Tuesday, June 3, 2014

Douglas-fir Beetle Outbreak in the Columbia River Gorge

Another bark beetle has shown up in big numbers in the eastern Gorge. This time, it’s Douglas-fir beetle attacking Douglas-fir trees -- and it’s no surprise.

In January, 2012, the Columbia Gorge was hit by a big ice storm that topped and toppled trees through the Eastern Gorge, including the Wind River and White Salmon River Valleys in Washington and the Hood River Valley in Oregon.

The blowdown created a lot of Douglas-fir beetle food. The Douglas-fir beetle developed large populations on the downed Douglas-fir trees during 2012.

The beetle has only one generation per year, so it has taken a couple of years to see the evidence of the spotty outbreak. In the spring of 2013, those beetles emerged to mass attack standing trees. Even though the beetles are very small (about a quarter inch), large numbers can coordinate an attack on standing trees to overwhelm the tree’s defenses.

These trees died last fall and are now drying out and turning color. The adult beetles already have left those trees to attack other susceptible trees in the area.

As the summer progresses, more Douglas-fir trees are expected to redden. In wet forests like those in the Gorge, outbreaks can be expected to last two to three years. Outbreaks can last longer in areas under drought, much like the California fivespined Ips pine beetle outbreak on eastern gorge Ponderosa pine trees.

Douglas fir beetles attack trees damaged by breakage, fire scorch, or root rot. Evidence that a Douglas-fir has been successfully attacked is the red-orange boring dust in bark crevices or on the ground around the tree.

The best management is to promote stand vigor by thinning. Prompt removal of damaged trees is also recommended. Individual high-value trees can be treated with a pheromone that sometimes prevents infestation. Attacks are most severe in unmanaged stands, on trees that are largest in diameter, and in dense stands.

Forest landowners and managers should watch for local workshops discussing management options in WA and OR in the summer and fall of 2014.

For more information, please contact Todd Murray (tmurray@wsu.edu, 509-427-3931) at the WSU Extension office or Glenn Ahrens, OSU Extension Forester at 503-655-8631 (glenn.ahrens@oregonstate.edu). Photo from Idaho Forest Products Commission.

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