Bark Beetles Attacking Drought-Stressed Douglas Fir Trees Across Washington
August 8, 2018
DNR Forest Health Experts Offer Advice
Washington residents have reported uncharacteristically high numbers of dead and dying Douglas fir trees to the Washington State Department of Natural Resources (DNR) this spring and summer. The culprits: drought and bark beetles.
The native bark beetle species involved are opportunists, also known as secondary bark beetles. They don’t normally have the capacity to kill live, healthy trees. Instead, they breed in dead or downed branches and then infest small diameter, thin-barked portions of trees that are stressed or dying from factors such as root rot, fire, or drought.
A doubling of public inquiries tipped off DNR scientists whose field observations have confirmed the public’s forest concerns. Those driving highways in South Puget Sound and lowlands around Blewett, Sherman, or White passes will likely spot the damage to Douglas fir trees.
“Bark beetle infestation is a visible manifestation of drought conditions, stressed trees, and unhealthy forestland,” said Commissioner of Public Lands Hilary Franz. “Unfortunately, this infestation increases the risk of wildfires in the forests that surround our communities. If you own forests or trees, do your part to make sure they are healthy so they survive and continue providing ecological benefits for you, your neighbors, and our state.”
Washington has had several years in a row of hot, dry summer weather. Douglas fir trees in particular have been heavily affected by these drought conditions, which showed similar — though less significant — signs of stress in 2012 and 2015. Some trees seem to be dying from drought alone. It’s this stress and easy access into the trees that has allowed beetle populations to grow larger than seen in recent years.
The primary symptoms of bark beetles are entirely red crowns in saplings, and red tops or scattered branches of red needles in larger trees. The damage is commonly visible in scattered areas throughout eastern Washington and in western Washington in dry lowland areas and sites with well-drained soils.
DNR expert advice:
- Keep forests thinned so trees have less competition for water.
- Trees in parks or landscaped areas can benefit from mulch and irrigation during prolonged drought, however take care not to overwater.
- Avoid fertilizing as this can increase foliage growth and the need for more water.
- Unlike for the Douglas-fir beetle, another common threat, there are no repellent pheromones available for preventing attacks from these beetles.
- Don’t use pesticides, which are not effective in controlling beetles in trees that have already been attacked.
- Those with small family forests in eastern Washington may wish to partner with the agency on a cost-share program to treat their private forest. The program allows DNR to reimburse private landowners for contractor work or equipment to improve the health of their forests.
DNR is currently flying aerial surveys across the state to assess the breadth of damaged trees. These annual flights help the agency better understand the extent of infestation and other forest health issues. Results will be available by November.
Commissioner Franz’s 20-Year Forest Health Plan for eastern Washington includes strategies to help improve the health of our forests and decrease such outbreaks, leaving more productive forests that can better withstand wildfires. Adopted last fall, the plan calls for treating 1.25 million acres of forest by 2037.
For more information on Washington’s forest health, go to www.dnr.wa.gov/ForestHealth.
DNR wildfire and agency leadership
The Commissioner of Public Lands, Hilary Franz, oversees the Department of Natural Resources and its responsibility to prevent and fight wildfires on 13 million acres of private, state and tribal-owned forestlands. This also includes managing 3 million acres of state trust lands, 92 state natural areas and statewide efforts to improve forest health across 13 million acres of private and public forests.
- The bark beetles causing the damage - identified from gallery patterns (the engravings made as they burrow under the bark) and adult specimens - include the Douglas-fir engraver (Scolytus unispinosus) beetle; the Douglas-fir pole beetle (Pseudohylesinus nebulosus); and another engraver beetle (Scolytus monticolae), which has no common name.
- Unlike the Douglas-fir beetle Dendroctonus pseudotsugae, which results in the crown of large diameter trees turning from green to red all at once, the species causing this damage produces a patchy pattern of red branches throughout the crown.
- As drought conditions persist, other bark beetle and wood borer species populations are also increasing.
- DNR has stewardship foresters within most parts of Washington that can assist landowners to improve the health of their forests www.dnr.wa.gov/sflo.
- For insect and disease management advice, contact DNR’s Forest Health Program at 360-902-1300 or firstname.lastname@example.org.
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