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AEB 403: Entomology
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Aphids are remarkable, evolutionarily exquisite creatures, and among the most successful insects.
Aphid evolution has been shaped through nutrient-driven selection and by the host plants on which
they feed, and aphids have responded by developing intricate life cycles and complex
polymorphisms. These sap-feeding hemipterans have coped with a hostile world through
developing an exceptionally high reproductive rate and passive wind-borne dispersal, a strategy
in which individuals are quite expendable, but survival and prosperity of their genes are
Social aphids form family groups composed of a reproductive female (a foundress or fundatrix)
and one or more generations of morphologically and/or behaviourally specialized soldier-like
nymphs, produced asexually by the foundress Often soldiers are associated with plant galls, but
many species form galls and do not produce soldiers, or soldiers are produced at other life cycle
stages in the absence of galls. As the name implies, the primary function of soldiers is defence
against natural enemies, although many exhibit other unusual adaptations. Some species have
castes of juveniles that are distinctly dimorphic, and produce sterile soldiers with enlarged
forelimbs for grasping their enemies, and/or sharp pronotal horns that they use to stab their
enemies. Others do have dimorphic castes, and juveniles are uniformly aggressive without
morphological specializations. In many species, soldiers pierce their enemies with the needle-like
mouthparts (stylets; Kurosu et al., 1994; and in some, the mouthparts themselves can be
dimorphic. (Note that in the aphid literature, distinctions are made about what does or does not
constitute a soldier. For simplicity, I will use ‘soldier’ and omit other caveats regarding aphid
sociality. Interested readers who wish to sample the diversity of social aphid biology. are directed
to the many enlightening papers from Shigeyuki Aoki and Utako Kurosu (e.g., Aoki and Kurosu,
2010; and references therein), and Takema Fukatsu and colleagues (e.g., Kutsukake et al., 2012;
Shibao et al., 2010; and references therein). General reviews and guides can be found in Blackman
and Eastop (1994), Stern and Foster (1996), Costa (2006), Pike et al., 2007; Pike and Foster (2008).
Their diagnostic eusocial feature is the presence of soldier morphs; Natural enemies are the
primary selective agent driving social evolution They do not provide direct brood care; By virtue
of their gall-forming habit, they occur in highly related family groups; . Although each insect has
certain tasks to perform, the entire hive or colony appears to function as one living organism. Their
evolutionary and ecological success is based upon the regulation of internal con´Čéicts (e.g. [2,3]),
control of diseases (e.g. [4,5]) and individual skills and collective intelligence in resource
acquisition, nest building and defence (e.g. [6,7]).
While aphids are among the most serious agricultural problem insects, only about 250
species are considered to be agricultural pests. Pest aphids may affect only a very specific
host (e.g., Brachycorynella asparagi, asparagus aphid), or group of related hosts, such as
crucifers (Brassicaceae) (e.g., Brevicoryne brassicae, cabbage aphid). Some, however, are
quite polyphagous (e.g., Aphis gossypii, cotton aphid; Myzus persicae, green peach aphid),
with an extremely wide host range.
They comprise anholocyclic clones, or biotypes, that differ in host preferences, ability to
transmit diseases, or resistance to pesticides. Aphids cause damage in several ways. They
can build to high population densities and damage plants directly, by removing enough sap
to cause withering and eventual plant death. If not washed off, aphid excrement, or
honeydew, can build up enough on plants to serve as a medium for the growth of sooty
molds, impairing photosynthesis and plant development, and eventually promoting other
fungal diseases. Salivary secretions of some aphids are phytotoxic, causing stunting, leaf
deformation, and gall formation. Even if the feeding effects of aphids are not apparent, they
may affect plant hormone balances, changing host metabolism to their advantage, thus
essentially hijacking the plant’s physiological functions.
