Antifeedant discrimination thresholds for two populations of western corn rootworm

Sensitivities to β-hydrastine, strychnine and picrotoxinin were compared between two populations of western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), differing in their susceptibility to cyclodiene insecticides. The antifeedants are antagonists of γ-aminobutyric acid (GABA) neuroreceptors that may mediate gustation and, in the case of picrotoxinin, interact directly with the target site for cyclodiene insecticides. In combination with cucurbitacin B, a specific Diabroticite phagostimulant, cyclodiene-resistant beetles were two- to four-fold less sensitive to hydrastine and strychnine. The discrimination threshold for picrotoxinin was higher for resistant beetles; however, overall dose-response profiles were similar. Beetles were more sensitive to hydrastine and strychnine in combination with L-alanine, a general phagostimulant of less potency than cucurbitacin B. Dose-response slopes for the alkaloid-alanine combinations were double those of respective cucurbitacin B slopes, which indicates that phagostimulatory input from L-alanine was more negatively affected by the alkaloidal antifeedants than the respective cucurbitacin B treatments. Picrotoxinin sensitivity was similar in mixtures with either phagostimulant. Comparison of chemosensillum responses to strychnine was inconclusive. Results are discussed in context of GABA receptor pharmacology.