Potential for Douglas fir beetle outbreaks in eastern Washington

High winds that damaged swaths of Washington forest in 2015 paved the way for a potential outbreak of the Douglas fir beetle in on the state’s eastside this spring.

Douglas-fir beetle egg and larval tunnels
Characteristic pattern of Douglas fir beetle egg and larval tunnels. Photo courtesy of Oregon Department of Forestry.

The Douglas fir beetle is a bark beetle that normally breeds in felled, injured, windthrown and root-diseased Douglas fir. It may also attack western larch but can only produce its brood in downed trees. At outbreak levels, this bark beetle can attack and kill large diameter, healthy Douglas fir trees. Outbreaks tend to occur after extensive windthrow events such as in 2015. Outbreaks may also occur after fire and extended drought.

Dense stands surrounding areas where windthrow, defoliation, fire and drought have occurred may be at high risk for an outbreak, particularly if those stands contain a 50 percent or more component of Douglas fir that are greater than 120 years of age and larger than 14 inches DBH (DBH = diameter at breast height; diameter of a tree bole 4.5 feet from the base).

The Douglas fir beetle has one generation a year. Brood that developed through 2016 will pupate and emerge as adults this spring. Once emerged, they will begin attacking standing trees surrounding the windthrow, as the windthrow is no longer habitable for them.

What can be done?

The best approach to prevent an outbreak this spring is to salvage any large diameter Douglas fir or western larch that were downed by the storms prior to the adult beetle flight, which should occur in April, depending on temperatures. Windthrown trees can also be burned or chipped on site if salvage is not an option. Time is running out; if you find you cannot take care of this material, the use of the anti-aggregate pheromone MCH is another option.

A pheromone is a chemical released by bark beetles that is used to affect the behavior of other beetles of the same species. Aggregating pheromones attract beetles, while anti-aggregates repel them. A bark beetle might use an anti-aggregate to prevent overcrowding within a tree. An anti-aggregate basically tells other beetles that there is no room for additional inhabitants in the tree.

MCH bubble capsule stapled to a tree
MCH bubble capsule stapled to a tree. Photo: US Forest Service.

The Douglas-fir beetle naturally produces an anti-aggregate to repel others. A synthetic version of this anti-aggregate, MCH, has been produced and is available for purchase through several online companies. MCH comes in a “bubble capsule” and can be used to protect individual live, high-value Douglas fir or even an entire stand. For individual tree protection, two bubble capsules can be stapled on either side of a Douglas-fir bole at approximately 6-8 feet from the ground for a tree less than 24 inches DBH. Four bubble capsules should be used for Douglas fir larger than 24 inches DBH. To protect a stand of Douglas fir, 30 bubble caps per acre can be evenly placed through the stand.

MCH should be hung prior to the beetle flight in April. Contact your local forest health specialist if you are considering this method of management. Additional information about this method can be found in the free publication, “Using MCH to protect trees and stands from Douglas-fir beetle infestation,” published by the US Forest Service.

This post by Melissa Joy Fischer, forest health specialist, Washington State Department of Natural Resources, first appeared in Forest Stewardship Notes, a free, quarterly e-newsletter published by DNR and Washington State Forestry Extension.

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