The aphid vectoring of stylet-borne and circulative plant viruses is the most serious problem
to agriculture posed by aphids. Stylet-borne viruses occur on the aphid’s epidermis and are
not aphid specific. These viruses are acquired quickly and transmitted during the aphid’s
probing of the plant’s epidermis. They are nonpersistent, however, and the aphid’s
infectiousness is lost upon molting. Circulative viruses, in contrast, live internally in the
aphid’s gut. The aphid must feed for a while to acquire these viruses, which require an
incubation period before they can be successfully transmitted. They are persistent, however,
and once infected, the aphid remains a vector throughout its life. The virus–aphid–plant
linkage is fairly specific for circulative viruses, and a given virus is transmitted by only one
or few aphid species. Virus-infected plants often show an aphid-attractive yellowing and
have increased free amino acids, so aphids benefit by virus transmission.
General characteristics of social insects
The following are the characters commonly possessed by all the social insects:
1. Parental care: Parental care is an instinct behavior whereby the young ones are provided
with food, shelter and defense by the parents as a part of the family relationship. The social
life in insects is linked with parental care. Parental care paves way for stronger association
between the parents and the young ones. Parental care includes activities like providing the
young ones with food, cleaning the nests, feeding the young and queen, removal of debris
and bodies, arranging eggs in proper chambers, protecting the queen from all adversities,
cooling the nest in summer season.
2. Rich nests: The nests of all social insects are rich in structure which helps in protection,
storage of food and maintenance of broods. The following table summarizes the nest
richness of various social insects:
3. Polymorphism: Polymorphism is the occurrence of several forms within the same species.
It refers to specialization of individuals within a species. In the animals exhibiting
polymorphism, individuals at the center of the colony develop gonads and reproduce
sexually. Individuals at the periphery expose themselves to danger of combat, and do not
reproduce sexually. This is also an example of altruistic behavior. The polymorphic
individuals are sometimes called super-organisms, as in polymorphic individuals the unit
for natural selection is not a single individual but the whole colony. The social insects are
the most prominent examples of super-organisms
4. Large population: All the individuals of the social insects species live in an integrated
manner and hence the term colony is commonly used to describe their complex society.
These colonies are matriarch or in other words, all the members of the colony are the
offspring of a single female and so all of them have similar genotype. Also these colonies
do not accept the members form other colonies of same species.
5. Extra populations: Some aphids, beetles, mites etc. are attracted into the nests of ants and
termites by the high temperature and surplus food. These extra populations are protected
and fed by the ants and termites. In return the ant and termite populations feed on a fluid
secreted by them. Sometimes intruders and thieves rob the social insects of their food. Some
beetles live in the nest of the ant and feed on the ant larvae. All these form the extra
populations of the social insects.
6. Cohesiveness of the colony: Cohesiveness is a measure of the attraction of the group to its
members and the resistance to leave it. The sense of team spirit and the willingness of its
members to coordinate their efforts are very important for the group to perform best as a
biological unit. All the members of the insect society live in a coordinated manner. As a
result of this various castes which differ in structure and function are formed. And these
castes cannot live independently. All the members of the colony work in cooperation and
they reap mutual benefit of the work done. The success of these insects is measured in terms
of the colony and not individually. The members of the castes are bound by chemical and
physiological mechanisms.
7. Liberal food facility: After laying the eggs, stingless bees and some other insects provide
sufficient mass of food for development of the larvae which hatch out of the eggs. This
phenomenon is known as mass provisioning of the food. At the same time, other social
insects daily feed their young ones continuously and extensively. The young ones are fed
until they metamorphose into adults. For example, in the ant colony army ants hunt insects
or flesh; pastoral ants feed on the honey dew produced by aphids. Also the pastoral ants
carry the aphids into the overwintering locations to protect them from predators. Harvesting
ants gather and store seeds in summer to tide them throughout the winter.
