David Choi

Feeding in Manduca Sexta

Abstract

The parasitic wasp Cotesia congregata is a natural parasitoid of the tobacco hornworm (sphingidae), an agricultural pest which feeds preferentially on the plant family Solanaceae. The female wasps oviposit by injecting eggs directly into the host's hemocoel where they subsequently hatch and develop until they are ready to emerge. In general, parasitization may have one of two effects upon feeding behavior of a host organism: an increase in food consumption consistent with nutrient deficiency or a decrease of food consumption consistent with depression after an immune challenge. While previous studies present evidence for an immune-activated suppression of feeding in Manduca sexta which coincides with the emergence of wasps through the host's cuticle, it is unclear how these hosts respond to the earlier stages of internal development of a parasite within its body. This study aims to elucidate the effect of parasitic infection on the feeding behavior of the third and fourth juvenile instars of Manduca sexta by comparing the mass of plant matter consumed within a limited amount of time by parasitized versus un-parasitized individuals. 

Introduction

            Clearly, it is to C. congregata’s advantage that M. sexta has a suppressed feeding response at the time of its emergence from its host. If such behavior were not suppressed, the host could simply consume the wasp cocoons that form on its body (Adamo 1998). However, it is most likely that it is not the actual wounds caused by emergence that affect M. sexta’s feeding suppression; rather, the increased levels of octapamine resulting from the immuno-response to the wounds are a more probable factor in effecting the suppression of feeding (Adamo 2005). Parasitized M. sexta also remove octopamine from their hemolymph 23 times more slowly than non-parasitized ones, which further indicates octopamine’s important, albeit not yet fully understood role, in suppressing feeding in M. sexta. However, it has been speculated that immune/wound reaction only increases the suppression of feeding, and that there are many other factors that could play a role. In order to obtain a better understanding of the effects of C. congregata’s effect on feeding suppression during earlier instars, we observed the feeding response of parasitized M. sexta during their 3^rd and 4^th instars.

Materials and Methods

            We isolated 40 M. sexta when they had hatched into their first instars. We placed each of the specimens in individual plastic cups (which we sterilized using bleach) and gave them each a block of food as devised by Bell and Joachim in 1976. We parasitized the M. sexta within 24 hours of their molt into the 2^nd instar by exposing them to the parasitoid wasp C. congregata. Most of the M. sexta were stung only twice, which was our intention, but some were stung three times, which we recorded on their respective labels.

            We then checked on the M. sexta at least once a day, waiting for them to molt into their 3^rd instars. Whenever we found specimens that had molted, we isolated them from their food for 3 hours. Then we placed the specimens into individual Petri dishes and divided them into 2 groups: the test group of specimens that had been parasitized and the control group that had not been parasitized. We then punched out uniformly sized discs of tobacco leaf and placed a single disc in each Petri dish with the M. sextas. Then we timed the M. sexta for ten minutes using a stopwatch as they ate their respective leaf discs. After the ten minutes had elapsed, we retrieved the remains of the eaten leaves being careful to keep the leaves of the parasitized group and the control group segregated. We then let the leaves dry out for two days, sometimes resorting to heating them in an oven in later trials of the experiment. After the leaves had dried, we weighed the masses of the eaten leaves.

            This experiment was repeated with M. sexta within 24 hours after they had molted into their 4 instar. Also, we realized that we had made an error in not recording the initial mass of each leaf disc we had punched out, which prevented us from normalizing our data in order to determine statistical significance. In order to compensate for this error, we punched out 8 leaf discs and calculated their average mass used that figure as our initial mass to compare the average masses of the leaves eaten by the parasitized M. sexta and control M. sexta.

Results

Fig. 1 Averages of remaining mass of leaf discs eaten by M. sexta

Discussion

            Although simply using the average of the dry weights of uneaten leaf discs is an inadequate substitute for running a T-test to check for statistical signficance, the data from the experiment seem to suggest that the parasitoid C. congregata suppresses feeding in M. sexta during both the 3^rd and 4^th instars of its host, since the parasitized M. sexta ate less, on average, than the control group in both instars. The data from the experiment also seem to suggest that suppression of feeding in parasitized M. sexta is a progressive phenomenon that increases in extremity as the parasitoid and/or host molts into its successive instars.

Acknowledgements

References Cited

Bell, R. A. and Joachim, F. A. (1976). Techniques for rearing laboratory colonies of tobacco hornworms. Ann. Entomol. Soc. Am. 69, 365-373

Adamo, Shelley A. (2005) Parasitic Suppression of Feeding in the Tobacco Hornworm, Manduca sexta: Parallels With Feeding Archives of Insect Biochemistry and Physiology 60, 185–197

Adamo, Shelley A. (1998) Feeding Suppression in the tobacco hornworm, Manduca Sexta: costs and benefits to the parasitic wasp Cotesia congregata Canadian Journal of Zoology 76, 1634-1640