Why are My Douglas-fir Trees Dying?

The Douglas-fir bark beetle is the most destructive beetle attacking Douglas-fir trees in the Seeley/Swan Valley. The potential loss of trees can be considerable and can affect both residential homeowners as well as commercial interests. Outbreaks develop in host trees following stand disturbances such as windthrow, drought, severe defoliation or wildfire, such as the Condon Mountain Fire in 2012. During outbreaks, groups of dead trees may total 100 or more. Yearly mortality in the northern region, which includes Montana, may extend into the millions of board feet.

Are Your Trees at Risk to Bark Beetles? What Should You Do?

Step 1: Identify your trees with a tree identification guide, available at Swan Valley Connections (SVC) or ask for assistance from SVC.

Step 2: Determine your trees' susceptibility.

This beetle is attracted to weakened trees. As more of the weakened trees (susceptible hosts) are killed, the attacking beetles are forced into adjacent healthier trees and eventually populations decline. Although a natural phenomenon, bark beetle infestations can become epidemic, killing large patches of healthy trees as well as valued individual trees. In pre-outbreak populations, mortality is confined to individual trees or small groups. The most susceptible trees are larger than 14 inches in diameter, older than 120 years, growing in dense stands, weakened by drought or are located near existing beetle-infested trees

Step 3: Determine if your trees have been attacked.

Evidence of this beetle is:

• Reddish-brown boring dust, or frass, found in bark crevices on the lower portion of the tree, or on the ground at its base if weather has not removed it.

• Streams of clear resin that appear about 20 to 35 feet off the ground where the beetle has bored into the tree.

• And, like the Mountain Pine beetle, egg galleries beneath the bark, which girdle the sapwood under the bark, will also confirm the presence of beetles. Spring emergence of adult beetles from the galleries varies with location and weather, but usually occurs from mid-April to early June. Beetles that have passed the winter as larvae complete their development in spring and early summer. Those emerge and attack host trees in mid-summer. Occasionally, drought will trigger a second flight of beetles.

• Unlike the Douglas-fir Beetle, Mountain Pine Beetle are identified by pitch tubes on the bark of lodgepole and ponderosa pine, where the healthy tree has secreted pitch into the bore hole to defend itself. The presence of blue stain in the tree is also unique to Mountain Pine Beetle.

Step 4: How to treat trees that have been attacked.

Most trees will not survive a Douglas-fir bark beetle attack. There are no chemical insecticides registered or recommended for killing bark beetles under the bark of infested trees.

Step 5: How to help prevent a Douglas-fir Bark Beetle attack on my property.

• Remove all trees that currently contain beetles and any down trees.

• Thin dense stands of trees, leaving the healthiest, most vigorous ones.

• The most effective treatment is the application of bubble caps with the chemical pheromone MCH. The bubble caps are placed on the north side of healthy Douglas-fir trees in April. The pheromones tell the attacking beetles that the tree is already occupied, which helps to repel other beetles. Bubble caps for Douglas-fir bark beetle, as well as those for Mountain Pine beetle, can be ordered at a reduced cost from SVC.

To help protect your trees from beetle mortality, you can order bubble caps from SVC. You must place your order by Thursday, March 30. Call 406-754-3137 or order online at SwanValleyConnections.org

SVC is also available for a free property assessment on how to reduce the fire risk on your Swan Valley property and improve the health of your trees.