Susceptibility of field populations of beet armyworm (Lepidoptera: Noctuidae) to commercial and experimental insecticides

Susceptibility of field-collected strains of beet armyworm, Spodoptera exigua (Hubner), to commercial and experimental insecticides was evaluated in a diet overlay bioassay by using 2-d-old larvae. Larvae were collected from cotton, Gossypium hirsutum L., fields in Alabama, California, Louisiana, Mississippi, and Texas, and from Rio Brave, Mexico. Several field strains exhibited reduced susceptibility to chlorpyrifos and thiodicarb, which are the only 2 labeled insecticides recommended for beet armyworm control in Louisiana. In chlorpyrifos bioassays, 7 of the 11 field strains had significantly higher LC(50)s than the reference strain from the Southern Insect Management Laboratory at Stoneville, MS. In thiodicarb bioassays, 3 of the 10 field strains had significantly higher LC(50)s than the reference strain. Generally, LC(50)s of field strains to the experimental insecticides chlorfenapyr, spinosad, and tebufenozide were comparable to those of the reference strain. In chlorfenapyr bioassays, a field strain from Red Cross, LA, had a significantly higher LC50 than the reference strain. In spinosad bioassays, 3 field strains had LC(50)s that were significantly lower than the reference strain. In tebufenozide bioassays, a field strain from Starkville, MS, had a significantly higher LC50 than the reference strain.