Toxicity bioassays with insecticide formulations used for control of Spodoptera frugiperda (Lepidoptera : Noctuidae)
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The fall armyworm, Spodoptera frugiperda, is an exotic pest of maize in Africa. It is polyphagous and has the ability to develop on a wide range of host plants with different nutritional indices. Rearing of insects is important for entomological research. After evaluation of several artificial diets, the most suitable artificial diet for rearing of S. frugiperda larvae was determined in this study. The following four diets were evaluated: Busseola fusca diet, Anticarsia gemmatalis diet, Stonefly Heliothis diet, Chillo partellus diet and maize leaves. The following fitness parameters were used to compare suitability of the different diets for S. frugiperda development: larval and pupal development, pupal mass, survival, adult eclosion, fecundity and fertility. The B. fusca artificial diet was determined as the most suitable for rearing of S. frugiperda. The nutritional composition of the respective diets differed as well as the water content. These factors affected the fitness parameters. The B. fusca diet is, however, not suitable for toxicological studies with insecticides incorporated into the diet since the temperature while preparing the diet is too high for incorporation of insecticides. The Stonefly Heliothis diet was therefore selected for rearing and use in toxicological studies. Monitoring of insecticide efficacy is used for proactive evidence-based resistance management. Baseline susceptibility of S. frugiperda should be determined to monitor its susceptibility to pesticides in future. Four bioassay methods viz. leaf dipping, topical application, insecticide overlay onto and incorporation in artificial diets were evaluated for use in toxicological studies with insecticides with different modes of entry and action. These insecticides were chlorantraniliprole (diamide), lufenuron (benzolureas), pyridalyl (unknown) and methomyl (carbamate). Dose responses of third-instar S. frugiperda larvae were evaluated with PoloSuite, and statistical parameters were analysed to determine the most appropriate bioassay for the different insecticide groups. The insecticide incorporation into artificial diet bioassay was identified as the most suitable for susceptibility evaluation of S. frugiperda to chlorantraniliprole and methomyl. For lufenuron, the most suitable method was the insecticide overlay onto artificial diet. No suitable bioassay could be developed for the evaluation of susceptibility to pyridalyl, which may be explained by its unknown mode of entry and action. The difference in suitability of bioassays determined for each insecticide showed that the mode of entry into the insect has a profound effect on the effectivity of bioassays and also on the estimate of the median lethal dose.