Generalist insect herbivores show host-plant dependent differences in both bi-trophic performance (i.e., growth) and parasitism frequencies. The relationship between bi-trophic performance and realized parasitism (i.e., parasitoid emergence) on a given host plant may vary according to factors including parasitoid attack rates and diet-based effects on immune function. Similar realized parasitism frequencies among host-plants might obscure host-based differences in immune function if attack rates greatly differ. Conversely, higher attack rates on certain host-plants may falsely imply an inferior immune function. In this study I test the hypothesis that host-plants affording the greatest larval growth also confer enhanced immunocompetence. I compared host-associated growth, realized parasitism frequencies, and melanization capability in the larvae of three lepidopteran species: Achatia distincta, Himella intractata, and Hyphantria cunea. Host-specific parasitism and performance data were obtained from caterpillars collected from Connecticut forests and reared in the laboratory from 2004-2009. I determined immune function by injecting caterpillars with glass beads and measuring the degree to which beads were melanized after 24 hours.
Results/Conclusions
No significant host-based differences were found for mean percent melanization of the beads or parasitism frequency, however host-associated growth differed significantly amongst hosts for all three species. Coefficient of variation of bead melanization within a single caterpillar varied significantly with host plant for H. intractata and A. distincta. Significant differences mediated by host plant on minimum and maximum melanization scores were also found for these species. These differences indicate that host-associated differences in immune function may be important in cases of multi- or super-parasitism, yet may not heavily influence cases of single parasitoids. For Prunus serotina, the host-plant for which I had data for all three caterpillar species, mean percent melanization was positively correlated with the overall parasitism frequency of each species. This suggests that greater overall parasitism risk selects for greater investment in the immune system.