Single Sensillum Recordings Reveal Antennal Responses of Monochamus alternatus and Monochamus saltuarius (Coleoptera: Cerambycidae) to Pheromones and Host Volatiles.

Monochamus alternatus Hope and Monochamus saltuarius Gebler (Coleoptera: Cerambycidae) are insect vectors of the pine wood nematode (PWN) on the Korean peninsula. Although they share an identical aggregation-sex pheromone, the two species exhibit different synergistic responses to host volatiles and other semiochemicals. In this study, we examined the electrophysiological responses of olfactory sensilla in both species to 22 semiochemicals, including pheromone, bark beetle pheromones, and host volatiles. Single-sensillum recordings (SSR) identified two distinct groups of basiconic-type olfactory sensilla, each associated with different functional groups. Sensilla basiconica A (SBaA) contained specialized olfactory sensory neurons (OSNs) that could be functionally distinguished into non-responsive, specialist like pheromone specialist, and generalist OSNs. In contrast, OSNs in sensilla basiconica B (SBaB) did not show clear functional classification into specific odor groups. Therefore, they were classified based on the number of compounds eliciting responses, as non-responsive neurons, specialists, and generalists. Notably, enantiomer-specific differences were observed among generalist OSNs. Most OSNs responded to both (+)- and (−)-α-pinene enantiomers, whereas a significantly higher proportion of OSNs showed exclusive responses to (−)-limonene compared to (+)-limonene. These results provide new insight into the olfactory mechanisms that underlie species-specific behavioral responses to semiochemicals in these economically important beetle vectors.