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 d iet that was surface-treated with several concentrations of selected i nsecticides (permethrin [Ambush], Bacillus thuringiensis variety kurst aki [Condor OF], thiodicarb [Larvin], chlorfenapyr [Pirate], emamectin benzoate [Proclaim], or spinosad [Tracer]). LC50 s (72 h) for field s trains were compared with a susceptible USDA reference strain to evalu ate 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 s trains and the USDA strain in the B. thuringiensis, thiodicarb, and em amectin benzoate bioassays. In the chlorfenapyr bioassays, only 1 fiel d strain from Winnsboro, LA, had a significantly greater LC50 than tha t 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 LC5 0 than that of the USDA strain. These data will serve as a historical database for monitoring soybean looper resistance to these compounds a nd should prove useful in the development of an insecticide resistance management program for this pest.