Dosage-mortality responses of third instars of beet armyworm (Lepidoptera : Noctuidae) to selected insecticides

摘要

Susceptibility of third instars of beet armyworm, Spodoptera exigua (Hubner), larvae to registered and experimental insecticides was evaluated in a diet overlay bioassay. Larvae were progeny of beet armyworms originally collected from cotton fields in Georgia, Louisiana, and Mississippi. Several field strains exhibited reduced susceptibility to the registered insecticides chlorpyrifos and thiodicarb compared with the ECOGEN laboratory strain. In chlorpyrifos bioassays, all field strains had significantly higher LC(50)s than the ECOGEN strain. In thiodicarb bioassays, five of the eight field strains had significantly higher LC(50)s than the ECOGEN strain. Five of the eight field strains had significantly higher LC(50)s than the ECOGEN strain in chlorfenapyr bioassays. In emamectin benzoate bioassays, five of the seven field strains had significantly lower LC(50)s compared with the ECOGEN strain. Although considerable variation was observed among field strains in methoxyfenozide bioassays, only the St. Joseph strain from Louisiana had a significantly higher LC50 than the ECOGEN strain. In both the spinosad and tebufenozide bioassays, 95 confidence limits (CL) could not be obtained for the ECOGEN laboratory strain; thus, comparisons were based on 90 CL. All field strains responded similarly to the ECOGEN strain in the spinosad bioassays, except for the strain from Tallulah, Louisiana, which had a significantly higher LC50. The strains from Tift County, Georgia, and Winnsboro, Louisiana, were the only field strains to have significantly higher LC(50)s than the ECOGEN strain in tebufenozide bioassays.