Insecticidal Activity and Mechanism of 7-Methylcoumarin Against Diaphorina citri Reveal the Role of Chemosensory Genes in Defense Response

The Asian citrus psyllid (Diaphorina citri) is a major vector of Huanglongbing (HLB), a devastating citrus disease worldwide. In addition to vectoring HLB, D. citri is a significant economic pest in its own right, causing direct damage to citrus by feeding on tender shoots, leading to leaf distortion, shoot dieback, and sooty mold infestation. Chemical control remains the primary strategy for managing D. citri populations, but heavy reliance on insecticides has led to insecticide resistance. Coumarin and its derivatives, as a natural product, are widely recognized for their insecticidal activities against various agricultural pests. In this study, the insecticidal activity of 11 coumarin compounds against D. citri nymphs was evaluated by bioassay, among which 7-methylcoumarin exhibited the most substantial efficacy. Moreover, the insecticidal mechanism of 7-methylcoumarin was assessed by a comparative transcriptome analysis, and 294 differentially expressed genes (DEGs) were identified, including 165 up-regulated and 129 down-regulated genes. GO enrichment analysis showed that DEGs were mainly involved in extracellular region/space, (chitin-based) cuticle development, and structural constituent of (chitin-based) cuticle terms, especially down-regulated genes related to cuticle proteins, chemosensory proteins and odorant-binding proteins, which were further verified by qPCR. Among chemosensory genes, only DcCSP2 was significantly up-regulated, indicating its potential involvement in the defense response of D. citri. Suppression of DcCSP2 via RNAi significantly increased susceptibility to 7-methylcoumarin in D. citri nymphs. Molecular docking revealed that 7-methylcoumarin binds within the internal hydrophobic cavity of DcCSP2. This finding provided a potential molecular mechanism underlying the action of coumarin compounds against pest insects and may contribute to facilitating the development of new strategies for managing insecticide resistance.