Ecological Immunology Mediated by Diet in Herbivorous Insects

A rapidly advancing area of ecological immunology concerns the effects of diet on animals' immunological responses to parasites and pathogens. Here, we focus on diet-mediated ecological immunology in herbivorous insects, in part because these organisms commonly experience nutritional limitations from their diets of plants. Nutritional immunology highlights nutrient-based trade-offs between immunological and other physiological processes as well as trade-offs among distinct immunological processes. This field reveals that nutrition influences the quality and quantity of immunological defense in herbivorous insects, and conversely that nutritional intake by herbivorous insects can be an adaptive response to the specific types of immune-challenge they face in the context of other physiological processes. Because the diets of herbivores challenge them physiologically with plants' secondary metabolites, another area of study analyzes constraints on immunological defense imposed by secondary metabolites of plants in the diets of herbivorous insects. Alternatively, some herbivores can use secondary metabolites as medicine against parasites or pathogens. Animal-medication theory makes an important contribution to ecological immunology by distinguishing prophylactic and therapeutic mechanisms of anti-parasite defense. Integrating ideas from animal-medication and nutritional immunology, we outline a conceptual framework in which the immunological role of the diet consists of mechanisms of prophylaxis, therapy, compensation, and combinations thereof. Then, we use this framework to organize findings from our own research on diet-mediated ecological immunology of woolly bear caterpillars. We show evidence that the woolly bear caterpillar, Grammia incorrupta (Hy. Edwards) (Lepidoptera, Erebidae, and Arctiinae), can employ both diet-mediated prophylaxis and therapy. First, increased consumption of carbohydrate-biased food prior to immune-challenge increased its melanization-response. Second, increased consumption of pyrrolizidine alkaloids (PAs) more than 24 h after parasitism by tachinid flies resulted in anti-parasite resistance. Caterpillars reduced feeding on protein-biased food within 24 h after immune-challenge, showing evidence of illness-induced anorexia. We synthesize our work to generate the hypothesis that a diet-mediated defense by the host against parasites acts as a temporally explicit, multi-stage process.