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Effects of Diet Manipulation on the Caterpillar Trichoplusia ni and its Parasitoid Copidosoma floridanum
Mark D. Johnson II, William C. Best and David M. Donnell
Department of Biology, The Citadel
Charleston, South Carolina
Summary of Results
• Mean pupal mass as a function of diet treatment:
Control = 0.278g; 1/6 = 0.219g and Grains-only = 0.210g
• Mean silk mass as a function of diet treatment:
Control = 0.0024g; 1/6 = 0.0014g and Grains-only = 0.0009g
• The average length of time from hatching to pupation for animals on
the control diet = 17 days and for those on the 1/6 diet = 28 days.
• Animals raised on the control and 1/6 diets pupated exclusively from
suspended positions in the rearing containers. Half of all T. ni reared
on the Grains-only diet pupated in the mock leaf matter. The other
half pupated from suspended positions in the rearing containers.
• The average soldier larvae count in parasitized T. ni reared on the
control diet was 27. The average reproductive larvae count was1192.
Conclusions
Analysis of the developmental values for caterpillars reared on the
different diet treatments indicates that both the 1/6 and Grains-only
diets proved an effective means of keeping the caterpillars alive while
greatly extending their larval period. Differences observed in mean
pupal weights suggest animals reared on the modified diets pupated
with less than optimal nutrient stores. This had an impact on silk
production, especially marked in the Grains-only animals, and may have
driven many of these animals to pupate in the mock “leaf matter”
provided rather than on the platforms or at the top of the rearing
containers as is typical for animals reared on the control diet. The
impact of a lower pupal weight on adult emergence times or
survivorship and fecundity was not assessed.
The average soldier counts obtained from the dissections of parasitized
animals reared on the control diet were in keeping with published
numbers. Ode and Strand (1995) found that male broods averaged 11
soldiers and female broods averaged 25 soldiers6. This suggests the
number of soldiers we obtain from dissections of parasitized animals
reared on the modified diets will be a reliable test of our recruitment
hypothesis.
Acknowledgments
The authors would like to thank The Citadel Foundation for their support
of research in the Department of Biology and Richard W. Zealy for his
help with poster configuration.
Goals
We plan to continue gathering life history data for caterpillars reared on
the different diets. Also, we plan to dissect parasitized animals reared
on the 1/6 diet to determine if there is an impact of diet on the relative
numbers of soldier and reproductive larvae.
References
1. Davidowitz, G, D’Amico, LJ, Nijhout, HF, 2004. The effects of environmental
variation on a mechanism that controls insect body size. Evolutionary Ecology
Research 6:49-62.
2. McEwen FL, Hervey GER,1960. Mass-rearing the cabbage looper, Trichoplusia ni.
Annals of the Entomological Society of America 53: 229-234.
3. Murillo, H, Hunt, DWA, Vanlaerhoven, SL, 2012. Trichoplusia ni (Lepidoptera:
Noctuidae), in field tomato crops in Southwestern Ontario. Journal of the
Entomological Society of Ontario 143:115-119.
4. Harvey JA, Corley LS, Strand MR, 2000. Competition induces adaptive shifts in
caste ratios of a polyembryonic wasp. Nature 406:183–186.
5. Shorey, HH, Hale, RL,1965. Mass-rearing of the larvae of nine noctuid species on a
single artificial medium. Journal of Economic Entomology 58: 522–524.
6. Ode, PJ, Strand, MR, 1995. Progeny and sex allocation decisions of the
polyembryonic wasp Copidosoma floridanum. Journal of Animal Ecology 64:213.
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Control Parasitized Caterpillar Dissections
Head Capsule Width Soldier Count Reproductive Count
1.9 9 1135
2.2 11 1017
2.2 14 1226
2.1 14 1188
2.0 17 943
2.1 25 1057
2.2 26 1139
2.0 36 1652
1.9 76 1281
Figure 2. A C. floridanum female parasitizing a T. ni egg (A). A
soldier (B) and reproductive larva (C) of C. floridanum.
Hypothesis
The substitution of spent grains in the diet of parasitized T. ni will cause
a decrease in caterpillar growth rates and mean pupal weights and
increase the recruitment of soldiers from the parasitoid brood.
Introduction
As in most holometabolous insects, the nutrients required to support
pupation and ensure adult fitness are acquired during the larval period.
