Development of diagnostic concentrations for insecticide resistance monitoring in soybean looper (Lepidoptera : Noctuidae) larvae using an artificialdiet overlay bioassay

Diagnostic concentrations for several standard and experimental insecticide s were determined for a laboratory reference strain of soybean looper, Pseu doplusia includens (Walker), using an insecticide diet overlay bioassay to evaluate the relative susceptibility of field (P) and F-1 generations of fo ur field-collected strains of third-, fourth-, and fifth-instar soybean loo pers in 1996 and 1997. Diagnostic concentrations were defined as concentrat ions that killed 90-95% of the susceptible individuals and were 5 ppm for p ermethrin, 1,300 ppm for thiodicarb, 60 ppm for chlorfenapyr, 5 ppm for ema mectin benzoate, and 60 ppm for spinosad. Field strains exhibited significa ntly greater percentage survival than the laboratory reference strain in th e permethrin bioassays in 1996 and 1997 in both the P and F-1 generation bi oassays and in the thiodicarb bioassays in 1997. Larvae exposed to diagnost ic concentrations of the experimental insecticides chlorfenapyr, emamectin benzoate, and spinosad usually did not exhibit significantly higher percent age survival than the reference strain.