Douglas-fir Damage May Be Caused by Secondary Bark Beetles

A picture of Douglas-fir trees killed by bark beetles and other factors.

This spring and summer, the Washington State Department of Natural Resources (DNR) received numerous reports of dead or damaged Douglas-fir trees throughout the state. Symptoms include entirely red crowns in saplings and red tops or scattered red branches in trees. The damage is more common in dry lowland areas and sites with well-drained soils.

DNR forest health specialists examined Douglas-firs with these symptoms and found unexpected levels of attack by several species of bark beetles such as Douglas-fir engraver, (Scolytus unispinosus), Douglas-fir pole beetle (Pseudohylesinus nebulosus), and another engraver beetle, Scolytus monticolae, that has no common name.

Picture of a Douglas-fir engraver.
Douglas-fir engraver. (Photo by Joseph Benzel, USDA APHIS, bugwood.org)

These beetle species are normally considered ‘‘secondary’ because they typically infest trees that are first weakened by a larger, primary issue such as root disease, fire damage or drought stress. Secondary beetles can damage trees under stress but normally lack the capacity to kill live, healthy trees.

Douglas-firs have been particularly affected by Washington’s abnormally hot and dry summers over the past three years. Back-to-back drought years are stunting the health of Douglas-firs, leaving them less able to fend off insect attacks.

These conditions have allowed secondary bark beetle species to establish in healthy Douglas-firs and boost beetle populations, causing significant damage or even killing some trees. Secondary bark beetles mostly prefer to attack small-diameter trees, yet recent investigations have found them in stems of larger diameter trees as well, which is historically uncommon.

Consider the following to prevent or manage secondary beetle attacks in Douglas-fir:

  • Keep forested stands thinned so that remaining trees can access more of the water stored in soils
  • Irrigate and mulch high-value yard or park trees during prolonged periods of drought
  • Avoid fertilizing trees, as this can increase foliage growth and the need for more water
  • Do your homework before applying pesticides:
    • Some systemic pesticides can be applied as a preventative measure for high-value trees
    • Pesticides are not effective at controlling beetles in trees that have already been attacked
    • Consider hiring a licensed pesticide applicator to ensure the proper selection, timing and application of pesticides

Unfortunately, pheromone treatments, such as those used to deter the more aggressive Douglas-fir bark beetle (Dendroctonus pseudotsugae), are not available to combat these minor bark beetle species.

For more information on forest health in Washington, go to www.dnr.wa.gov/ForestHealth or contact DNR’s forest health staff at 360-902-1300.

Article by Glenn Kohler, DNR Forest Entomologist, glenn.kohler@dnr.wa.gov