Toxicity of plant defensive compounds has been shown to weaken herbivore immunity, and parasitoids are considerably more successful on herbivores with weak immune responses. There is a great deal to be learned about the causes and consequences of variation in herbivore immunity in diverse tropical communities. We are investigating three general questions about tropical herbivore immunity: 1) Does phytochemical diversity of a host plant genus predict immune response in caterpillar species? 2) How do immune responses of caterpillars on the same tropical host genus vary? and 3) How do caterpillar immune responses to different host species vary across large geographic scales? To answer these questions we used the diverse plant genus, Piper (Piperaceae), as it is well studied and has diverse defensive compounds and hosts complex arthropod communities. We worked at two sites that already have 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 as phytochemical diversity increased, caterpillar immune response weakened. Additionally, we found Quadrus species to have a stronger immune response compared to Eois – something that might be caused by the fact that Eois caterpillars are more specialized than Quadrus species. Overall, Piper species had variable impacts on caterpillars, with a few Piper species having unusually large effects on the immune response. Our results show that plant secondary defense compounds can be detrimental to herbivore immune systems, which can influence parasitoid success. Moreover, our study highlights the importance of plant chemistry mediating tri-trophic interactions, which is a key component of tropical diversity.