Emamectin benzoate disrupts molting of larvae via interfering ecdysteroid biosynthesis in Spodoptera frugiperda

The fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith) is a highly destructive pest of many crops. Emamectin benzoate (EMB) is a major insecticide for the management of FAW, however potential sublethal effects and regulating mechanisms on the development of FAW remain unknown. In this context, we exposed FAW to two EMB concentrations (LC10 or LC30) and then assess sublethal effect(s) on FAW development and reproduction, as well as evaluated potential regulating mechanisms. We showed that EMB prolonged the larval development time, pupal duration time and adult longevity, and increased pupal weight and fecundity/female, while lowered pupation rate (compared to control group). EMB also impaired FAW larvae molting and reduced levels of ecdysteroids in third-instar larvae and 1st-day pupae. Two new ecdysteroid biosynthesis genes, Neverland (SfNvd) and Shroud (SfSro) were identified from FAW. The spatial and temporal expression patterns and the changes in mRNA levels under EMB exposure of SfNvd and SfSro, along with five Holloween genes, were monitored. Six genes, including SfCYP306A1, SfCYP302A1, SfCYP307A1, SfSro, SfCYP314A1, and SfCYP315A1 were significantly induced, and SfNvd were decreased after exposure to LC10 or LC30 of EMB at specific developmental stages, while SfCYP307A1 was increased by LC10 in pupa and decreased by LC10 and LC30 of EMB in adults. These results suggest that EMB inhibits ecdysteroid biosynthesis by regulating ecdysteroid biosynthetic enzyme genes, thus resulting in ecdysis failure and disruption of larval development.