Research Summary Specialist and generalist caterpillars are different in many aspects. My research highlights these differences in a tritrophic context by focusing on plant chemistry and natural enemies. For example, quite often, specialist caterpillars are physiologically constrained to feed on plants with specific leaf chemistry. In addition, they have adapted to sequester plant compounds in their tissues thereby becoming toxic to predators. In contrast, generalist caterpillars may encounter several different classes of plant chemistry over the course of a day. Given these differences in host plant chemistry, one question that I am interested in addressing is: what are the physiological differences between specialist and generalist caterpillars? To answer this question, I have focused on the insect immune response, which is one of the most important defenses caterpillars have against natural enemies. Using an immunoassay, I have shown how host plant chemistry differentially affects a specialist caterpillar (Junonia coenia, buckeye) and a generalist caterpillar (Grammia incorrupta, woolly bear). In the case of the buckeye, individuals ingesting and sequestering high concentrations of plant compounds are immunocompromised. With the woolly bear, there is evidence that plant compounds help the immune response; however, this question is still under investigation. From these data, I hypothesize on how natural enemies may be influencing the evolution of the insect immune response.