The use of foliar insecticides has historically been a common management strategy for lepidopteran pests in cotton, Gossypium hirsutum (L.). Bioassays to establish initial toxicity levels and surveys of changes in insecticide susceptibility for field insect populations are critical components of an insecticide resistance management (IRM) program. A novel insecticide in the anthranilic diamide class, rynaxypyr® (chlorantraniliprole), has demonstrated significant levels of toxicity to several lepidopteran targets in preliminary field screening trials. The objectives of this study were to evaluate three bioassay methods (insecticide-treated diet, topical application, and adult vial test) for potential use in future insecticide resistance surveys and to determine the baseline toxicity of rynaxypyr® to selected lepidopteran pests. Rynaxypyr® was highly toxic to bollworm, Helicoverpa zea (Boddie), fall armyworm, Spodoptera frugiperda (J.E. Smith), and tobacco budworm, Heliothis virescens (F.), in all three laboratory bioassay procedures. Larvae of the three species demonstrated similar susceptibility to rynaxypyr® in the insecticide-treated diet bioassay with LC50 values ranging from 0.02-0.09 ppm. Rynaxypyr® susceptibility among the three species was also observed in the topical application assay with LD50 values ranging from 0.52 to 1.52 µg/g larval weight. There were no significant differences in rynaxypyr® susceptibility between the laboratory tobacco budworm (LSU lab TBW, 1.21 μg/vial) and field-collected fall armyworm (Louisiana FAW, 1.71 μg/vial) colonies in the adult vial test (AVT). No evidence of rynaxypyr® cross-resistance was detected in pyrethroid-resistant bollworm and tobacco budworm colonies. The consistent results generated with the bioassay procedures suggest that all methods can be used for future rynaxypyr® resistance surveys of these target pests.
