LsToll Gene Mediates Antibacterial Immunity and Developmental Regulation in Loxostege sticticalis

Toll-like receptors (TLRs) are conserved pattern recognition receptors essential to insect innate immunity. However, the functions of TLRs in Loxostege sticticalis, a destructive agricultural pest, remain poorly characterized. In this study, the full-length coding sequence of the L. sticticalis Toll receptor (LsToll) was identified and characterized to analyze its molecular features. Structural analysis showed that LsToll possesses typical Toll family features, including an extracellular domain containing 19 leucine-rich repeats (LRRs), a transmembrane helix, and a highly conserved intracellular Toll/interleukin-1 receptor (TIR) domain. LsToll transcript levels were significantly upregulated after bacterial challenge. RNAi-mediated silencing of LsToll significantly reduced larval tolerance to bacterial infection and increased mortality. Notably, LsToll suppression also induced severe developmental abnormalities, including molting obstruction, pupation failure, and defects in wing expansion in newly emerged adults. Transcriptome analysis after RNAi identified 5230 differentially expressed genes (DEGs), which were significantly enriched in insect hormone biosynthesis and metabolic pathways. Biochemical assays further confirmed that LsToll knockdown decreased 20-hydroxyecdysone (20E) titers and increased juvenile hormone III (JH III) titers. These results suggest that LsToll contributes to antibacterial defense and normal development in L. sticticalis. Its involvement in both survival and development indicates that LsToll may serve as a promising molecular target for sustainable pest management strategies.