The interplay between nutrient balancing and toxin dilution in foraging by a generalist insect herbivore

Food mixing by herbivores is thought to balance nutrient intake and possibly dilute secondary metabolites characteristic of different host plant species. Most empirical work on insect herbivores has focused on nutrient balancing in laboratory settings. In this study, we characterize food mixing behaviour of the caterpillar Grammia geneura (Strecker) (Lepidoptera: Arctiidae) in nature and use the observed patterns to design ecologically relevant experiments that reveal the relative importance of these processes in food-switching behaviour. Our design involved both choice and no-choice experiments with chemically defined diets in which primary nutrients and secondary metabolites were manipulated in tandem. We analysed two stages in the process of food-switching behaviour: leaving food and accepting new food. In nature, an individual's rate of leaving host plants was positively associated with its probability of rejecting plant species most recently eaten, but not related to its probability of accepting different host plant species. Furthermore, an individual's leaving rate was negatively related to its average feeding bout duration. This relationship resulted partly from variation in the response of individuals to nutrient imbalance and partly from shortened feeding bouts prior to switching, suggesting that a decline in feeding excitation preceded searching for food that differed from that most recently eaten. Laboratory experiments with synthetic diets indicated the importance of secondary metabolites in the decline in feeding excitation prior to switching. Preference for new food depended strongly on secondary metabolites in a manner consistent with toxin dilution. This is the first experimental evidence for the process of toxin dilution in caterpillars, and for the combined influence of nutrients and secondary metabolites on their foraging patterns in nature.