Baseline Toxicity Data of Different Insecticides against the Fall Armyworm Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) and Control Failure Likelihood Estimation in Burkina Faso

The fall armyworm (Spodoptera frugiperda) is a worldwide maize pest originating from the American continent. It invaded Africa during 2016, causing important economic damages, forcing African countries to take urgent actions to tackle this new invasive pest. In Burkina Faso, several chemical insecticides were promoted, but farmers have quickly and repeatedly reported control failures. In this work, we collected seven fall armyworm populations in as many maize-producing areas of Burkina Faso. Following the approved IRAC leaf bioassay protocol, we evaluated the susceptibility of third instar larvae to seven commercially available insecticide formulations, including various modes of action: methomyl and chlorpyriphos-ethyl (acetylcholinesterase inhibitors), deltamethrin and lambda-cyhalothrin (sodium channel modulators), emamectin benzoate and abamectin (chloride channel activators) and Bacillus thuringiensis (a microbial disruptor of insect midgut membranes). Lethal concentrations (LC50), resistance ratios (RR50) and relative toxicity were calculated for each population and active ingredient. LC50 values for all S. frugiperda populations were, in order of importance: emamectin benzoate (0.33–0.38 µg/l), methomyl (18–73 mg/l), abamectin (58–430 mg/l), chlorpyrifos-ethyl (199–377 mg/l), deltamethrin (70–541 mg/l) and lambda-cyhalothrin (268–895 mg/l). LC50 of the B. thuringiensis formulation ranged from 430 to 614 MIU/l. Lambda-cyhalothrin was the least efficient of the tested chemical pesticides, and emamectin benzoate the most efficient (relative toxicity ×2 712 969). Methomyl (×49), abamectin (×15), deltamethrin (×13), chlorpyriphos-ethyl (×4) were also more toxic than lambda-cyhalothrin. Based on these results, we conclude that emamectin benzoate, methomyl and chlorpyriphos-ethyl insecticides are the most efficient for the control of the fall armyworm in Burkina Faso. We discuss the importance to implement a national-level resistance survey for this major pest, which would allow rapid and efficient adaptation of the control strategy.