Levels of Parasitism on
Lepidopteran Larvae of Eastern
Long Island
By
Grant Monahan
2008
Introduction: Lepidopteran larvae have several types of natural enemies, which can
be characterized as invertebrate predators, vertebrate predators, and parasitoids.
These enemies have a significant impact on their population dynamics (Holmes et.
al., 1979). As a result caterpillars have evolved many different types of defensive
systems to protect themselves from predators and parasitoids. Rota & Wagner
(2008) characterized defensive strategies into four distinct categories:
morphological, chemical, physiological, and behavioral. Morphological adaptations
include cryptic coloration, which conceals larvae from enemies, and hairs and
spines, which discourage predators from eating them. Chemical strategies include
projection of toxic or noxious compounds from glands or via regurgitation.
Physiological adaptations respond to the presence of internal parasitoid larvae or
eggs and includes encapsulation in which large numbers of blood cells bombard the
invader cutting it off from vital nutrients (Chapman 1998), and diet change where
caterpillars begin feeding on toxic plants in order to increase their probability of
survival (Karban & English-Loeb 1997). Behavioral defenses are designed to thwart
enemies and include biting, thrashing, dropping from the host plant when
threatened, and frass throwing. Many species of Lepidoptera larvae combine these
basic defensive strategies for further protection (Rota & Wagner 2008).
In recent years, there is evidence that different defensive strategies have
different effects on parasitoids. Gentry and Dyer (2002) found that parasitic flies
very rarely attack solitary feeding caterpillars because they lay microtype free
roaming eggs on, or in close proximity to the caterpillar. Parasitic wasps are most
affected by behavioral defenses such as biting, thrashing, and dropping, and
chemical defenses such as regurgitation. Vertebrate predators such as birds hunt
using visual cues. The most effective defenses against visual hunting are cryptic
coloration and bright warning coloration. Once a caterpillar has been spotted the
most effective defenses are hairs and spines that contain venom, and obstruct
edibility. Invertebrate predators are most affected by shelter building behavior,
which protects the caterpillar while it feeds. It therefore seems likely that there is
no generalized defensive strategy that works against all enemies. The defenses that
have evolved thus represent a compromise between the costs of mounting a defense
and the losses due to each type of predator. Larvae may have defenses against one
kind of enemy but be relatively vulnerable to other types.
Every time a predator kills a caterpillar any developing parasitoids within
are also killed. Thus, Lepidopteran larvae that are well defended against vertebrate
predators would seem to make ideal hosts for parasitoids. In our study we focused
on parasitoids that occur on Lepidopteran larvae found on eastern Long Island. The
purpose of our study was to look for parasitoids in caterpillars found on Prunus
serotina, and see if we could find any patterns. The main pattern of interest to us
was whether caterpillars that are protected from vertebrate predators are targeted
by parasitoids. The purpose of our study was to collect as many species of
caterpillars as possible, rate them on their protection against vertebrate predators,
and compare rates of parasitism on our specimens. We explored these 2 questions:
(1) Are larvae that have evolved defenses against predators more likely to be
parasitized because they are relatively free of predation by vertebrates? (2) Which
defensive strategies do parasitoids target to increase the survival of their eggs? To
address these questions we reared multiple species of Lepidopteran larvae in the
lab to see which species were more highly prone to parasitoids.
Materials and Methods: Larvae were collected on eastern Long Island, New York,
during the summer and fall of 2008. This area is characterized by temperate
broadleaf and mixed forest with oaks (Quercus spp.) or pitch pine (Pinus rigida) as
the dominant tree species. We collected branches from various Locations on
eastern Long Island including Hither Hills State Park and Theodore Roosevelt
County Park in Montauk and Vineyard Fields and Poxabogue County Park in
Bridgehampton. Low growing branches of Prunus serotina were brought to the lab,
and arranged in five-gallon buckets of water. We used various visual methods to
locate the caterpillars on the branches, which included a preliminary inspection,
locating areas of caterpillar activity by looking for newly damaged leaves or piles of
frass each morning. When frass was located, we searched the leaves in the
surrounding area. If no caterpillars were found, we would clean the table of frass,
marking the spot with tape and resume our search the following morning.
Throughout the experiment our methods of locating caterpillars changed. If
the source of a frass pile could be localized to a single branch, we cut it from the
main branch into a smaller more manageable size, and put it in 1000-milliliter
beaker. This made the searching process easier because it allowed us to locate the
exact branch the caterpillar was feeding on.
