Toxicity of plant defensive compounds weakens herbivore immunity, and parasitoids are considerably more successful on herbivores with weak immune responses. In diverse tropical systems, there is still a great deal to be learned about the causes and consequences of variation in herbivore immunity. We are investigating two general questions about tropical herbivore immunity: 1) How does phytochemical diversity influence herbivore immunity and levels of parasitism and how are these relationships affected by diet breadth? And 2) How do these effects vary across different herbivore species and different locations? To answer these questions we used the diverse plant genus, Piper (Piperaceae), which has diverse defensive compounds and hosts complex arthropod communities. We worked at two sites for which we have constructed a large record of plant-herbivore-parasitoid interactions on Piper in Costa Rica and Ecuador. We collected the specialist herbivores, Eois (Geometridae) and Quadrus (Hesperiidae) for immune assays. In these assays we measured the phenoloxidase response of the caterpillars, which is one of the most important components of the insect immune system.
Results/Conclusions
Similar to previous studies on Piper, we found that phytochemical diversity was an important predictor for herbivore immunity, herbivore parasitism, and diet breadth for specialist caterpillars, but that the specifics of these relationships differed between sites. In Costa Rica, immune function for more specialized herbivores was negatively associated with the phytochemical diversity of the Piper host plant, and rates of parasitism decreased with higher immune function. The opposite relationship was true for specialist caterpillars in Ecuador. Furthermore, phytochemical diversity did not affect herbivore immunity and parasitism for the more generalized herbivores. Results also indicated that herbivore diet breadth is an important factor mediating herbivore immunity and parasitism success for Eois both in Costa Rica and Ecuador. These patterns contribute to a growing body of literature that demonstrate strong cascading effects of phytochemistry on higher trophic levels, which is a key component of tropical diversity.