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.