Evidence that (+)-endo-brevicomin is a male-produced component of the southern pine beetle aggregation pheromone

Previous research indicated that the aggregation pheromone of the southern pine beetle, Dendroctonus frontalis, is produced only by females, the sex that initiates attacks. We provide evidence indicating that secondarily arriving males augment mass aggregation by releasing the attractive synergist (+)-endo-brevicomin. Healthy pines artificially infested with both sexes of D. frontalis were significantly more attractive to conspecifics than trees infested solely with females. Coupled gas chromatography-electroantennographic detection (GC-EAD) analyses of volatiles isolated from male beetles revealed substantially greater olfactory sensitivity by D. frontalis to endo-brevicomin than to any other component. The threshold of detection of both sexes for (+)-endo-brevicomin was four orders of magnitude lower than for its antipode and at least one order of magnitude lower than for either enantiomer of frontalin, the major female-produced aggregation pheromone component. Pairing with a female in a gallery stimulated individual male beetles to produce hundreds of nanograms of (+)-endo-brevicomin. (+)-endo-Brevicomin was detected in a small percentage of female D. frontalis, whereas (-)-endo-brevicomin was never detected in either sex. In field trapping bioassays, we confirmed that (+)-endo-brevicomin is a potent synergist for attractive combinations of frontalin and pine turpentine. However, (+)-endo-brevicomin failed to attract D. frontalis either when presented alone or in combination with turpentine. We postulate that mass colonization of host trees by D. frontalis is mediated by distinct semiochemicals from both sexes rather than females alone. Our discovery of a key aggregation pheromone component in such an apparently well-studied species implies that the pheromone models of other bark beetles could benefit from systematic reexamination using newer technologies. Additionally, baits fortified with (+)-endo-brevicomin may enhance pest management strategies that exploit attractants for D. frontalis.