Beet Armyworm (Lepidoptera: Noctuidae) Adult and Larval Susceptibility to Three Insecticides in Managed Habitats and Relationship to Laboratory Selection for Resistance
A field bioassay that measures adult male susceptibility was used to document resistance to fenvalerate, permethrin, and methomyl in beet armyworm, Spodoptera exigua (Hubner), in California, Baja California Norte (Mexico), and Sinaloa (Mexico). At the LC50, the highest levels of resistance (R) were typically found at sites in Monterey County, Calif. (resistance [R] ratio of 11 and 29 for fenvalerate and methomyl, respectively); Kern County, Calif. (R ratio of 10 and 19 for fenvalerate and methomyl, respectively), and the Del Fuerte and Guasave valleys of Sinaloa (R ratio of 22, 7, and 16 for fenvalerate, permethrin, and methomyl, respectively). Geographic and temporal variability in resistance followed this trend: overall variation among regions > variation among sampling dates at the same site within a region ≥ variation among sites in a region within three consecutive days. Concurrent with field bioassays, larval susceptibility was measured with topical application bios says in the laboratory. Resistance was detected in larvae. Larval and adult susceptibility were significantly correlated for fenvalerate and methomyl (r = -0.84 and -0.78, respectively). Selection experiments with fenvalerate and methomyl on larvae confirmed potential for increases in resistance. For fenvalerate, adult susceptibility was similar to larval susceptibility in selection experiments, agreeing with the above adult-larval correlations. This correspondence was not apparent for methomyl.