Evidence for manipulation in caterpillar feeding behavior by a parasitoid wasp

Host manipulation theory predicts that a change in host behavior increases parasite fitness.  Variation in host diet mediates host-parasite interactions and the way in which parasites control host dietary factors can play a critical role in parasite success. Grammia incorrupta is a grazing generalist caterpillar that is frequently parasitized by the specialist wasp Cotesia nr. phobetri. Cotesia wasps are known to manipulate the biochemistry and physiology of their hosts. Wasp-parasitized caterpillars select a carbohydrate-biased diet.  The preference for carbohydrates might be an adaptive response of wasps to optimize nutrient intake.  Alternatively, the result could be due to random host foraging behavior resulting in the breakdown of nutrient regulation. It is also unclear if altered host feeding behavior is beneficial for the developing wasp. We hypothesize that wasps manipulate the foraging behavior of host caterpillars to optimize nutrient intake and enhance wasp performance. We investigate the role that nutrients play in foraging behavior of parasitized caterpillars in a choice experiment using several combinations of a carbohydrate-biased diet and a protein-biased diet.  To test parasitoid performance on the altered nutrient intake, parasitized caterpillars were given diets consisting of either the nutrient intake of a parasitized or an unparasitized caterpillar.

 Parasitized caterpillars defended a carbohydrate-biased diet, while unparasitized caterpillars defended a nutrient equal diet. Wasp larvae were larger from hosts that ate the self-selected diet (carbohydrate-biased) of parasitized caterpillars, relative to those hosts given the self-selected diet of unparasitized caterpillars. This study provides evidence for manipulation in caterpillar feeding behavior to increase parasitoid wasp performance.