Development of discriminating concentrations for insecticide resistance monitoring in the soybean looper.
Discriminating concentrations of several standard and experimental insecticides were determined for an insecticide-susceptible strain of Pseudoplusia includens [Chrysodeixis includens] (soyabean looper) using an insecticide diet overlay bioassay. These concentrations were used to evaluate the relative susceptibility of field and F1 generations of 3 strains of C. includens larvae collected from soyabean fields in Iberia, Pointe Coupee and Franklin parishes in Louisiana in 1996. Field strains exhibited significantly higher percent survival compared with the susceptible reference strain (USDA) when exposed to permethrin (Ambush), Bacillus thuringiensis subsp. kurstaki (Condor XL) or thiodicarb (Larvin) as larvae directly from the field or as FI laboratory-reared larvae. Soyabean looper larvae collected from Bt-cotton had higher survival when exposed to B. thuringiensis subsp. kurstaki than the susceptible reference strain. Larvae from field strains exposed to the discriminating concentration of chlorfenapyr (Pirate) and emamectin benzoate (Proclaim) did not exhibit significantly higher survival than that of the reference strain. In the emamectin benzoate bioassays, larval survival for two field strains of the field generation was significantly lower than that of the reference strain. In the Tracer bioassays, two strains in the field generation bioassays and one strain in the F1 generation bioassay had survival significantly higher than the susceptible reference strain. These differences may have been due to natural variation in the soyabean looper population, but need further investigation.