COS 8-8
Cascading effects of diet induced variation in the immune response of two specialist herbivores
Understanding sources of variation in the animal immune response is an important goal in the emerging field of ecological immunology. In this study, we sought to understand the influence of plant diet as a source of variation in the immune response of two specialist caterpillars, Eois apyraria and Eois nympha (Geometridae) and whether this variation is manifested by parasitism success. Caterpillars were field collected in a lowland tropical rainforest in Costa Rica and assigned to one of two plants in the genus Piper (P. imperiale, P. cenocladum). Piper is a diverse, pantropical genus and has a tight, possibly coevolved relationship with caterpillars in the genus Eois. After reaching fifth instar, larvae were bled to collect hemolymph and the strength of the immune response measured using a colormetric assay of phenoloxidase (PO) enzyme activity. Parasitism data used in the analysis came from 20 years of accumulated data from a long-term rearing project at La Selva Biological Station.
Results/Conclusions
We found that overall E. apyraria had a higher immune response than E. nypmha. Variation by plant species showed that both Eois species had a significantly stronger immune response on P. imperiale compared to P. cenocladum. These data were supported by the parasitism data, which showed that overall parasitism rates were higher on P. cenocladum. Interestingly E. nympha individuals were far less likely to be found on P. imperiale and had significantly lower survivorship to adults when feeding on this host. In contrast E. apyraria could be found on either host and survivorship was not affected by host plant species. Prior work with the chemistry of P. imperiale and P. cenocladum shows that they differ by key amide compounds, and it is possible that the differences between the immune response and parasitism rates are linked to the secondary chemistry of these two plants. In conclusion, we can speculate that the secondary chemistry of these two Piper plants may have cascading effects to the third trophic level via the insect immune response. Follow-up studies are underway to understand how plant chemistry and the insect immune response may structure parasitoid communities.