| Two major conceptual approaches for understanding the evolutionary ecology of insect-plant interactions, the plant-insect paradigm and the tri-trophic paradigm, have focused primarily upon dietary specialists and their host-plants. Here, I attempted to evaluate the utility of both paradigms for explaining the maintenance of food-mixing by the individually polyphagous caterpillars of Grammia geneura (Strecker) (Lepidoptera: Arctiidae). First, I conducted three experiments testing the hypothesis that individual G. geneura caterpillars perform better on mixed-plant diets than on single-plant diets due to improved physiological efficiency of food utilization. However, caterpillar performance was not always superior on mixed-plant diets. In the one case in which food-mixing improved performance, the host-plant species included in the mixture were individually of low suitability. Behavioral observations of individual caterpillars both in the above experiments, in nature, and in two further laboratory experiments with chemically-manipulated, synthetic diets supported the idea that such dietary benefits resulted from dilution of plant secondary metabolites, achieved behaviorally via the physiological mechanisms of neophilia and post-ingestive feedbacks on feeding.;I also investigated the possibility that food-mixing was maintained by the unpredictable availability of high-quality host-plant species. A field survey of caterpillar feeding preference, frequency of parasitism, and host-plant availability suggested that this variation in food availability combined with the increased risk of parasitism incurred by individuals experiencing prolonged development (e.g. by searching excessively for a rare, preferred host-plant species) should favor polyphagy, and reinforce opportunistic food-mixing. However, because individual caterpillars showed a tendency to leave nutritionally superior host species for nutritionally inferior ones, I tested the idea that individuals ate some host-plant species for defense against parasitoids. Two experiments showed that diet modified the survival of parasitized caterpillars, and that at least one pair of host-plant species revealed a trade-off between their nutritive and defensive value to caterpillars. Taken together, the experiments in this study underscore the importance of the tri-trophic approach toward understanding the pattern and process of foraging in generalist as well as specialist herbivores. |