When a caterpillar was located, we placed it in a clear plastic pint container
with slightly moist paper towel and leaves from the host plant. The paper towel and
leaves from the host plant were changed every three to four days. Each caterpillar
was given an identification number, and data was recorded on location, and date
collected. Caterpillars were categorized by their apparent feeding strategy: silk
shelter, leaf fold, leaf miner, leaf roller, leaf tier, etc. We kept larvae until the pupae
eclosed, noting date of pupation and eclosion. Those that had not eclosed by
November 19 were placed in a large plastic container, which was sealed tight. The
large container was placed outdoors in an area of mild temperature pupation for 14
days and then held outdoors to pass the winter. The pupae will be brought back to
the lab in April to await eclosion. If no parasitoids were present at the adult stage
the specimen was considered to have no parasitoids. The adults were pinned for
voucher specimens and are in the collection of H.D. McGuinness.
Results: During the study we found 84 caterpillars (Table 1). Of these
approximately one-third were free-living larvae, the remaining two-thirds
constructed some form of shelter. The most common shelter encounter was leaf
folds, which account for 36% of the larvae. The remaining third was comprised of
larvae that built silk shelters (13%), leaf rollers (7%), leaf tiers (6%), and leaf
miners (5%).
Table 1: Number of larvae encountered according to feeding strategy
Feeding Strategy
Amount
Percent (%)
Leaf Fold
30
36%
Silk Shelter
11
13%
Larva
28
33%
Leaf Miner
4
5%
Leaf Roller
6
7%
Leaf Tier
5
6%
In this study we were able to identify 15 species from 8 families of
Lepidoptera (Table 2). The most commonly encountered species was Machimia
tenoriferella. The most commonly encountered family was Limacodidae, which was
represented by 5 species.
Table 2: Identified species encountered during the study organized by family
Family
Species
# Of Individuals
Limacodidae
Parasa chloris
2
Acharia stimulea
1
Tortricididia testacea
2
Phobetron pithecium
1
Lithacodes fasiola
1
Oecophoridae
Machimia tentoriferella
9
Sphingidae
Paonias myops
4
Hemaris diffinis
1
Orgyia leucostigma
2
Lymantriidae
Halysidota tesselaris
1
Papiliondae
Papilio glaucus
2
Saturniidae
Automeris io
1
Notodontidae
Schizura unicornis
1
Tortricidae
Unidentified Acleris
1
Out of the 84 caterpillars collected 34 died (40.48%), out of the 34 dead caterpillars
2 were parasitized (6%). The parasitoids as yet are an unidentified Hymenoptera
and an unidentified Diptera. Although the caterpillar species could not be identified
before the parasitoid emerged they both utilized silk shelters. All of the silk shelter
caterpillars reared were Machimia tentoriferella implies that the two parasitized
caterpillars were of the species Machimia tentoriferella. Out of 84 caterpillars 12
caterpillars (14.29%) eclosed before winter diapause, out of these 12 caterpillars 9
where of the species Machimia tentoriferella, 2 where of the species Orgyia
leucostigma, and 1 unidentified adult of the Acleris family.
Discussion: In our study both caterpillars that parasitized utilized a silk shelter.
Whereas it is impossible to draw conclusions from such a small sample size, this
data is consistent with our hypothesis because silk shelters are an effective defense
against vertebrate predators (Rota & Wagner 2008). These shelters can function as
an extension of the caterpillar’s nervous system similar to a spider’s that acts as a
warning system. If anything touches the silk, the caterpillar receives an advanced
warning, giving the caterpillar a chance of escape. The flaw to this system is in the
early instars the silk shelter has not yet been fully developed. If parasitoids target
silk shelter caterpillars for egg deposition during the early instars there would little
webbing in the way, making it easy for the parasitoid (Rota & Wagner 2008). Silk
shelters may make caterpillars more prone to parasitism because the shelter can be
used by parasitoids for chemical and visual signals of caterpillars, making it easier to
locate habitually (Gentry & Dyer 2002).
Even though we did not collect enough data to fully answer our original
questions, we were able to see that larvae protected from vertebrate predators are
likely to be parasitized. We found two caterpillars with parasitoids, but there may
be more that will emerge next year during the pupation process, so our total is likely
to increase. Unless there is a large number of parasitoid species that emerge overwinter, it seems likely that our rate of parasitism is going to be very low, under 10%.
This suggests that the work should be repeated perhaps on late season caterpillars,
or that Long Island is relatively parasitoid free.
References
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Pergamon Press; 1998. Available from:
http://books.google.com/books?id=vOkIvV0MrvYC&pg=RA1PA126&lpg=RA1PA126&dq=caterpillar+encapsulation&source=web&ots=Rv
FlSinl1w&sig=aG0czgPt5Gbwjql_U12eHkix0A&hl=en&sa=X&oi=book_result
&resnum=8&ct=result#PRA1-PA124,M1.
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