Costs of toxin ingestion versus costs of sequestration in a community of specialized herbivores

When multiple insect herbivores specialize on well-defended plants, they often show convergent adaptations to a class of phytochemical defences. However, herbivore communities are typically composed of taxonomically diverse species with different feeding modes and life histories, and thus not all benefits and costs of adaptation are predicted to be the same. Using common milkweed (Asclepias syriaca), which produces toxic cardenolides, we investigated convergence in insect performance and cardenolide sequestration in the native community of specialist beetles, caterpillars and bugs, feeding on roots, leaves and seeds, respectively. We found minimal correlations between defence of different plant parts, or in the performance of the insect species, suggesting organ-specific plant defence and distinct insect offence strategies. None of the three species was affected by ingesting cardenolides, despite known differences in counter-adaptations. Nonetheless, all species showed a cost of sequestration, which probably emerged from physiological limits. Although variable among species, our findings provide evidence that the evolutionary benefits of sequestration are accompanied by costs, revealing a general constraint among diverse species coevolving with a shared host plant.