OPTIMAL DOSE OF AN ANTIAGGREGATION PHEROMONE (3-METHYLCYCLOHEX-2-EN-1-ONE) FOR PROTECTING LIVE DOUGLAS-FIR FROM ATTACK BY DENDROCTONUS-PSEUDOTSUGAE (COLEOPTERA, SCOLYTIDAE)

The Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins antiaggregat ion pheromone, 3-methylcyclohex-2-en-1-one (MCH), was applied to stand s of Douglas-fir, Pseudotsuga menziesii (Mirbel) France, at high risk for infestation to determine the lowest effective dose for protecting live trees. MCH was applied at rates of 50, 100, and 150 bubble capsul es per hectare (20, 40, and 60 g/ha, respectively) in 1991, and 15, 30 , and 50 bubble capsules per hectare (6, 12, and 20 g/ha, respectively ) in 1995. Mean release rates throughout the beetle flight periods in 1991 and 1995 were 1.63 and 1.23 milligrams per capsule per day, respe ctively. For both years, catches of Douglas-fir beetles in pheromone-b aited traps located at the plot centers were significantly lower on al l MCH-treated plots compared with untreated plots, but there were no d ifferences among tile 3 doses of MCH. In contrast, MCH had no effect o n the numbers of 3 predators [Thanasimus undatulus, (Say), Enoclerus s phegeus F., and Temnochila chlorodia (Mannerheim)] collected in the tr aps during either year. In 1994, the percentage of Douglas-fir greater than or equal to 20 cm diameter al breast height (dbh) that were mass attacked was significantly lower on MCH-treated plots compared with t ile untreated control, and all 3 doses were equally effective. In 1995 , there were no significant differences in the percentage of mass-atta cked trees among the treatments. These results demonstrate that MCH ap plied at rates as low as 20 g/ha (50 bubble capsules per hectare) can reduce the probability that high-risk Douglas-fir will became infested .