Dietary anti-oxidants as a first line of defense against parasitoid infection in Grammia incorrupta

Background/Question/Methods

By adjusting their intake of various plant species, grazing herbivores have the potential to use a diversity of phytochemicals in several ways for growth and defense. Grammia incorrupta is a grazing caterpillar that ingests and sequesters plant secondary metabolites called pyrrolizidine alkaloids from a subset of host-plant species to self-medicate against parasitoids. However this behavior is only observed during late stages of parasitoid infection. More recent experiments have shown that during early stages of infection G. incorrupta prefers to eat a plant of high food quality for caterpillar growth (Malva parviflora), suggesting the possibility that nutrients mediate host-parasitoid interactions. An initial follow-up experiment showed no evidence that immune-challenged caterpillars altered their macronutrient intake. Malva plants also contain compounds known to have high anti-oxidant activity, suggesting the alternative possibility that anti-oxidants mediate host-parasitoid interactions. We hypothesize that caterpillars with early-stage parasitoid infections prefer plants with chemicals that enhance the immune system as a first line of defense against parasitoid infections. Here we consider the plant secondary metabolite, rutin, as a candidate anti-oxidant compound. In this study, we experimentally test the feeding response to rutin by immune-challenged and control caterpillars. The immune challenge consisted of injected glass beads, known to trigger an encapsulation response. Caterpillars were assigned to a bead-injected, sham-injected (ringer solution only), or a control treatment that received no injection and then placed on a 24-hour feeding assay.

Results/Conclusions

Immune-challenged caterpillars consumed the same amount of rutin-treated food as caterpillars in the two control groups. These results do not support the hypothesis that caterpillars increase ingestion anti-oxidant compounds during immunologically intensive early stages of parasitoid infection. We conclude that further testing of this hypothesis is necessary before dismissing it. In addition, we cannot rule out the possibility that parasitoids manipulate host caterpillar feeding behavior during early stages of infection, thus causing host-plant preference (e.g., consumption of Malva) that benefits parasitoid rather than host fitness.