Sublethal doses of broflanilide prevents molting in the fall armyworm, Spodoptera frugiperda via altering molting hormone biosynthesis

Broflanilide is a novel insecticide with a unique mode of action on the insect GABA receptor and is registered worldwide for the control of agricultural pests. It shows high efficacy in controlling the fall armyworm (FAW) Spodoptera frugiperda, which is a destructive pest to various crops. FAW was exposed to sublethal concentrations of broflanilide to determine its impact on insect development. Sublethal doses (LD10 and LD30) caused failure of ecdysis, reduced body length of larvae, malformation of pupae, and vestigial wing formation in adults. Also, broflanilide at LD30 significantly reduced the amount of molting hormone (MH). After exposure to LD10 or LD30 broflanilide, expression of five Halloween genes, which participate in MH biosynthesis, were found to be altered. Specifically, the transcript levels of SfrCYP307A1 (Spook), SfrCYP314A1 (Shade) and SfrCYP315A1 (Shadow) in 3rd day larvae were significantly decreased as well as SfrCYP302A1 (Disembodied) and SfrCYP306A1 (Phantom) in 5th day pupae. In contrast, the transcript levels of SfrCYP302A1 in 3rd day larvae, SfrCYP307A1 and SfrCYP314A1 in 5th day pupae, and SfrCYP306A1, SfrCYP307A1 and SfrCYP315A1 in 0.5th day adults were significantly increased. Our results demonstrate that broflanilide caused the failure of ecdysis in FAW possibly by influencing the intake of cholesterol through inhibition of feeding and also via altering expression of genes important for MH biosynthesis.