8. Finally the leaf cutter ants or fungus growing ants grow their own crop of fungi. These ants
cut and carry the leaves underground to serve as a substrate for growing fungi. These ants
feed on the fungi. Protection of the colony and self: Protection of the individual self is done
with the help of protective devices like stings and jaws. Stings are present in most of the
bees and also in few ant species. Also well-developed jaws are present in stingless bees,
soldier ants and termites. Sometimes for the protection of the colony or nest, few guards are
posted at certain locations. These guards protect the nest and attack the intruders. Finally
the nests are made in such protective locations which have the possibility of rapid exit
during danger. Insects also take care that their nests have numerous side exits.
9. Trophallaxis: Exchange or sharing of food between the insects of different species is called
as trophallaxis. For example, termites and ants feed each other from mouth to mouth.
Similarly young ones exchange food with the adults. Also beetles, aphids and coccids are
fed by ants and then in return ants drink a fluid secreted by them. This is a form of mutual
feeding. Trophallaxis is an important phenomenon in determination and regulation of castes
in termite colony. During trophallaxis, ectohormones with certain inhibitory substances are
passed on to the young nymphs and this prevents them from developing into individuals of
same sex or caste. Consequently the number of individuals in a particular caste is
10.Communication: Communication is an interesting feature found both in social and
nonsocial insects. Insects use chemical, visual, tactile and auditory signals as a means of
communication with each other. Chemical communication occurs with the help of body
secretions called pheromones. Pheromones pass out of the body and help in regulating and
coordinating the colony activities. For example, ants deposit substances on their way which
act as trail markers and help them return to their homes after foraging trip. Similarly visual,
tactile, and auditory signals help in various activities of the colony.
11. Swarming: The behaviour of the insects to come out of the nest in large numbers to relieve
the overcrowding is called as swarming. It takes place during spring or early summer
seasons. Swarming occurs for feeding and migration. It is also a means of the colony
reproduction or in other words founding of new colonies. The queen and the males mate
during swarming flight and this is also known as nuptial or marriage flight.
Biological Pest Control
Many insects are accidentally introduced to environments through commerce or
transportation of private goods. Although current quarantine laws are in place to prevent
accidental introduction of new pests, serious new pests do find their way into the country.
An example is the Russian Wheat Aphid. A non native pest arriving by mistake, without its
natural enemies that kept it in check in its native country, will cause damage, once it
establishes itself. The practice of biological control has resulted from the need to find and
import natural enemies from other countries. Biological Control (Biocontrol) is the
application of raising and discharging natural rivals (predators and pesticides) to find and
eradicate those insects and mites considered pests. These natural enemies reestablish the
accepted balance of nature and are risk free to humans and don’t damage the environment.
Biological control’s purpose is not to completely eliminate pests but keep the pests at low
enough levels for successful crop production.
Principles of Biological Control
Because all insects and pests have some natural enemy, the enemies must be managed.
Management of natural enemies is achieved through controlling importation, directing
conservation, and monitoring growth. The goal of biological control is for the natural enemy
introduced, to establish itself and continue to provide control without assistance from the
gardener or farmer. Standards for biological control include: 1. One living organism is used
to control another living organism. 2. Some control organisms require a limited host range
and are therefore considered host specific. 3. Biological control agents affect the organisim
either directly or indirectly. • Direct – kills the pest • Indirect – weakens the host so they
can not reproduce at a normal rat
Three types of applied Biocontrol that man can influence:
1. Augmentation Man can increase the native agents for control. Normally, there is a lack
or absence of natural enemies occurring in the early pest season. Man can release the
natural enemy early in the season to ensure that when the pests first appear natural
enemies will not be scarce.
2. Classical Biological control Man can introduce exotic biological control agents from
their native home into the areas where exotic pests have established themselves and
survived and multiplied due to the absence of natural enemies. This way, the control
agents will reestablish equilibrium to keep the pest under control. This method is referred
to as an old association.
3. Neoclassical biological control Man can introduce an exotic biological control agent that
previously did not have an association with the pest. The new biological control agent
can establish itself and prey on the pest. This is referred to as a new association
Natural Enemies
There are four types of natural enemies that can be used in biological control.
