Douglas-fir beetles enter the trees in early spring after the weather begins to warm. The eggs are laid in vertical 'galleries' in the phloem layer of the inner bark. The larvae form groups of horizontal galleries that alternate on opposite sides of the egg gallery. The larvae eventually girdle and kill the trees. The tiny beetles require about one year to fully develop. Beetles that emerge in spring were laid as eggs during the previous spring. Sometimes a second attack occurs in mid- to late-summer.

Douglas-fir beetle outbreaks are identified by the red-orange boring dust caught in the bark crevices on the tree's trunk and on the ground at the base of the tree. Mature trees greater than 12 inches in diameter are most likely to be attacked. Often the insects enter the trees high up the trunk where it is difficult to spot the boring dust. Once the boring dust has been noticed the trees are usually already dead even though the needles may still be green.

Factors affecting Douglas-fir beetle outbreaks
Dense stands of trees 120 years old or older on moist sites are most likely to be susceptible to Douglas-fir beetle outbreaks. Yet, the beetles will sometimes also attack young trees in densely stocked stands. Epidemics are almost always associated with disturbances, such as blowdown, fire, defoliation or drought. Beetles are attracted to the downed and weakened trees. Douglas-fir that have been stressed by root disease are more likely to be attacked (see Armillaria Root Disease). Western larch trees may also be attacked during Douglas-fir beetle outbreaks, but brood development in larch is rare.

Douglas-fir beetle management options, salvage or sanitation
Douglas-fir beetles prefer recently fallen trees to live trees. Removing weakened or downed trees prior to Douglas-fir beetle attacks or before the beetles emerge from infected trees can be effective. Firewood should be split to dry and should not be stacked near live trees.

Douglas-fir beetle trap trees
When beetle populations are present, a trap can be made in early spring, before April 1, by dropping a group of 3-5 large trees (15 inches in diameter or greater) to lure the beetles. The trap trees should be felled into a shady area where they are unlikely to dry out. They should not be bucked into short pieces or de-limbed. The trap trees should be removed soon after the beetles fly, in about mid-July, and certainly no later than April 1 of the following year.

Preventative thinning
Thinning dense older stands that have trees greater than 14 inches in diameter can reduce the risk of attack as long as the trees haven't been affected with Armillaria root disease.

Pheromones
Douglas-fir beetle pheromones are available in small packets. They have been shown to protect trees if properly applied. Swan Ecosystem Center may be able to offer the pheromone packets at a reduced cost. Contact SEC in January to be included. The packets are attached to trees in April or May, depending on weather. For more information and a resource for detering Douglas-fir beetle, visit our Landowner Assistance page.