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

(Dendroctonus ponderosae)

Hosts: Lodgepole pine, ponderosa pine, western white pine, whitebark pine

Quick ID:

  • Egg galleries long and straight, with the grain (vertical)
  • Larval galleries at 90 angle from egg gallery
  • Yellow pitch tubes on trunk
  • Red boring dust in bark crevices
  • Pitch streamers on trunk
  • Foliage discolored

Mountain Pine BeetleField Identification

Tree: Trees from 4 to 5" in diameter and up may be attacked. Attacks usually occur in the middle of the trunk. Yellow pitch tubes are found on the trunk at the point of entry, and red boring dust may be found in bark crevices and at the base of the tree. Pitch streamers on the trunk may also indicate point of entry. Egg galleries within the inner bark are long and straight, and follow the grain. These galleries will be packed with frass (boring dust and insect droppings). Generally there is a bend at the bottom of the egg gallery. Although there may be several egg galleries on the same tree, they do not cross. Larval galleries branch off of the egg gallery at a 90 angle and are 1 to 2" in length. As the successful attack proceeds, foliage discolors from dark green to light greenish yellow and then to reddish brown. The wood of successfully attacked trees becomes discolored by bluestain fungi.

Insect: Adults are about 1/5" in length, stout, black, cylindrical beetles. Larvae are about 1/4" long, white and resemble grains of rice. They are found in separate tunnels off the egg gallery; pupal cells with pupae may be found at the end of these tunnels.

May be confused with: Red turpentine beetle, western pine beetle.

Life cycle: Mountain pine beetles are both a primary and secondary pest, infecting both apparently healthy and disease-weakened trees. In the endemic (constantly maintained) population, beetles preferentially attack trees that are weakened by root diseases, but once an epidemic gets started beetles attack healthy trees as well. Mountain pine beetles show a strong preference for pines of large diameter more than 80 years in age. In July and August, when trees are most likely to be under stress from water deficiency, adults emerge, females find a new host, and bore through the bark to the cambial area. There, they emit pheromones that attract males and other females. The other females that are attracted will also colonize the tree; this "mass attack" strategy overcomes the tree's defenses. Mating occurs under the bark, then the female constructs an egg gallery and deposits eggs in niches on either side. The gallery becomes packed with frass. When the larvae hatch in 7-10 days, they mine separate feeding tunnels. When fully grown, the larvae hollow out a pupal cell at the end of the tunnel and pupate. New adults bore their way out and the cycle is repeated. One generation per year is the rule in Washington. Insects overwinter in trees as larvae and adults.

Predisposing agents: A strong association has been found between root diseases and bark beetles, where disease weakens trees and makes them targets for bark beetle attack. Agents of stress such as drought, overcrowding, overtopping, and defoliation can also weaken trees. Overmature stands are ripe for insect attack, as they are not vigorous and have a thick phloem layer, which beetles prefer. Low-elevation stands are harder hit than high-elevation stands as the warmer temperatures at low elevations are more favorable for insect survival; likewise, unusually warm summers aid bark beetle development. Outbreaks of mountain pine beetle typically get
started in overstocked stands or in stands on poor sites, then spread to healthy timber. Essentially, the success or non-success of bark beetle attack has to do with the physiological condition of the tree. Healthy, vigorous trees are able to overwhelm attacking beetles with pitch, while trees that have been stressed may not have the available resources for massive pitch production. However, during outbreak conditions an otherwise healthy tree may simply be overwhelmed by sheer numbers.

Impact: Mountain pine beetle has historically been the most damaging of the bark beetles. According to the USFS, in 1990 they were responsible for the death of 289,800 trees totaling 6.5 million cubic feet over 186,600 acres in the state of Washington. In 1991, 298,400 trees were lost totaling 5.6 million cubic feet over 155,422 acres. Outbreaks when mountain pine beetle attacks reach epidemic levels have been recorded in the west since 1894. Outbreaks can last for more than ten years, and mountain pine beetles are generally in an epidemic condition on at least one of their hosts somewhere in the west.

Management: Control methods have shifted away from direct control (e.g. spraying, felling, burning) and towards prevention of outbreaks. This course of action was chosen after thoroughly exploring direct control measures for nearly a century and arriving at a simple conclusion: They don't work. 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.

Once a mountain pine beetle outbreak begins to spread, it can be stopped by thinning the stand ahead of the edge of the outbreak. This is because outbreaks expand on a tree to tree basis where the incoming beetles switch their attacks from a recently attacked-stem to the next largest tree. More importantly, infestations can be prevented by thinning stands before crown closure, an operation that not only increases the vigor of the residual stand, but also prevents the spread of an outbreak if individual trees have been attacked.

Mountain pine beetles are a natural part of western ecosystems, and for this reason will never be completely eradicated (nor should they be, as they serve to create small stand openings which are important for biodiversity of both flora and fauna). As such, the death of a few trees on your property doesn't necessarily mean an epidemic is getting started; check your trees for root disease symptoms. To maintain mountain pine beetles at their normal levels, predisposing factors for outbreak must be removed. Some of these, such as environmental stresses, are not possible to control. However, many stresses are related to stand management practices. First and foremost, two situations need to be addressed: root disease centers and overstocked stands. More details about treatment for root disease centers have been given in other WSU Cooperative Extension "Forest Health Notes;" in summary, they need to be identified and planted with resistant species. Overstocking causes trees to compete for water, light and nutrients, and thus weakens their defenses against bark beetle attack. 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 low-intensity 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 standing trees.
  • Salvage logging is fine for beetle-killed trees except in root disease areas where that could increase the severity of 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 mountain 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 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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