1. Parasitoid A parasitoid is an organism that spends a significant portion of its life
history attached to or within a single host organism which it ultimately kills (and
often consumes) in the process. They are similar to typical parasites except in the
fate of the host. In a typical parasitic relationship, the parasite and host live side by
side without lethal damage to the host. The parasite takes enough nutrients to thrive
without preventing the host from reproducing. In a parasitoid relationship, the host
is killed before it can produce offspring. This type of relationship seems to occur
only in organisms that have fast reproduction rates (such as insects or mites).
Parasitoids are also often closely coevolved with their hosts.
Insects Parasitoids
An insect parasitoid is an insect parasite that destroys its host. It has an undeveloped
life stage. They are valuable as natural enemies because they develop on or within a
single insect host and prevent any further development of the host after initial
parasitization, eventually killing the host. This typically involves a host life stage
which is immobile (e.g., an egg or pupa), and almost without exception they live
inside the host. Insect parasitoids only damage a specific life stage of one or several
related groups.
Characteristics of insect parasitoids
• Specific in their choice of host
• Smaller than their host
• Only females search out hosts
• Different parasitoid species attack different development stage of the host • Adults
are free living, mobile and may be predatory
• Undeveloped usually kill the host.
Usefulness as Biocontrol
An insect predator will instantly kill or immobilize their prey. A pest attacked by a
parasitoid will die more gradually. The presence of parasitoids may not be obvious,
even though they may be effective. In some cases, it is necessary to dissect or raise
samples of pest insects to determine if any adult parasitoids appear. Because
parasitoids are often more susceptible to chemical insecticides than their hosts,
caution must be used.
Bathyplectes spp. Are small, non-stinging wasps that are parasitoids of the alfalfa
weevil, a serious pest of alfalfa in the Midwest and elsewhere. They were introduced
to North America in 1911, from Italy by the U.S. Department of Agriculture as part
of a biological control effort against the alfalfa weevil
2. Predators
Insect predators are established in practically all agricultural and native habitats. The
following are natural enemies of cabbage pests.
1. One of the benefits, of insect predators is that they are found all around and
throughout plants, including below ground, as well as, trees and shrubs. Some
predators are specific in their choice of prey, while others are not and they need a
wide range of prey. Because they destroy their prey quickly, their successes are
easily recognized. However, specific predators may not be identified due to their
mobility. Once they feed, they move on.
Major characteristics of arthropod predators:
• The adults and young are usually generalists
• Normally larger than their prey
• They destroy or consume many prey
• They will attack both immature and adult prey
Ladybugs, and in particular their larvae which are active between May and July,
are voracious predators of aphids such as greenfly and blackfly, and will also
consume mites, scale insects and small caterpillars.
Hoverflies are another very welcome garden predator. Resembling slightly
darker bees or wasps, they have characteristic hovering, darting flight patterns.
There are over 100 species of hoverfly whose larvae principally feed upon
greenfly, one larva devouring up to fifty a day, or 1000 in its lifetime. They also
eat fruit tree spider mites and small caterpillars. Adults feed on nectar and pollen,
which they require for egg production.
Aphids, also known as greenfly/blackfly Wax scales on a lemon tree branch
Hoverflies can be encouraged by growing attractant flowers such as the poached
egg plant marigolds or phacelia throughout the growing season.
Dragonflies are important predators of mosquitoes, both in the water, where the
dragonfly nyads eat mosquito larvae, and in the air, where adult dragonflies
capture and eat adult mosquitoes. Community-wide mosquito control programs
that spray adult mosquitoes also kill dragonflies, thus removing an important
biocontrol agent and can actually increase mosquito populations in the long term.
Other useful garden predators include lacewings, rove and ground beetles, aphid
midge, centipedes, predatory mites, as well as mega fauna such as frogs, toads,
hedgehogs, slowworms and birds
Usefulness as Biocontrol
Although most beneficial predators consume large amounts of pest insects during
their life span some predators are more effective at controlling pests than others.
