A battle for control in a host-parasitoid system?

Variation in host diet mediates host-parasite interactions and it is important to assess which dietary factors play a critical role in host defense and parasite success. Grammia incorrupta is a grazing caterpillar that ingests and sequesters plant secondary metabolites (PSMs) called pyrrolizidine alkaloids (PAs) from a subset of host-plant species to self-medicate against generalist tachinid parasitoids. Cotesia nr Phobetri  is one specialist wasp parasitoid that attacks Grammia incorrupta. When Grammia eat a mix of plants, including a plant high in PAs, they have reduced mortality from Cotesia.  However, caterpillars parasitized by Cotesia show a preference for a nutritious plant rather than one with PAs. We hypothesize that Grammia acquire diet-mediated resistance against Cotesia through therapeutic self-medication with PAs.  Alternatively, Cotesia wasps manipulate the foraging behavior of host caterpillars to optimize nutrient intake and enhance wasp performance.  We experimentally test the feeding response of parasitized and unparasitized caterpillars to PAs.  We also look at host and parasitoid performance on diets that vary in [PA].  We investigate the role that nutrients play in foraging behavior of parasitized caterpillars in a choice experiment using a carbohydrate-biased diet and a protein-biased diet.  To test parasitoid performance on the altered nutrient intake, parasitized caterpillars are given diets consisting of either the nutrient intake of a parasitized or an unparasitized caterpillar.  We find that wasp-parasitized caterpillars have a suppressed PA feeding response relative to unparasitized caterpillars.  PA consumption by parasitized caterpillars does not confer resistance. Contrary to this result, a high [PA] host diet results in a decreased parasite burden.  In the nutrient choice experiment parasitized caterpillars eat less overall and favor the carbohydrate-biased diet, resulting in an altered nutrient intake. However, we find no difference in parasitoid performance on host diets of contrasting nutrient intake. We suggest that parasitized caterpillars cannot successfully implement therapeutic self-medication against the specialist wasp.  Apparent parasitoid manipulation of host diet does not seem to translate into increased parasitoid fitness. This study provides evidence for an ongoing co-evolutionary tug of war between Grammia and Cotesia and calls for a better understanding of the interplay between nutrients and PSM and how they affect host-parasite interactions.