Caterpillar chemical defense: sources of variation and effects on an endoparasitoid

Background/Question/Methods The role of sequestered chemical defenses against predators is well-established, yet their importance for endoparasitoids is much less well understood.  These two guilds of natural enemies may respond differently to secondary compounds they encounter in their prey or hosts.  In this study, we examined the multi-trophic roles of variation in one group of such chemical defenses, iridoid glycosides.  These compounds are important in the unpalatability of sequestering herbivores to both vertebrate and invertebrate predators.  To assess the effects of sequestration of iridoid glycosides on parasitoids, we analyzed iridoid glycoside content of a specialist insect herbivore, larvae of the Catalpa sphinx, Ceratomia catalpae (Sphingidae) and its host plant, Catalpa bignonioides (Bignoniaceae).  For Catalpa, we also compared the iridoid glycoside content of both damaged and undamaged trees.  To assess the effects of sequestered iridoid glycosides on parasitoids, we compared the iridoid glycoside content of host caterpillars with the success of the specialist hymenopteran parasitoid, Cotesia congregata (Braconidae). 

Results/Conclusions

Overall, leaves of Catalpa trees attacked by C. catalpae contained less of its two major iridoid glycosides, catalpol and catalposide, than leaves of unattacked trees.  Furthermore, increasing catalpol and catalposide concentration in leaves was associated with increased catalpol sequestration by C. catalpae larvae.  For the parasitoid, however, there was no relationship between the iridoid glycoside content of the caterpillar host and C. congregata parasitism success, including field parasitism rate, brood size, and larval survival to adulthood, despite finding evidence that parasitoid larvae consume catalpol.  Our results suggest that iridoid glycoside sequestration is not an effective defense against this endoparasitoid, although sequestration of iridoids effectively deters predators.  These results suggest that different guilds of natural enemies may exert different selective pressures on herbivores to defend themselves against attack.