Responses of catalpa sphinx and its primary parasitoid to varying levels of host plant iridoid glycosides

The catalpa sphinx, Ceratomia catalpae (Lepidoptera: Sphingidae), occurs in the eastern US and feeds exclusively on Catalpa spp., which contain iridoid glycosides, a class of secondary metabolites, consisting mainly of catalpol and catalposide. Many catalpa trees are heavily infested and defoliated by catalpa sphinx caterpillars each year whereas others are untouched. Likewise, most populations of catalpa sphinx caterpillars are heavily parasitised by the braconid wasp, Cotesia congregata, but some populations remain unparasitized. We hypothesized that differences in caterpillar infestations and parasitism could be explained by differential concentrations of iridoid glycosides. Gas chromatography was used to quantify iridoid glycoside concentrations in trees at four locations. Results demonstrated that catalposide concentrations vary among trees and that trees with higher levels of catalposide were more heavily defoliated than those with lower levels. Ovipositional preferences of catalpa sphinx moths were assayed by offering moths a choice of two trees with high or low catalposide levels within an greenhouse enclosure. Preliminary results suggest that moths preferred trees with high rather than low catalposide concentrations. Parasitoid responses to high or low catalposide trees were assayed by quantifying searching responses of 2-day old females to leaf discs. Preliminary results suggest that female searching times on high and low catalposide leaf material are similar. This work is still ongoing.