ANTIFEEDANT DISCRIMINATION THRESHOLDS FOR 2 POPULATIONS OF WESTERN CORN-ROOTWORM

Sensitivities to beta-hydrastine, strychnine and picrotoxinin were com pared between two populations of western corn rootworm, Diabrotica vir gifera virgifera LeConte (Coleoptera: Chrysomelidae), differing in the ir susceptibility to cyclodiene insecticides. The antifeedants are ant agonists of gamma-aminobutyric acid (GABA) neuroreceptors that may med iate gustation and, in the case of picrotoxinin, interact directly wit h the target site for cyclodiene insecticides. In combination with cuc urbitacin B, a specific Diabroticite phagostimulant, cyclodiene-resist ant beetles were two- to four-fold less sensitive to hydrastine and st rychnine. The discrimination threshold for picrotoxinin was higher for resistant beetles; however, overall dose-response profiles were simil ar. Beetles were more sensitive to hydrastine and strychnine in combin ation with L-alanine, a general phagostimulant of less potency than cu curbitacin B. Dose-response slopes for the alkaloid-alanine combinatio ns were double those of respective cucurbitacin B slopes, which indica tes that phagostimulatory input from L-alanine was more negatively aff ected by the alkaloidal antifeedants than the respective cucurbitacin B treatments. Picrotoxinin sensitivity was similar in mixtures with ei ther phagostimulant. Comparison of chemosensillum responses to strychn ine was inconclusive. Results are discussed in context of GABA recepto r pharmacology.