4,000 years ago in China it was discovered that by
boiling a cocoon, the filament of silk used to construct it became loose and
could be unwound, fortunately in a single strand. The diametre of silk fibre
is 450 - 820 µm. The thread produced by winding several of these filaments
together could be woven into a soft, lustrous, easily dyed fabric. The silk
thread is elastic, resistant and a non-conductor of heat and electricity. It
has a good tensile strength. The silkworms are cultured for the commercial
production of silk. In India, all four commercial varieties of natural silk,
i. e. , mul berry (produced by Bombyx mori reared on mulberry leaves), tasar
(produced by Antheraea mylitta reared on Terminalia tomentosa, Terminalia
arjuna and Shorea robusta), eri (produced by Samia cynthia ricini reared
on Ricinus communis) and muga (produced by Antheraea assama reared
on Machi/us) are commercially cultivated. The silkworms feed on leaves of
their host plants, and close, continuous attention and great care are
required to rear them and harvest silk. Many of the steps in silk production
require hours of tedious hand labour, a factor that has allowed the silk
industry to flourish in Asia (95% of the world yield). Most of the
commercial silk is produced by Japan (70% ). The production of silk in
China and India is 1 5% and 1 .5%, respectively. However, silk has been
produced in more than twenty countries throughout the world . The silk is
used in textile industry, for surgical sutures, parachutes, fishing leaders, etc.
[ VII] Lac
Lac is a secretion of integumental glands of the scale insect Kerria lacca
(Homoptera, Coccoidea), which inhabits a large number of trees in India,
Thailand and Myanmar. From this secretion, commercial shellac is
manufactured. India produces over 60000 tonnes of lac every year which is
about 65-70% of the world' s total output. The thousands of minute nymphs
(crawlers) after hatching wander about on shoots and settle on the tender
branches (now called as settlers) in close proximity to one another. They
Apiculture, Sericulture and Lac Culture
{ 459
pierce the rostrum inside shoot and begin to suck the sap. A day or two
after settling down, they secrete resin over their body from the
integumental glands and eventually cover themselves with lac, which serves
as a protective shield. When they mature, the females remain wingless and
sedentary, whereas winged males emerge and inseminate the females. After
oviposition the eggs hatch, the young crawlers move to fresh twigs of the
host plant and begin to feed, renewing the cycle. The twigs covered with
mature females are referred to as brood lac and are used to inoculate
healthy plants in lac cultivation. Harvesting of lac is done by removing
branches covered with lac (stick lac) and grinding them. The ground lac
(seed lac) is washed, bleached and dried in the sun, then heated in cloth
bags over open charcoal fires. As the lac melts, it is squeezed onto the
floor and quickly pressed and stretched into thin sheets, which are then
flaked (shellac). About 20,00,000 insects produce one kilogram of lac. Lac
is used as stiffener in making shoes; making shoe polishes, artificial fruits
and flowers ; lithographic ink; electrical insulation; protective coverings for
wood, paper, fabric, wax emulsions, wood fillers, scaling wax and buttons;
glazes on confections ; coffee bean burnishing; paints; cements and
adhesives, shellac varnishes and mouldings, photographic products,
Phonographic
records; playing card finishes; dental plates, pyrotechnics;
.
foundry work and hair dyes.
[ VIII] Natural dyes
Cochineal and lac dyes are natural dyes produced by insects. Cochineal is
a product of a scale insect, Dactylopius coccus ( =Coccus cacti), which lives
and feeds on the prickly cactus, Opuntia coccinellifera in Mexico, Peru,
Chile, Honduras, Spain, Canary Islands etc. In India, another species
Dactylopius (=Coccus ) tomentosus occurs on Opuntia dillenii. When the
insects are fully developed, they are brushed off the host plant and killed
by hot water and then sun dried. The dried insects are then ground and
are marketed. Cochineal is a red pigment containing about 1 0% pure
carminic acid, and has been used widely as a permanent dye for colouring
beverages and dyeing wool, silk and leather. It is also used in medicines
for treating whooping cough. Approximately 1 ,50,000 insects are required
to produce a kilogram of dye. It is now getting out of market with the
availability of cheaper synthetic colouring agents.
Lac dye is obtained as a by-product from the wash-water of lac
industry. It contains mainly laccaic acid, a water soluble red dye. Lac
dye is used for dyeing of wool, silk and cotton. It is also used by
Indian womenfolk for a/ta or mahavar and in certain other cosmetics.
[ IX] Insect galls
Several kinds of insect as a result of ovipos1t10n inside the plant tissues
make galls in the host, thus injuring the plants. But certain galls are
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Apiculture, Sericulture and Lac Culture
powerful
vegetable
astringent,
tonic
and
antidotes
for
certain
poisons.
Some other galls have been the source of various pigments used for dyeing
wool, skin, hair, leather, and so on, and for the production of permanent
inks. Galls are the richest source of tannic acid
(30-70% ), a substance
widely used in tanning, dyeing, and preparation of inks.
Insect Used in Medicine
Several insects have been reported to have medicinal properties for last
several hundreds of years. The medicinal properties of honey, propolis, lac,
cochineal etc. have already been dealt above. Certain insect products like
cantharidin and venom of bees and wasps are widely used in medicine. The
maggots of blow flies have been used in sores to remove tissue debris.
[ I] Cantharidin
Cantharidin
is
derived
from
the
bodies
of blister beetles
(Coleoptera:
Lytta vesicatoria , the Spanishfly,
found throughout Europe. In India, the blister beetle, Mylabris cinchorii
Meloidac).
The
best
known
species
supplies
about
twice
the
species.
When
taken
internally,
quantity
is
of cantharidin
cantharidin
acts
as
as
compared to
a
strong
other
urogenital
irritant. For this reason it has been used as an aphrodisiac (e.g., in cattle
breeding), and to cure certain urogenital diseases. It is a very dangerous
substance and is no longer used in humans.
[ II] Apitherapy
Apitherapy (also known as bee venom therapy) is the medicinal use of
honey bee venom which is secreted by poison gland of workers. It contains
1 8 active substances. Melittin, the most prevalent substance, is one
( 100 times more potent
at least
of the most potent anti-inflammatory agents known
than
hydrocortisol).
Adolapin
is
·another
strong
anti-inflammatory
substance, and inhibits cyclooxygenase ; it thus has analgesic activity as well.
Apamin inhibits complement C3 activity, and blocks calcium-dependent
potassium channels, thus enhancing nerve transmission. Other substances,
such as Compound
X, Hyaluronidase, Phospholipase A2, Histamine, and
Mast Cell Degranulating Protein (MS DP), are involved in the i nflammatory
response of venom, with the softening of tissue and the facilitation of flow
of the
other
substances.
neurotransmitters
Apitherapy
such
as
pain
and
respond
:
(i)
can be
in
to
bee
(ii)
useful
in
bursitis,
venom
there
are
measurable
Norepinephrine and
both rheumatoid
s