Mormon crickets maximize nutrient intake at the expense of immunity

Carbohydrates and protein comprise two of the major macronutrients and many animals regulate their dietary intake of both. In the field, the carbohydrate (C) to protein (P) intake of Mormon crickets Anabrus simplex Haldeman (Orthoptera: Tettigoniidae) is indicative of a nutritional imbalance affecting both migration and immunity. In the present study, dietary choice experiments in the laboratory are used to investigate the preferences of Mormon cricket nymphs and adults for C and P. Diets of differing C : P ratios and amounts are presented in pairs to permit Mormon crickets to reach an intake target of C : P from four unique starting points. After the last pair of diets is removed, phenoloxidase (PO) and anti‐bacterial activity are assayed. Both males and females at the adult and nymphal stages show a strong preference for the diet richest in macronutrients, with an equal preference for C or P. When given a choice between a high C diet or a high P diet, Mormon crickets select both at random, balancing their daily intake of C and P. Weight gain is dependent on the mass of P consumed, with a conversion factor greater than four times that of C consumed. As predicted, Mormon cricket nymphs and adults that consume more P have higher titres of total phenoloxidase and, in addition, lysozyme‐like anti‐bacterial activity is independent of dietary treatment. In nature, omnivores might consume an excess of one macronutrient because they often find the other through active searching of their local habitat. However, environmental change and interspecific or intraspecific competition can challenge the ability of an organism to encounter the required nutrients on a local scale, contributing to long‐distance migratory behaviours.