Parasites often manipulate the physiology and behavior of their hosts in ways that benefit parasite fitness. Host feeding behavior represents a potentially powerful target of parasite manipulation because host diet is likely to have strong effects on parasite fitness. Here we investigate how a parasite’s manipulation of its host’s nutrient intake benefits the parasite. The parasitoid wasp Cotesia nr. phobetri specifically attacks the grazing caterpillar Grammia incorrupta. Previous work by others shows that Cotesia wasps manipulate extensively the biochemistry and physiology of their hosts. In our study system, we found that wasp-parasitized caterpillars consistently selected a carbohydrate-biased diet, which increased the body size of the developing wasps. This result provides evidence for adaptive parasite manipulation of host feeding behavior. We further hypothesized alternative mechanisms for this benefit to parasitoids: maximizing the nutrient supply to host tissue (fat body) that serves as a critical food source for parasitoid growth, and/or limiting nutrients required for maximal recovery of host immune function. In the latter case, we reasoned that the feeding duration of parasitoids might be limited by the rate at which the host’s immune response recovers to full strength. We analyzed total lipid content as a proxy for fat body as well as immune function of parasitized and unparasitized caterpillars. Our preliminary results show quantitatively more lipid in hosts on the carbohydrate-biased diet after wasp emergence. We also found changes in the immunological melanization response of parasitized caterpillars over time on different diets. These results show how parasite manipulation of host nutrient intake can accrue multiple physiological benefits to the parasite.