HERBIVORE FORAGING IN CHEMICALLY HETEROGENEOUS ENVIRONMENTS: NUTRIENTS AND SECONDARY METABOLITES

Abstract

We provide an exemplar study for investigating the manner and extent to which plant secondary metabolites (PSMs) influence nutritional regulation in herbivores selecting among multiple foods. Two experiments were performed using the African mi- gratory locust, Locusta migratoria (L.). In both cases, locusts were given access to multiple synthetic foods that varied in their concentration of the two most strongly regulated nutrients (protein and digestible carbohydrate) and a carbon-based PSM, tannic acid (TA). Insects in the first experiment were given two suboptimal but complementary foods: a high-protein, low-carbohydrate food and a low-protein, high-carbohydrate food. Tannic acid was then added to one food type but not the other, to both, or to neither food type. Here we could see how the addition of TA to a food with a specific protein: carbohydrate profile influenced nutritional regulatory responses. In a second experiment, locusts were given a high-protein, low-carbohydrate food and a low-protein, high-carbohydrate food, both of which contained TA. A third TA-free food containing one of five different protein: carbohydrate ratios was also provided. This experiment provided an opportunity to measure the extent to which a TA-free resource would be incorporated into the diet in relation to its nutrient content. Results indicated that the extent to which locusts regulated their protein: carbohydrate intake depended on the protein: carbohydrate composition of the TA-free foods in their environ- ments. It was evident that TA is more effective as a feeding deterrent than as a post- ingestive toxin, but its effectiveness as a feeding deterrent is strongly linked to the protein: carbohydrate composition of the food in which it occurs. We discuss these findings within the context of plant defense theory and models of foraging behavior.