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Common Forest Health Agents

Several forest health agents common to the Inland Northwest are Douglas-fir beetle, pine bark beetles, dwarf mistletoe, pine engraver beetles, fir engraver beetle, western gall rust, root diseases, stem decays, and physical damage from weather and animals. Eliminating impacts to individual trees is nearly impossible, but minimizing their scope and impact throughout a forest area is achievable. A brief description of a few insects and diseases follows.

Douglas-fir Beetle

Douglas-fir beetle outbreaks are usually initiated by catastrophic events such as blow down or winter breakage. Downed or weakened trees are attacked and beetles build up large populations. The next year, new generations emerge and attack susceptible trees in surrounding stands. Damage in standing trees is greatest in dense stands containing a high percentage of large, mature Douglas-fir.

Salvage of down or weakened Douglas-fir is a primary tool in preventing Douglas-fir beetle outbreaks. When attacks have already occurred removing standing green or faded infested trees will help reduce or prevent further damage in the area. The risk of Douglas-fir beetle damage is reduced when dense mature stands are commercially thinned.

Pine Bark Beetles

Four different pine bark beetles affect the pine trees in the Inland Northwest area—western pine beetle, mountain pine beetle, red turpentine beetle, and pine engraver beetle. The beetles generally favor trees that are water stressed. Trees can become water stressed during a drought or by having too many trees in an area (overstocked). The bark beetle bores through the bark and lays its eggs in the cambium layer between the bark and the wood; the cambium is full of sugar and nutrients that feed the larvae.

Trees killed by bark beetles can often times be recognized as red trees in the stand that appear suddenly. A tree can turn from green to red within weeks. However, other indicators of bark beetle such as pitch tubes, boring dust, or frass on the bark of the tree would have been present for months. Red trees, themselves, are usually not a forest health risk. They are an indicator of what has happened in the stand and what may happen in the future.

The western and mountain pine beetles are considered major tree killers in Northern Idaho and Eastern Washington. Both prefer trees greater than 6 inches in diameter. Trees that they attack usually die.

Root Diseases

Root diseases are the most damaging group of tree diseases. Diagnosis and identification is based on:

  • Circular groups of dead and dying trees. Root diseases tend to kill a few trees each year. Look for dying trees at the edge of a group with dead trees towards the center.
  • Thinning tree crowns. Crowns of root diseased trees fade in color, thin from the inside of the tree crown towards the edge. Diseased trees may produce a stress cone crop, though much of the seed is not viable. Young trees are killed more quickly than older ones.
  • Symptoms and signs in roots and root crowns. Trees with advanced root disease may have basal resin flow, wood discoloration and decay, and presence of fungal tissue.

Root disease is managed by promoting the establishment and growth of resistant tree species. Not all conifer species are equally susceptible to root disease. Many young stands can be grown to merchantability if disease tolerant species are favored.

Dead and dying trees can be salvaged; however, rates of disease spread and tree mortality may not be reduced. There is some evidence that partial cutting increases the rate of mortality in root-diseased stands.

Recognizing the various insects and diseases on your forest land and striving to keep forest land healthy is a worthwhile goal. Salvage of blowdown timber from last winter and springs wind events is very important. Please call Northwest Management, Inc. if you have any concerns about the health of your forestland. We can assist you in managing your property to achieve your management objectives.

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