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 eval uated in a diet overlay bioassay by using 2-d-old larvae. Larvae were collected from cotton, Gossypium hirsutum L., fields in Alabama, Calif ornia, 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 refe rence strain from the Southern Insect Management Laboratory at Stonevi lle, MS. In thiodicarb bioassays, 3 of the 10 field strains had signif icantly higher LC(50)s than the reference strain. Generally, LC(50)s o f field strains to the experimental insecticides chlorfenapyr, spinosa d, and tebufenozide were comparable to those of the reference strain. In chlorfenapyr bioassays, a field strain from Red Cross, LA, had a si gnificantly higher LC50 than the reference strain. In spinosad bioassa ys, 3 field strains had LC(50)s that were significantly lower than the reference strain. In tebufenozide bioassays, a field strain from Star kville, MS, had a significantly higher LC50 than the reference strain.