For this reason, the duration of the larval stages is a function of diet
quality and the critical weight required of the animal before it commits to
pupation1. The cabbage looper Trichoplusia ni undergoes five larval
instars before pupating into an adult moth2. During this time the
caterpillar is exposed to a large guild of parasitoids3. The caste-forming,
polyembryonic wasp Copidosoma floridanum deposits its eggs into
those deposited by T ni moths. Some of the resulting wasp embryos
undergo precocious morphogenesis to give rise to a soldier caste of
larvae that defend the host resource against competitors. The
remaining embryos develop into a reproductive larval caste that
develops into adult wasps. The number of soldiers in a wasp brood
appears responsive to the presence of other parasitoids4. Given the
potential for a prolonged host larval period to lead to multiparasitism of
caterpillars previously parasitized by C. floridanum, we want to
determine if the relative number of C. floridanum soldier larvae in a
brood increases with the duration of the host larval period. As a prelude
to this investigation, we undertook the development of two caterpillar
diets by substituting different amounts of spent grains from the brewing
industry for more nutritious ingredients in the established diet used for
T. ni culture.
Abstract
The cabbage looper Trichoplusia ni spends most of its time feeding in
preparation for the rigors of pupation and metamorphosis. The duration
of the larval stages is a function of diet quality and the critical weight
required of the animal before it commits to pupation1. The eggs of T. ni
moths are parasitized by the caste-forming, polyembryonic wasp
Copidosoma floridanum. The resulting wasp brood develops
throughout the host’s larval stages giving rise to soldier larvae that
defend the brood against other parasitoids and reproductive larvae that
carry the germline. Given that protracted host feeding stages are
anticipated to increase the rate of secondary parasitism in the field, we
want to determine if the relative number of C. floridanum soldier larvae
in a brood is responsive to the duration of the host larval period. As a
prelude to this investigation, we developed two caterpillar diets by
substituting different amounts of spent grains from the brewing industry
for more nutritious ingredients in the established diet used for T ni
culture. Both of the new diets significantly decreased caterpillar
development rates as well as their mean pupal size and silk mass. The
diets also influenced caterpillar pupation location. We have started
collecting data for the rates of C. floridanum soldier production in
caterpillars reared on both the control and modified diets.
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B
Photograph: Mary Aegerter
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Figure 1. The larval (A), pupal (B) and adult (C) stages of T. ni.
Photograph: John L. Capinera
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Photograph: James L. Castner
B
Photograph: John L. Capinera
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Figure 8. C. floridanum soldier and reproductive counts following dissection of
caterpillars reared on the control diet.
Objective
We want to determine the effects of diet quality on T. ni development parameters and to evaluate the effect of a prolonged caterpillar
development period on soldier production in caterpillars parasitized by the caste-forming, polyembryonic wasp, C. floridanum.
Figure 7. (A) and (B) 4th instar caterpillars feeding on the
control diet in their rearing container.(C) T. ni fed the
Grains-only diet pupating on the side of the rearing
container.
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Materials and Methods
• Newly emerged parasitized and unparasitized caterpillars were fed
either the established diet5 (= control) or the 1/6 diet (1 part control to
5 parts spent grains from a local brewery). Head capsule widths were
measured every day to monitor larval stages.
• Half of the unparasitized 3rd instar caterpillars reared on the control diet
were moved onto a diet consisting of spent grains and pinto beans
(Grains-only).
• Unparasitized caterpillars entering the 4th instar were placed in clear
plastic rearing containers with feeding platforms and paper towel strips
serving as mock leaf matter. The timing and location of pupation was
recorded.
• Pupal and silk weights were measured from each of the unparasitized
animals.
• Parasitized animals feeding on the control diet were dissected after
undergoing a supernumerary molt to a 6th instar. The number of C.
floridanum reproductive and soldier larvae were counted.
Figure 3. Bar graph showing the effects of the different diets on pupal weights in T. ni.
Values shown are means (Bars = S.D.). The pupal weights of animals reared on the
control diet were significantly higher than those of animals reared on the 1/6 diet (p=.0042
by students two sample T-test) and on the Grains-only diet (p=.0001). Weights did not
differ between animals reared on the two modified diets (p=.5626) .
Figure 4. Bar graph showing the effects of the different diets on silk production in T. ni.
Values shown are means (Bars = S.D.). Silk weights for animals reared on the control diet
were significantly higher than those of animals reared on the 1/6 diet (p=.0216 by students
two sample T-test) and the grains only diet (p=.0001). Silk weight differences between
animals reared on the two modified diets were also significant (p=.0431).
•
Figure 6. (A) Bar graph showing the effects of the different diets on pupal
weights in T. ni. Values shown are means (Bars=S.D.). Mean time to pupation
for animals reared on the control diet was significantly lower than that of animals
reared on the 1/6 diet (p=.0060 by a students two sample T-test).
(B) Bar graph showing the effects of different diets on pupation location.
Feeding on the Grains-only diet correlated with a significant shift away from
pupation locations that require suspension from silk webbing (S) towards
locations in the mock leaf matter (LM = strips of paper towels) provided in the
bottom of the rearing containers.
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Figure 5. Images depicting the effects of diet quality on developmental
milestones in T. ni caterpillars of the same age. (A) Comparison of an
unparasitized animal reared on the1/6 diet (left) to an unparasitized animal
reared on the control diet (right). (B) A parasitized animal reared on the control
diet compared to a parasitized animal reared on the1/6 diet. (C) The pupa and
silk of an unparasitized animal switched to the Grains-only diet in the 3rd instar
(left) and that from an animal reared on the control diet (right).
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