Geometrical and optical isomerism of pheromones in two sympatric Dryocoetes species (Coleoptera: Scolytidae), mediates species specificity and response level.
In field-trapping experiments in British Columbia, adults of Dryocoetes confusus were highly attracted to a 5:1 mixture of (±)-exo- and (±)-endo-brevicomin. Adults of the sympatric species D. affaber were most attracted to a 1:1 blend of these semiochemicals (either (±):(±) or (+):(+)), and it is suggested that both geometrical isomers are pheromone components in these species. In laboratory bioassays and further field experiments, attraction of D. confusus was greatest when the (+) enantiomers of both geometrical isomers of brevicomin were presented in a 9:1 ratio. Responses by male D. confusus to attractive mixtures were reduced in the presence of (-)-exo-brevicomin. It is suggested that exploitation of the complete range of variability in pheromone structure (both geometrical and optical isomerism) would allow for optimization and regulation of response levels within a species and also could maintain reproductive isolation among sympatric congeneric species primarily through production and response to species-specific blends.