Chemosensory proteins‐mediated immune response to <i>Beauveria bassiana</i> infection in <i>Monochamus alternatus</i>

Monochamus alternatus Hope, commonly known as the pine sawyer beetle, is a devastating pest in coniferous forest ecosystems. While insect olfactory-related genes have been implicated in pathogen defense, the potential involvement of chemosensory protein (CSP) genes in M. alternatus immune responses to the entomopathogenic fungus, Beauveria bassiana remains largely unknown. In this study, we identified 15 CSP genes in M. alternatus based on transcriptome, seven of which exhibited altered expression following B. bassiana infection. Quantitative real-time PCR (RT-qPCR) analysis showed significant upregulation of MaCSP8 and MaCSP15 at 3 d post-infection. Tissue-specific profiling revealed predominant expression of MaCSP8 in larval heads and epidermis, whereas MaCSP15 expression was highest in the fat body. Crucially, RNA interference (RNAi)-mediated silencing of MaCSP8 and MaCSP15 significantly increased larval susceptibility to B. bassiana infection. Furthermore, knockdown of the two MaCSPs resulted in significant downregulation of key immune effectors such as Dorsal, Defense, Attacin, and Coleoptericin, suggesting modulation of the Toll pathway. Taken together, MaCSP8 and MaCSP15 were defined as critical components of the antifungal immune response in M. alternatus larvae, likely functioning through the regulation of antimicrobial peptide (AMP) production. This study provides novel insights into CSP-mediated innate immunity, establishing it as a promising target for developing next-generation bio-insecticides and enhancing fungal biocontrol strategies against forestry pests.