This winter’s heavy snow and wind have knocked down many trees across the state. This has set up perfect conditions for an outbreak of Douglas-fir beetle unless landowners act quickly to either remove the downed or damaged trees or apply an insect pheromone to drive away the pest.
Forest Entomologist Christine Buhl is with the Oregon Department of Forestry. Buhl said that while the beetle (Dendroctonus pseudotsugae) is native to the Northwest, its population can soar when living large-diameter Douglas-fir trees are thrown to the ground.
“Normally this pest is scattered on the landscape wherever Douglas-fir grows,” said Buhl. “However, they tend to concentrate in tree stands where there has been a lot of storm damage.”
“The beetles first attack downed Douglas-firs and then move to nearby standing Douglas-fir trees that are stressed, injured or less vigorous,” said Buhl. “Attacked trees that cannot drown out infesting beetles with pitch die and turn red the same year as the attack or the next spring.”
Tell-tale signs of this beetle’s presence include frass. This is sawdust-like material the beetle ejects from their galleries underneath the tree’s bark, Buhl explained.
Buhl said landowners can prevent Douglas-fir beetle outbreaks by removing large-diameter downed trees before the insects take flight in the spring looking for new homes.
“Before April remove any downed Douglas-firs greater than 10 inches in diameter at a point about chest height,” she advised. “Trees with low vigor should also be removed to reduce stand susceptibility to outbreaks. Large amounts of cull trees created by logging also should not be left on site, particularly in shade.”
Buhl said the beetles will generally not attack trees that have been dead for over a year.
“If removal is delayed or not possible,” said Buhl, “apply the repellant pheromone called MCH to prevent infestation.”
A naturally occurring beetle repellent, MCH (methylcyclohexenone), can be applied to downed logs or standing green trees to prevent Douglas-fir beetle attacks. Buhl said MCH pouches are most often applied in a grid pattern within a stand to effectively deter infestation. Beetles approaching treated areas are fooled by the pheromone into sensing that beetles already occupy the site and will pass it by.
“This technique is useful in parks, camps or habitat conservation areas where salvage is not possible, but there is a desire to preserve the remaining standing trees,” said Buhl. “It’s also helpful in forestlands where downed wood can’t be removed before April for one reason or another. However, it will not make beetles that have already infested a tree leave. Once they are in, they’re in.”
Learn more about Douglas-fir beetle at the Oregon Department of Forestry’s Forest Health webpage.