Geometrical and optical isomerism of pheromones in two sympatric Dryocoetes species (Coleoptera: Scolytidae), mediates species specificity and response level

In a field-trapping experiment, western balsam bark beetles, Dryocoetes confusus Swaine, were highly attracted to a 5:1 mixture of (±)-exo-and (±)-endo-brevicomin. Beetles in the sympatric species D. affaber (Mann.), were best attracted to a 1:1 blend of these semiochemicals [either (±):(±) or (±):(±)], suggesting 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. 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.