Insect herbivore stoichiometry: Why macronutrient balance matters

Organismal stoichiometry investigates the movement and balance of elements between organisms, but little is known about how food macronutrient concentration and balance affects the flow of nutrients into an insect herbivore. In this study newly molted 6th-stadium Schistocerca americana nymphs were fed diets with various amounts and ratios of protein (p) and digestible carbohydrate (c) until they molted. For each individual, consumption, mass gain, and development time was measured, and then each insect was analyzed for nitrogen (N), carbon (C), and phosphorus (P) content. Results showed development was best on diets containing balanced p:c ratios, but that growth was best on a balanced diet with high total macronutrient content (42% and 70%, respectively). Food consumption decreased as total macronutrient content increased, and was 2 times greater on the balanced diets with low total macronutrient content (14%). In terms of elemental flow from the food to the insect, three patterns were observed. First, C-gain was correlated with the amount of digestible carbohydrate in the diet. Second, N-gain was best on diets that were balanced and contained 21% or more digestible carbohydrate. Third, P-gain differences were small between treatments, but were best on balanced diets with 21% or more digestible carbohydrate. These results demonstrate the importance of maintaining nutritional balance, and suggest that while nitrogen can be a critical limiting growth factor for insect herbivores, the ability of insect herbivores to efficiently utilize ingested nitrogen is linked intimately to the amount of digestible carbohydrate in the diet.