Background/Question/Methods Mormon crickets (Anabrus simplex) form large migratory bands that march over rangeland in the western United States in search of nutrients. Immune defense is particularly relevant to survival in migratory bands, but little is known about the role of nutrition in insect immunocompetence, particularly in the wild. We hypothesized that immune defenses are compromised in Mormon cricket bands due to nutrient limitations. To determine general constituents lacking in their diet, we presented captive members of the band with both protein rich and carbohydrate rich diets.

Results/Conclusions Members of a migratory band in Utah preferred the protein diet, indicating a protein deficiency like that found in Idaho. In contrast, members of the Nevada band preferred the carbohydrate diet, and showed little interest in proteins. These two kinds of nutrient deficiency were associated with different kinds of immunodeficiency. In the protein-deficient band, a protein diet enhanced phenoloxidase (PO) activity, an enzyme involved in wound healing and fighting foreign invasion. PO activity was unaffected by the dietary treatments in the carbohydrate-deficient band. In the carbohydrate-deficient band, feeding on carbohydrates enhanced the crickets' ability to encapsulate foreign particles and lyse bacteria, whereas these abilities were unaffected by the dietary treatments in the protein-deficient band. Shortly after feeding on protein or carbohydrates, Mormon crickets exhibited measurable effects on the immune system. The difference in components of the immune system that are enhanced by the contrasting dietary constituents suggests that PO activity requires protein whereas encapsulation and antibacterial activity require carbohydrate fuels. Thus there may not be a common currency for the generalized immunity of insects. In the general framework of ecological nutrition, insects may require a balanced diet to maximize defense against invasion.