Effects of larval exposure to sublethal concentrations of methoxyfenozide in Spodoptera frugiperda (J.E. Smith)

Methoxyfenozide is an ecdysone agonist, the most new class of insect growth regulators. Most studies of the toxicity of ecdysone agonists on lepidopteran pests have been conducted during the larval stages, and little has been published regarding its sublethal effects in the surviving individuals. In this study, the lethal and sublethal effects of methoxyfenozide on the fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) were assessed. For this, fifth instars of this pest were continuously fed, until pupation, with artificial diet containing 0.24 and 0.35 mg of active ingredient/kg diet, which correspond to the LC10 and LC25 values, respectively, for this insecticide. Larval mortality reached 8% and 26% in the low and high concentration groups, respectively, on the seventh day of the experiment. A progressive larval mortality of 12% for the LC10 and 60% for the LC25 was observed before pupation. Treated larvae exhibited lower pupal weights, higher pupal mortality, presence of deformed pupae, and more deformed adults than untreated larvae. The incorporation of methoxyfenozide into the diet had a significant effect on the timing of larval development. Both male and female treated larvae lived about seven days longer than the controls for both concentrations tested. Finally, S. frugiperda adults that resulted from fifth instars treated with methoxyfenozide were not affected in their sex ratio. Our results suggest that the combination of lethal and sublethal effects of methoxyfenozide may have important implications for the population dynamics of the fall armyworm.