Weakness in the band: nutrient-mediated trade-offs between migration and immunity of Mormon crickets, Anabrus simplex

Mormon crickets, Anabrus simplex, form large migratory bands that march over rangeland in the western United States in search of nutrients. Immune defence is particularly relevant to survival in migratory bands, but little is known about the role of nutrition in insect immunity, particularly in nature. We hypothesized that immune defences are compromised in Mormon cricket bands because of nutrient limitations. Members of a migratory band in Nevada, U.S.A., preferred carbohydrate diets over proteins. After feeding on the carbohydrate diet, migratory velocity was less and the ability to encapsulate foreign particles and lyse bacteria was greater than for Mormon crickets consuming protein. Less locomotory demand for lipids may result in greater antibacterial activity. Total phenoloxidase (PO) activity also increased following feeding on carbohydrates, whereas spontaneously active PO was not different between the two diets. These results were very different from those of a band in Utah, U.S.A., that preferred the protein diet and that had enhanced spontaneous PO activity after protein supplementation. Haemolymph of Mormon crickets from the Nevada band was sampled 18 h after the diet treatments, whereas that from the Utah band was drawn 4 h after treatment. Either the difference in immune measures was due to the difference in sampling time, or spontaneous PO activity was protein-limited whereas encapsulation and antibacterial activity required carbohydrates. Currencies for the generalized immunity of insects may differ, and constraints on immunity in a given environment depend on which macronutrients are in short supply.