Attraction of Scolytus unispinosus bark beetles to ethanol in water-stressed Douglas-fir branches

Three similar-sized branches on each of seven Douglas-fir (Pseudotsuga menziesii (Mirb.) France) trees with a southern exposure were selected randomly to receive a water-stress, defoliation, or control treatment. All water-stressed branches were attacked by Scolytus unispinosus LeConte at 12-24 days after imposing the treatments, resulting in a significantly higher mean density of gallery holes (107 m(-2)) than in the defoliated or control branches, which were not attacked. Needles and woody tissues from stressed branches, sampled after being attacked, contained significantly higher ethanol concentrations (ranging from 2.91 to 15.26 mu mol g(-1) fresh weight) than tissue from defoliated or control branches (0.005-0.12 mu mol g(-1) fresh weight). Ethanol concentrations did not differ between defoliated and control branches. The water content in woody tissues and needles of stressed branches was 40.9 and 28.1% of the amount in control branches, respectively, when sampled for ethanol analysis. Woody tissues in defoliated branches remained moist, with 91.3% of the water in controls. Drying needles from water-stressed branches also lost 18.3-33.7% of their total monoterpenes compared to needles on the controls. The mechanism for ethanol synthesis in water-stressed branches appears to be distinctly different from that of logs overwintering on the forest floor. Ethanol synthesis in stressed branches probably was initiated by cytoplasmic acidification as a result of damage to cellular membranes from drying and heating. The ethanol that accumulated in needles and woody tissues of stressed branches functioned as a primary host attractant for S. unispinosus at our Willamette Valley, Oregon, site. Published by Elsevier Science B.V.