Drier Weather Leaves Trees Susceptible to Bark Beetles

May 10, 2018

Dying trees are a sign of bark beetle attacks.
Credit US Forest Service

Given the warm end to last summer and drier than normal winter, the Oregon Department of Forestry expects a higher chance of trees dying from drought or secondary attack by bark beetles this summer. The agency encourages tree and forest owners to watch for continuing drought symptoms like dead tops, dead branches, thinning canopies and stressed cone crops.

Oregon Department of Forestry Entomologist Christine Buhl said, “Above average temperatures this spring mean beetles may come out sooner than normal. If this happens, beetles may attack trees sooner than usual.”

Officials say plants and animals are still experiencing the effects from the 2012-2015 drought. Drought damaged trees may have collapsed vascular tissues and fine-root dieback. These tissues and roots absorb much of the water a tree needs. This damage takes time to repair and trees may not fully recover.

Last summer ended with a long stretch of higher temperatures and days without much rain. This winter has also had less than normal rain and snow, so soils are not as saturated as expected.

“Given the recent drought and below average rain and snow this winter, trees will likely be drought-stressed this season - unless precipitation boosts back to normal levels,” Buhl said. “What we need is a longer, lighter rain to allow trees time to soak up the water.”

Most bark beetles are part of a healthy ecosystem. Bark beetles pick off the sick or less vigorous trees that are competing with healthier trees. But, when dry conditions stress trees, they’re more susceptible to attack and bark beetle populations can increase to unsustainable levels. When the natural balance is off and the beetles can overcome the defenses of healthier trees it presents a problem.

The department advises tree and forest owners to plan ahead and consult with an arborist or forester to help healthy trees survive, despite lower precipitation. This can include removing some trees or competing weeds to allow the remaining trees to get enough food and water during dry spells. Avoid fertilizing and planting less drought-tolerant trees such as Douglas fir in areas historically dominated by more drought-tolerant trees such as oak and pine. 

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For more information visit: http://www.oregon.gov/ODF/Documents/ForestBenefits/Drought_2016.pdf