The effects of food macronutrient content on an insect herbivore: a fitness landscape approach

Background/Question/Methods

The plants that insect herbivores eat can be highly variable with respect to their protein and digestible carbohydrate content.  Here we explore the effects of food protein-carbohydrate balance and concentration on feeding behavior, performance and food utilization in the generalist grasshopper Melanoplus differentialis.  We conducted two separate experiments. In the first experiment we presented final instar nymphs with two foods that differed in their protein-carbohydrate content, and measured the extent to which individuals actively regulated their protein-carbohydrate intake, and whether regulation was similar across the different treatments.  In the second experiment we presented insects with one of nine foods that differed in their ratio and/or amount of protein and carbohydrate, and measured consumption, performance and nutrient utilization.  For this second experiment, the results are presented as detailed fitness landscape.  This visualization technique provides a comprehensive overview of how protein-carbohydrate content affects an insect herbivore, and a powerful link between lab and field studies.

Results/Conclusions

In the first experiment, we found that grasshoppers tightly regulated their protein intake across all treatments.  However, when the available foods had low total nutrient content, carbohydrate intake decreased; these insects also gained less mass.  In the second experiment, where insects were constrained to one of nine foods, grasshoppers initially consumed larger quantities of carbohydrate-biased food.  Over the entire stadium, however, insects on the most dilute diet ate the most food.  Development time decreased as macronutrient concentration increased, but mass gain and growth rate were best on diets that were:  1) concentrated and 2) carbohydrate biased.  Interestingly, food utilization, measured as the amount of food digested (food eaten minus frass production), was best on diets with low protein and moderate carbohydrate content.  We interpret our results in relation to the protein-carbohydrate landscape occupied by insect herbivores in the field.