| Dispersing coniferophagous bark beetles (Coleoptera: Scolytidae) search efficiently for a suitable brood tree. Much debate has focused on whether bark beetles discriminate between host and nonhost trees while flying or if they must land on and test each tree. Gas chromatographic-electroantennographic detection analyses revealed numerous compounds in the bark of black cottonwood, Populus trichocarpa, trembling aspen, P. tremuloides, paper birch, Betula papyrifera, bigleaf maple, Acer macrophyllum, red alder, Alnus rubra, and Sitka alder, Alnus viridis spp. sinuata, that were antennally-active to: mountain pine beetle, Dendroctonus ponderosae , Douglas-fir beetle, Dendroctonus pseudotsugae, spruce beetle, Dendroctonus rufipennis, the pine engraver, Ips pini, and western balsam bark beetle, Dryoceotes confusus . Frontalin and conophthorin, two scolytid pheromones, were found for the first time in plants. In field trapping experiments conophthorin had a consistent, dose-dependent disruptive effect on the response by D. pseudotsugae and I. pini to attractant-baited traps. It had an inconsistent effect on D. ponderosae and D. confusus and no effect on D. rufipennis. A group of compounds, generally common to angiosperm trees, disrupted the response of D. pseudotsugae, D. ponderosae, I. pini, and D. confusus to attractant-baited traps, usually in an additive and redundant fashion. For D. pseudotsugae the most responsive species, disruptive activity was found in the presence conophthorin, two aliphatic green leaf alcohols [1-hexanol and (Z)-3-hexen-1-ol], guaiacol and benzyl alcohol, and three aliphatic aldehydes [hexanal, (E)-2-hexenal, and nonanal] but not in two aromatic aldehydes [benzaldehyde and salicylaldehyde]. Generally, the most bioactive blend reduced the response level to 4.1—13.5% of the response to attractant-baited control traps, demonstrating that some coniferophagous bark beetles can at least perceive and avoid angiosperm trees while in flight. No compound or group of compounds was disruptive for D. rufipennis. Operational trials challenged D. ponderosae to attack pheromone-baited pines treated with various formulations of nonhost volatiles alone or in combination with the antiaggregation pheromone, verbenone. In each of five experiments aggregation was strongly disrupted. A prototype operational blend comprised of (E)-2-hexen-1-ol, (Z)-3-hexen-1-ol, benzyl alcohol, guaiacol, nonanal, salicylaldehyde, benzaldehyde, conophthorin, and verbenone was tested in a 60-replicate experiment with no baits. In this experiment 20.0% and 11.7% of the untreated control and treated trees, respectively, were attacked. However, the mean attack densities on treated trees were too low to kill the trees. Nonhost angiosperm bark volatiles have considerable potential for protecting trees from attack by coniferophagous bark beetles. |
