Responses of field-collected strains of soybean looper (Lepidoptera: Noctuidae) to selected insecticides using an artificial diet overlay bioassay

Third-instar F-1 progeny of several field-collected strains of soybean looper, Pseudoplusia includens (Walker), were exposed to artificial diet that was surface-treated with several concentrations of selected insecticides (permethrin [Ambush], Bacillus thuringiensis variety kurstaki [Condor OF], thiodicarb [Larvin], chlorfenapyr [Pirate], emamectin benzoate [Proclaim], or spinosad [Tracer]). LC50 s (72 h) for field strains were compared with a susceptible USDA reference strain to evaluate possible tolerance to these insecticides. Significant differences were found among LC50 s of all field strains and the susceptible USDA reference strain in the permethrin bioassays and among several field strains and the USDA strain in the B. thuringiensis, thiodicarb, and emamectin benzoate bioassays. In the chlorfenapyr bioassays, only 1 field strain from Winnsboro, LA, had a significantly greater LC50 than that of the USDA strain. In the spinosad bioassays, the only field strain with a significantly different LC50 than that of the USDA strain was the strain collected from Hamburg, LA, and this strain had a lower LC50 than that of the USDA strain. These data will serve as a historical database for monitoring soybean looper resistance to these compounds and should prove useful in the development of an insecticide resistance management program for this pest.