Some species play an important role in the containment of pests and others
provide good late season control. Some play a minor roll by themselves but their
contribution greatly influences overall pest mortality.
A good example of the potential number and diversity of predators in a crop
comes from an Agricultural survey on cotton crops in Arkansas. More than 600
species of predators in 45 different families of insects and 23 families of spiders
and mites have been documented, in Arkansas cotton. In the northeastern United
States, eighteen species of predatory insects (not including spiders and mites)
have been found in potatoes. Within a single acre, there may be thousands of
predators in addition to many parasitoids. Although the effect of any one species
of natural enemy may be minor, the combined influence of predators, parasitoids,
and insect pathogens can be significant
2. Pathogens
A pathogen or infectious agent is a biological agent that causes disease or illness
to its host. Disease causing organisms such as bacteria, viruses, protozoa and
fungi can infect insects and mites. These natural occurring organisms can multiply
to cause disease outbreaks or epizootics on pest populations. Under the right
conditions (high humidity, high pest numbers), an outbreak of epizootics can
eliminate an insect population. Diseases are an essential and normal control for
some insect pests. Microbial insecticides, biorational, or bio-insectices are
pathogens that have been mass produced and are accessible in commercial
formulations for use in standard spray equipment.
While some microbial insecticides are still experimental, others have been
available for use for several years. Gardeners and commercial growers have
widely used formulations of the bacterium, Bacillus thuringiensis or Bt. One of
the benefits of using microbial products is that they do not directly affect
beneficial insects and none are hazardous to wildlife or humans. Most insect
pathogens target certain groups of insects at certain life stages.
Chemical insecticides may bring about quicker results as microbial insecticides
can take longer to destroy or weaken a target pest, which may limit their use for
crops that can maintain some insect damage. In order for the use of microbial
insecticides to be most effective they must be applied at the correct life stage of
the pest. Because the use of microbial insecticides is compatible with the use of
predators and parasitoids this aids in spreading some pathogens through the pest
population. Although microbial insecticides are considered non-toxic to humans,
safety precautions must be followed to minimize any exposure.
Characteristics of insect pathogens:
• Use will result in killing, reducing reproduction, slowing growth, or shortening
the life of pests
• Specific to target species or specific life stages
• Effectiveness is often dependent upon other factors, such as host abundance or
environmental conditions
• Degree of control may not be predictable
• Normally slow acting; adequate control may not take place for several days or
Usefulness as Bio-control
Due to the fact that effectiveness is often dependent upon other factors and the
proper deployment appears to be crucial, disease control in the field is likely to
be less successful then in the laboratory where ideal conditions are in place.
Continued research needs to be carried out, as this may be another effective
method for control.
Biological Control advantages
Biological Pest control is a very specific strategy , whatever the predator is
introduced will only control the population of the pest they are meant to target ,
making it a green alternative to the chemical or the mechanical control methods ,
whereas the weed killing chemicals can destroy fruit-bearing plants , The
biological control allows the fruit to be left uninterrupted while the weeds are
destroyed .
Decreases Costs to Consumers
Natural enemies introduced from Europe have greatly reduced populations of the
cereal leaf beetle. This not only saves money for farmers (>$19M/year) by
reductions in pesticide use, but also helps consumers, who end up paying less for
bread and other cereal products.
Results in Permanent Control
Once established, biological control agents exert long-term control on the target
pest without further input. The alfalfa blotch leafminer has virtually disappeared
from alfalfa fields in the northeastern U.S. since parasites introduced by BIIR
became established in the 1970's, saving farmers $20 million each year
Protects Forest & Shade Trees
Biological control helps protect valuable shade trees and forest ecosystems from
invasive plant pests. Biological control of gypsy moth, a major defoliator of many
tree species, helps to maintain healthy landscapes and reduce harmful effects of
pest outbreaks on wildlife.
