Effects of protein and carbohydrate on an insect herbivore: the vista from a fitness landscape

Protein and carbohydrates are important nutrients driving the growth of herbivores; however, their content in plants is highly variable. Multiple studies have explored their effect on herbivores, but only one other study (using a caterpillar) has provided a comprehensive overview that includes a simultaneous evaluation of their ratios and concentrations. In the present work, we ran two experiments using nymphs of the generalist grasshopper Melanoplus differentialis. Grasshoppers and caterpillars differ in a number of important ways, which might affect their feeding and physiological responses to foods with variable content of protein and carbohydrates. First, in a choice experiment, we measured performance and related this to the self-selected intake of nutrients. No differences were found for duration of development across treatments, but gain in mass was lower on a diet of low macronutrient concentration. Consumption of protein was always tightly regulated, but intake of carbohydrate was significantly reduced when consuming diluted food. In the second experiment, insects were constrained to one of nine diets and we plotted performance and consumption using a fitness-landscape approach that mimics the natural variation of nutrients in plants. We found significant effects of protein and carbohydrate content on gain in mass and in duration of development. The concentration of macronutrients in the food had more pronounced effects than did the protein-to-carbohydrate ratio. The protein-carbohydrate content also significantly affected the intake of food and energy (calories), production of frass, and digestive efficiency. On foods with low macronutrient concentration consumption was high, but digestive efficiency was low. Our results suggest that insects will favor protein-biased foods when the total macronutrient content of available foods is low, and that in the short-term compensatory feeding responses can overcome nutritional deficits and/or imbalances. However, over the long term, insect herbivores might pay substantial costs when eating foods that are nutritionally suboptimal.