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Forest Health Notes:
A Series for the Non-Industrial Private Forest Landowner

(Dendroctonus brevicomis)

Host: Ponderosa pine, rarely lodgepole pine

Quick ID:

  • Egg galleries winding, branched, crossing
  • Small yellow pitch tubes
  • Red boring dust in bark crevices and at base of tree
  • Larval galleries short, entering bark
  • Large patches of bark often removed by woodpeckers
  • Foliage discolored

Western BeetleField Identification

Tree: Small yellow pitch tubes mark the point of entry of the beetle. Red boring dust may be found in the bark crevices and at the base of the tree. In the phloem (inner bark region), egg galleries wind and branch randomly, crossing over the galleries of other beetles. These galleries will be packed with frass (boring dust and insect droppings). Sapwood markings from larval galleries off the egg gallery will be short as the larvae quickly enter the bark to feed. On a successfully attacked tree, the foliage will discolor first to yellowish-green, then to reddish-brown. By that point, the trunk may have many small round emergence holes indicating the beetles have gone.

Insect: Adults are 3 to 5 mm in length and dark brown, with a cylindrical shape. Larvae are white and resemble grains of rice with a yellow-brown head. Larvae may be found in the outer bark of infested trees.

May be confused with: Mountain pine beetle

Life cycle: Western pine beetles attack and kill living trees. In an endemic (normal) population, adults preferentially choose weakened trees, especially those experiencing water stress. Once an epidemic outbreak gets started vigorous trees may be attacked as well. Western pine beetles prefer trees at least 10" in diameter. Adult insects emerge and attack trees late in spring or in early summer. Females attack the tree, bore through the bark to the phloem (inner bark), and emit pheromones that attract both other females, who also colonize the tree, and males. Attack
usually occurs on the midbole. "Mass attack" of the tree by many beetles overcomes the tree's resistance. Trees resist colonization by pitching out and immobilizing the beetles. Generally, trees that are not under water stress can fend off an attack. During an epidemic, however, all trees can be overwhelmed. Beetles mate inside the tree and the females construct winding, "S-"shaped egg galleries. Niches are cut into the sides of each gallery and eggs are laid singly, about 1/4" apart.
These niches and the egg gallery itself are packed with frass behind where the adults are working. Larvae hatch in 7 to 10 days and begin mining the phloem at right angles to the egg gallery. After a quarter of an inch or less, the larvae enter the bark where they develop through a number of larval stages. At last, a pupal chamber is hollowed into the outer bark, the larva pupates, and a new adult emerges. Unlike mountain pine beetles, western pine beetles may have two generations develop to maturity and a third overwinter as larvae in the same year.

Predisposing agents: Western pine beetles have greater success in attacking trees that are under water stress. Root rot infections may predispose trees to water stress during drought. Beetles selectively attack trees that have been weakened by drought, lightning strike, or other agents that interfere with the water balance of the tree. A tree is in water balance when as much water is taken up by the roots as is transpired. Trees out of water balance are preferentially attacked as these trees cannot mobilize strong defenses to expel beetles. Historically, epidemics occur as a result of prolonged drought, as drought weakens trees across large areas. Warm temperature aids development of larvae, hence a hot summer will permit faster maturing of a generation and a greater likelihood of multiple generations in a single summer.

Impact: As it prefers ponderosa pine, western pine beetle is somewhat less damaging than mountain pine beetle which preys upon all kinds of pines. According to the USFS, in Washington in 1991 the western pine beetle was responsible for the death of 21,679 trees over 28,159 acres resulting in a loss of 1.267 million cubic feet of lumber.

Management: Methods of control have shifted away from direct control (felling, burning, etc.) and towards indirect methods that discourage beetle habitat and keep populations at endemic (normal) levels. It is possible to prevent infestation with penetrating sprays on individual, high value trees such as those in campgrounds and near houses, but they need to be applied before the tree is infected and the cost of such treatments is prohibitive for any large-scale application.

Western pine beetles are a natural part of western ecosystems; they evolved here together with the pines they feed on. In the forest, they kill slow-growing, drought-stressed trees and act as natural thinners, opening up the stand for new trees and wildlife. Even outbreak conditions are "normal" as cycles of drought have been predisposing factors for beetle epidemics for thousands of years, as have root diseases. For this reason bark beetle attacks will never be eliminated (nor should they be). The death of a few trees on your property doesn't necessarily herald the beginning of an outbreak; check the trees for signs of root disease (there are other WSU Cooperative Extension "Forest Health Notes" dealing with symptoms of root diseases).

To maintain western pine beetles at their normal levels, predisposing factors for outbreak must be removed. Environmental stresses are not controllable, but many stress factors are associated with stand management practices and therefore may be changed. One important situation that can be addressed is having too many trees to the acre, or stand overstocking. Overstocking creates inter-tree competition for water, light and nutrients, and reduces their ability to resist attack by bark
beetles. A judicious thinning can show good results in ponderosa pine stands even 90 years of age. To minimize stand stresses and maintain vigorous growing conditions, stand managers should: (adapted from Berryman: Forest Insects, 1986).

  • Choose tree species that are adapted to the area on which they'll be planted.
  • Harvest trees in a way that mimics natural processes, such as cutting small patches to mimic a fire (for pine and larch management) or selective or shelterwood cutting (for grand fir and Douglas-fir management).
  • Remove diseased and unhealthy trees and logging debris, and minimize damage to residual trees. Salvage logging of beetle-killed trees is fine except in root disease areas where that may worsen the severity of root diseases such as Armillaria and Annosus.
  • Encourage diversity in species and age classes.
  • Use thinning, fertilization, prescribed fire, etc. to maintain stand diversity and vigor.
  • Prevent trees from becoming overmature by harvesting on time.

Patch cutting 6 to 10 acre blocks every few years and managing these as small even-aged stands helps keep the total number of older trees low and creates a variety of age classes that discourages western pine beetle attack. It has additional benefits for wildlife by creating small openings and edges. This may not, however, be a good strategy if trees at the edges of the cut are heavily infested with dwarf mistletoe and the species to be planted or naturally regenerated is the same species.

Note: Use pesticides with care. Apply them only to plants, animals, or sites listed on label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.



Forest Health Notes were prepared by Donna Dekker-Robertson, Peter Griessmann, Dave Baumgartner, and Don Hanley, Washington State University Cooperative Extension. The assistance of Robert L. Edmonds and Robert I. Gara, University of Washington College of Forest Resources, is gratefully acknowledged.

Department of Natural Resource Sciences
Washington State University Cooperative Extension
Pullman, Washington 99164-6410

Insect or disease treatments should always be part of an overall forest management planning process. It's a good idea to have a forest management plan for your property. This tool will help you prevent problems and keep your forest healthy. For information on forest stewardship educational programs, contact your local WSU Cooperative Extension office. For information on technical assistance and financial incentive programs, contact the Washington Department of Natural Resources. The Natural Resources Conservation Service will assist landowners in developing conservation plans.

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