INFLUENCE OF AGGREGATION INHIBITORS (VERBENONE AND IPSDIENOL) ON LANDING AND ATTACK BEHAVIOR OF DENDROCTONUS-BREVICOMIS (COLEOPTERA, SCOLYTIDAE)

The influence of the aggregation inhibitors verbenone and ipsdienol on the response of western pine beetle, Dendroctonus brevicomis, to attr active host trees was investigated. Paired ponderosa pine trees (Pinus ponderosa) were baited with aggregation semiochemicals to stimulate m ass attack. One tree in each pair received an inhibitor treatment cons isting of five sets of two verbenone and two ipsdienol dispensers spac ed 1 m apart vertically along the tree bole. Beetle landing was monito red with sticky traps on the tree bole, and attack density was assesse d from bark samples removed four or seven days after baiting. The inhi bitor treatment resulted in a significant reduction of both the number s of beetles landing on trees and the density of attacking beetles com pared to control trees (without inhibitors). The ratios of beetle land ing density to attacking density were not different between inhibitor- treated and control trees, nor were the vertical distributions of beet les landing or attacking, suggesting that beetle behavior was primaril y influenced at a longer range, prior to landing on the tree. Although the application of verbenone and ipsdienol did not prevent D. brevico mis from attacking baited trees, our results suggest that when applied to unattacked (and unbaited) trees, their effectiveness at reducing t he attack pressure might allow trees having a certain amount of resist ance to survive attack by pioneer beetles.