Costs of sequestration in the buckeye caterpillar (Junonia coenia)

Understanding the costs of herbivore specialization remains a major goal in Ecology and Evolutionary Biology. Theory suggests that specialization comes with trade-offs due to ecological, behavioral, or functional costs. However, these costs have been difficult to detect, complicating efforts to demonstrate trade-offs. The buckeye caterpillar (Junonia coenia) is a diet specialized herbivore, feeding only on plants with iridoid glycosides, which it subsequently sequesters as a defense against predators. We fed buckeye caterpillars a diet with high iridoids present and a diet with low iridoids present. Caterpillars feeding on high iridoid diets had a decreased immune response, and an increased respiration response, indicating a physiological cost to feeding on high concentrations of iridoid glycosides. We hypothesize that the act of sequestration is metabolically costly, and that caterpillars investing resources into sequestration will not be able to mount a strong immune response. Even though sequestered toxins can help defend herbivores from some predators, sequestration may decrease an herbivores defense against more specialized enemies, such as parasitoids. High metabolic rates associated with chemical defense may ultimately translate into evolutionary costs for specialization.