Susceptibility of Spodoptera frugiperda to chemical insecticides and control failure likelihood estimation in Burkina Faso

The fall armyworm, Spodoptera frugiperda Smith (Lepidoptera: Noctuidae) has recently invaded sub-Saharan African countries. It has been causing significant losses to maize, pearl millet, sorghum, rice, and other crops since 2016. Several insecticides have been promoted, but farmers have reported control failures.

In this work, we collected seven fall armyworm populations in as many maize-producing areas of Burkina Faso. We used the approved IRAC foliar bioassay protocol to evaluate the susceptibility of third instar larvae to six chemical insecticides commonly used by maize farmers, including different modes of action (active ingredient): acetylcholinesterase inhibitors (methomyl, chlorpyriphos-ethyl), sodium channel modulators (deltamethrin, lambda-cyhalothrin), chloride channel activators (emamectin benzoate, abamectin). Lethal concentrations and control failure likelihood (CFL, using Guedes' 2017 formula) were calculated for each population and active ingredient.

For all seven populations of S. frugiperda, LC80 values are significantly higher than the manufacturer's recommended rates and CFL values are high, 66-100% for both pyrethroids (deltamethrin, lambda-cyhalothrin) and abamectin. In contrast, the LC80 values for emamectin benzoate emamectin benzoate (0.001 mg/l), methomyl (43 - 278 mg/l) and chlorpyrifos-ethyl (284 - 659 mg/l) are lower than the recommended rates. Emamectin benzoate, methomyl and chlorpyriphos-ethyl insecticides have a negligible risk of control failure (i.e. their CFL values are below 0%) and are the most efficient for the control of the fall armyworm in Burkina Faso. These active ingredients should be alternated in the control of the pest to delay the development of resistance.