Enhances Nature Conservation
Invasive plant species such as Phragmites have had disastrous effects on wetlands
and other natural habitats in North America. ARS scientists are exploring
possibilities for using biological control agents to reduce the harmful
environmental effects of these invaders.
Biological pest control does not have adverse effect on the human health or the
environment , It is self-sustaining , It can be cost effective , When the cost of
testing & introducing control agents has been met , the on-going costs are small ,
There is no need to find & identify every individual weed to be treated , An
effective agent will search out all suitable plants of the weed .
Development of the host resistance is not a problem , The biological pest control
is compatible with most other control techniques ( except sometimes the use of
the insecticides & the herbicides ) , The biological control reduces the
competitiveness & reproductive capacity of the weed , making it more
The biological control method of pest management does not use the chemicals ,
It uses many organisms that are either the predators or the parasites to the pest ,
The pest is the organism that causes damage to the people and their crops , The
biological control should be implemented whenever possible because it does not
pollute the environment . Combats Invasive Pest Species
The great benefit of this method is its selectivity , There is a restricted danger of
damage to non target plant species , Biological control does not create new
problems , like conventional pesticides , Selectivity is the most important factor
regarding the balance of the agricultural ecosystems because a great damage to
non target species can lead to the restriction of natural enemies’ populations .
The biological control agent (BCA) can be deployed in the agricultural ecosystem
, so as not to damage non target pests , depends on appropriate host specificity
tests which determine the potential host range .
The ability to self-perpetuate is an interesting advantage of the biological control
method , BCAs will increase in number & spread , Because BCAs are selfpropagating & dispersing , pest control is self-perpetuating too , This is quite
important regarding the economic feasibility of the biological control
Another benefit of the biological control method is the environmental safety of
BCAs , The pest is unable (or very slow) to develop the resistance , The biological
control can be cost effective , Its effectiveness is based on the selfperpetuation &
self-propagation , So , if we establish a control agent in a specific area , it will
reduce the target pest in an acceptable threshold for quite long time .
The financial benefit of the biological control is greatest in the cases when there
is no other option , The biological control is very effective in the inaccessible
areas , The cost efficiency of this method is that the yield benefit of the biological
control is less than yield achieved by the agrochemicals , but the primary cost of
BCA is lower than the chemical pesticides .
The biological control is the attractive alternative to the agrochemicals , the use
of environmentally friendly alternatives to the chemical pesticides are absolutely
required in agriculture , No chemicals are used , so , there’s less pollution ,
disruption of the food chain & risk to the people eating the food that has been
sprayed .
BCAs are more susceptible to the environmental conditions than the chemical
control , This consequently causes the fluctuations to the pest populations , It
reflects to the product quality , to the crop yield and of course to the price of the
product on market , if the annual harvest of the crop is not stable , it will affect
the grower’s income stability .
The biological control method is environmentally safe , This method offers less
risk of the residues in food chain , biological control is most suited for exotic pest
that are not closely related to indigenous beneficial species , Biological control is
less expensive than the chemical control
DeBach, P. 1964. The scope of biological control. p. 3-20. In Biological Control
of Insect Pests and Weeds (P. DeBach, editor). Chapman and Hall Ltd., London.
844 pp.
Doutt, R. L. 1964. The historical development of biological control. p. 21-42. In
Biological Control of Insect Pests and Weeds (P. DeBach, editor). Chapman and
Hall Ltd., London. 844 pp.
van den Bosch, R., P. S. Messenger, and A. P. Gutierrez. 1982. An introduction
to biological control. Plenum Press, New York and London. 247 pp.
Van Driesche, R. G. and T. S. Bellows, Jr. 1996. Biological control. Chapman
and Hall, New York. 539 pp.
Varley, G. C., G. R. Gradwell, and M. P. Hassell. 1974. Insect population
ecology - an analytical approach. Univ. Calif. Press, Berkeley and Los Angeles.
212 pp.
United States Department of Agriculture Agricultural Research Service
Beneficial Insects Introduction Research Unit
What is Biological Control?