We tested the hypothesis that nonhost conifers contain compounds that repel coniferophagous bark beetles (Coleoptera: Scolytidae) during host selection in four experiments (n = 10) involving paired trees baited with aggregation pheromones. Mountain pine beetles, Dendroctonus ponderosae Hopkins, and Douglas-fir beetles, Dendroctonus pseudotsugae Hopkins, were tested for discrimination between their respective hosts, lodgepole pine and Douglas-fir, and spruce beetles, Dendroctonus rufipennis Kirby, and western balsam bark beetles, Dryocoetes confusus Swaine, for discrimination between interior spruce and interior fir. Both host and nonhost conifers in a pair were baited with the same aggregation pheromone. Most baited host trees were successfully attacked and contained galleries with eggs or young larvae. Neither D. rufipennis nor D. confusus attempted to establish galleries on nonhosts. A few attacks were initiated on nonhosts by D. pseudotsugae and D. ponderosae, but most did not reach the phloem tissue, and in no case were they numerous enough to have produced a significant source of aggregation pheromone. Thus employing pheromone-baited nonhost trap trees would not be an effective management tactic. Higher trap catches in unbaited multiple funnel traps within 1 m of nonhost trees than in control traps 12.5 m away also indicated that there was no strong long range repellence caused by nonhost volatiles. Although this study was not designed to evaluate primary attraction to host trees, the lack of strong repellence from nonhost conifers partly supports the hypothesis of random landing followed by close range olfactory or gustatory rejection of nonhosts.
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