Relative Toxicity and Risk Assessment of Commonly Used Insecticides for the Management of Spodoptera Frugiperda (J. E. Smith) (lepidoptera: Noctuidae).

Insecticide resistance in the fall armyworm, Spodoptera frugiperda, is increasing and forms a serious barrier to sustainable agriculture through the reduction of pest control efficacy and alteration of the insect's biology. The toxicity of seven commonly used synthetic insecticides to six field populations of the pest was evaluated using a controlled diet incorporation bioassay. Most insecticides resulted in only low to moderate resistance (RR = 6.56-8.75), but very high levels of resistance to thiodicarb, especially in the PuL population (RR = 97.88), were confirmed by resistant ratio analysis to be the least toxic agent (RR = 88.71). In contrast, spinetoram was the most effective insecticide, eliciting the highest mortality (95.13%) and the lowest fecundity (2.93 eggs/female/day); the chronic toxicity of the insecticide also caused a significant delay in most crucial developmental stages, such as egg hatching, larval growth, and pupation, which were only marginally affected by thiodicarb. In addition, all the effective treatments, except thiodicarb, significantly reduced all the major parameters of population growth, such as rm, Tc, Ro, and the female proportion. The findings call for continuous monitoring of resistance and justify the rotation of such potent insecticides as spinetoram in the resistance management of S. frugiperda.