Allelochemicals (plant chemicals not involved in growth/development) play an essential role in aiding insects that store these chemicals in their ability to thwart predators and parasitoids. Relatively little research has addressed the ability of polyphagous (feeding on several different food types) herbivorous insects to self-medicate by selecting diets that moderate infection intensity (self-medication). In the specific case of Manduca exta [Tobacco Hornworm] (Sphingidae), ingestion of the alkaloid nicotine hinders the success of the parasitoid Cotesia congregata (Braconidae), yet whether tobacco hornworms witch to a diet of high nicotine concentration when infected is unclear. In this study, the ability of M. sexta to discern between diets and to choose a high-nicotine diet to battle infection, and thus self-medicate, was analyzed. The relationship between nicotine in the diet and the insect’s encapsulation response (immune response against invading bodies) was also studied. I predicted that M. sexta would exhibit behavioral plasticity and choose diets of higher nicotine concentration as a response to infection, and that higher nicotine concentrations would be correlated with a greater ncapsulation response. 5th instar (the final molting stage before pupation) M. sexta were injected with glass silica beads and subjected to a series of dietary choice tests. They were subsequently dissected to retrieve beads and rate of melanization was determined for each caterpillar. While injected M. sexta differ in their dietary choices compared to healthy larvae, nicotine does not drive a trend in dietary choice. No relationship between nicotine concentration and melanization was found, although healthy caterpillars did consume significantly more food than those that were injected. M. sexta exhibit behavioral plasticity in diet choice, but no clear relationship between allelochemicals and encapsulation was found.
McNamara, Sean, "Self Medication as a Response to Parasitoid Infection in the Tobacco Hornworm, Manduca sexta" (2011). Undergraduate Honors Theses. 18.