Bitter gustatory receptor modulates the immune response of Coptotermes formosanus against Metarhizium anisopliae

Many termite species are classified as pests, and current control methods primarily rely on chemical insecticides, which pose environmental risks. Biological control using Metarhizium anisopliae offers a promising alternative, but its efficacy is limited by the termite's complex social and physiological defense mechanisms. Gustatory receptors (GRs) are known to mediate insect behaviors such as feeding and avoidance, yet their roles in antifungal immunity remain poorly understood. In this study, we identified 14 GR genes from the Coptotermes formosanus genome, a globally significant termite pest.Their functional roles were investigated using RNA interference. Knockdown of CfGr3, a putative bitter gustatory receptor, significantly reduced termite survival and impaired avoidance behavior in response to M. anisopliae. Furthermore, CfGr3 suppression suppressed the humoral immune response, as shown by decreased expression of antimicrobial peptides (termicin and lysozyme) and higher fungal load. Transcriptome analysis identified the crustacean cardioactive peptide receptor (CCAP-R) as a downstream target of CfGr3. Knockdown of CCAP-R similarly compromised avoidance behavior and immunity of termites. Our findings indicate that CfGr3 plays a critical role in coordinating behavioral and immune defenses against fungal infection.These results advance understanding of social immunity in termites and identify potential molecular targets for improved biocontrol strategies.