Antifeedant Effects of Some Novel Terpenoids on Chrysomelidae Beetles: Comparisons with Alkaloids on an Alkaloid-Adapted and Nonadapted Species

Structure–dose–feeding deterrency relationships were compared between the Colorado potato beetle, Leptinotarsa decemlineata (Say), and the western corn rootworm, Diabrotica virgifera virgifera LeConte, using 15 alkaloids, terpenoids, and phenolic derivatives. The former species, a specialist herbivore on selected alkaloid-rich Solanaceae species, was on average 100-times less sensitive to the antifeedant effects of alkaloids, but more similarly sensitive to the terpenoids and phenolics than the latter species, a generalist flower herbivore predominantly on Graminae, Cucurbitaceae, and Compositae species. Antifeedant ED₅₀ values for the potato beetle and corn rootworm, each from closely related subfamilies of Chrysomelidae, ranged over four orders of dose magnitude among the 15 compounds with major species differences in stereosensitivity to -hydrastines and analog sensitivity with the silphinenes. Extremes in sensitivity ranged from silphinene, a rare tricyclic sesquiterpene that is 53 times more active on the potato beetle to aconitine, which is 430 times more antifeedant to the corn rootworm. Among silphinene and its two hydrolysis derivatives, there was not a strong correlation between antifeedant potency and injected toxicity for the two beetle species, but there was correlation between behavioral activity and galeal taste cell electrophysiological threshold and frequency responses. That all of the established GABA- and glycinergic compounds tested were antifeedant for both species suggests a shared molecular mechanism for antifeedant taste chemoreception in these divergent Chrysomelidae species. Moreover, the wide differences in antifeedant sensitivities among these and other chrysomelids to a suite of ligand-gated ion channel antagonists implicate a common protein neuroreceptor type with extraordinary heterogeneity in beetle taste.