Toxicity of four insecticides to Spodoptera frugiperda in South Africa

Abstract

The Fall Armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), is described as one of the most important noctuid pests in the Americas. The invasion of the FAW in South Africa during 2017 poses a serious risk, especially for maize production. With 350 larval host plants reported, it is also described as one of the most destructive pests in agriculture. The high dependency on chemical control to effectively manage this pest has resulted in the development of resistance to numerous chemical classes. Since there is no baseline insecticide susceptibility data to the insecticides registered in South Africa, one of the objectives of this study was to estimate baseline susceptibility of three FAW populations to emamectin benzoate, spinetoram/methoxyfenozide and chlorpyrifos, using a leaf dip bioassay method as prescribed by the Insecticide Resistant Action Committee (IRAC). All three populations exhibited high susceptibility to emamectin benzoate and spinetoram/methoxyfenozide, while moderate to high levels of tolerance to chlorpyrifos were found. One of the many insecticides registered for FAW control in South Africa is pyridalyl, an insecticide which is considered to have a novel mode of action. Although pyridalyl has been reported to effectively control FAW in the Americas, no specific method is prescribed by which FAW susceptibility to pyridalyl can be evaluated in bioassays. In this study different bioassay methods, viz. leaf dipping, topical application, diet incorporation and diet overlay, were evaluated to find the most suitable method to assess the toxicity of pyridalyl to S. frugiperda larvae. The topical application bioassay proved to be the most suitable method for susceptibility evaluations of FAW. Fall armyworm was also shown to be highly susceptible to pyridalyl, and all these insecticides can therefore be included in an insect resistant management program for this pest in South Africa.