Odorant receptor CaspOR39 underpinned cineole-mediated oviposition repellency in the rape stem weevil Ceutorhynchus asper.

Herbivorous insects rely on olfactory cues to select oviposition sites on host plants, which is a process critical for their reproductive success. The rape stem weevil, Ceutorhynchus asper Roel. (Coleoptera: Curculionidae), is a major pest that causes significant damage to rapeseed and other cruciferous crop production, yet the chemical and molecular mechanisms underlying its oviposition preference remain unknown. Our behavioral assays revealed that C. asper females strongly avoided pre-infested rapeseed plants. Five monoterpenes (p-cymene, 1,8-cineole, ocimene, camphor, and α-pinene) were identified as herbivore-induced plant volatiles of C. asper-infested plants, with their emission levels significantly higher than those in healthy plants. Among these five compounds, subsequent electroantennogram (EAG) screening revealed that only 1,8-cineole triggered significant responses in C. asper and showed a concentration-dependent sensitivity. Behavioral assays showed that female C. asper exhibited significant behavioral and oviposition avoidance when exposed to 1,8-cineole. We identified CaspOR39 as the specific odorant receptor for 1,8-cineole, with molecular docking for prediction of the binding affinity and key residues. RNAi of CaspOR39 abolished female EAG responses, as well as behavioral and oviposition avoidance of 1,8-cineole. These results revealed a sex-specific chemical sensing mechanism: female C. asper can utilize CaspOR39 to detect 1,8-cineole as a key cue of plant infestation, driving adaptive oviposition behaviors of this beetle. Our results provide mechanistic and molecular insights into CaspOR39-mediated oviposition behaviors of C. asper, and a basis for the development of semiochemical-based strategies for management of this weevil.