Insecticide-induced sublethal effect in the fall armyworm is mediated by miR-9993/miR-2a-3p - FPPS/JHAMT - JH molecular module.

The sublethal effect of insecticides can affect the population dynamics of pests by changing the physiological or behavioral changes, which poses a serious threat to the sustainable control of crop pests in the field. However, the molecular regulation mechanism that mediates the sublethal effect of insecticides on crop pests remains unsolved. Here, we show that the sublethal effect of spinetoram and cyantraniliprole on Spodoptera frugiperda is mediated by the molecular module of microRNA-9993/microRNA-2a-3p - farnesyl diphosphate synthase gene (FPPS)/juvenile hormone (JH) acid methyltransferase gene (JHAMT) - JH. Spinetoram prolonged the duration of larvae and pupae, decreased the weight of pupae, while cyantraniliprole prolonged the duration of larvae and decreased the emergence rate. Similarly, injection of the juvenile hormone analogue (JHA) methoprene significantly prolonged the developmental duration of larvae and pupae, decreased the pupal weight and emergence rate. This sublethal phenotypic change was due to the upregulation of key JH synthesis genes, including FPPS and JHAMT, mediated by spinetoram and cyantraniliprole, resulting in an increase in JH titer. Furthermore, it was confirmed by small RNA sequencing, dual luciferase analysis and agomir-miRNA injection, miR-9993 and miR-2a-3p that it could bind FPPS and JHAMT respectively, and regulated the expression level of FPPS and JHAMT to affect the titer of JH, thus changing the phenotype of S. frugiperda. Collectively, these results provide insights into the mechanism of insecticide regulation of sublethal effects of pests, expand our understanding of development-related miRNAs, and reveal key factors involved in JH signaling pathways that support sublethal effects of insecticides.