DOSAGE-MORTALITY RESPONSES OF 3RD INSTARS OF BEET ARMYWORM (LEPIDOPTERA, NOCTUIDAE) TO SELECTED INSECTICIDES

Susceptibility of third instars of beet armyworm, Spodoptera exigua (H ubner), larvae to registered and experimental insecticides was evaluat ed in a diet overlay bioassay. Larvae were progeny of beet armyworms o riginally collected from cotton fields in Georgia, Louisiana, and Miss issippi. Several field strains exhibited reduced susceptibility to the registered insecticides chlorpyrifos and thiodicarb compared with the ECOGEN laboratory strain. In chlorpyrifos bioassays, all field strain s had significantly higher LC(50)s than the ECOGEN strain. In thiodica rb bioassays, five of the eight field strains had significantly higher LC(50)s than the ECOGEN strain. Five of the eight field strains had s ignificantly higher LC(50)s than the ECOGEN strain in chlorfenapyr bio assays. In emamectin benzoate bioassays, five of the seven field strai ns had significantly lower LC(50)s compared with the ECOGEN strain. Al though considerable variation was observed among field strains in meth oxyfenozide bioassays, only the St. Joseph strain from Louisiana had a significantly higher LC50 than the ECOGEN strain. In both the spinosa d and tebufenozide bioassays, 95% confidence limits (CL) could not be obtained for the ECOGEN laboratory strain; thus, comparisons were base d on 90% CL. All field strains responded similarly to the ECOGEN strai n in the spinosad bioassays, except for the strain from Tallulah, Loui siana, which had a significantly higher LC50. The strains from Tift Co unty, Georgia, and Winnsboro, Louisiana, were the only field strains t o have significantly higher LC(50)s than the ECOGEN strain in tebufeno zide bioassays.