Less Rain, More Bark Beetles

Spring 2004

Bark beetles have the potential to devastate many Inland Northwest forests. These tiny insects can be responsible for vast areas of tree mortality as well as the deaths of scattered trees in an otherwise green forest.


In fact, if you see a dead pine or fir tree (the most common species in our region), it is likely that bark beetles played a key role in its demise. How do bark beetles work? In essence, bark beetles are opportunists that take advantage of trees under stress. Trees can be stressed for a variety of reasons, including lack of moisture, overcrowding, or the presence of another pest, such as root disease.

Recently, limited moisture has been the biggest factor in bark-beetle caused tree mortality. This point is well illustrated by the two accompanying graphs. Graph 1 indicates how much precipitation the Coeur d’Alene, Idaho, area has received in the last few years, while Graph 2 shows the population levels of a common bark beetle (fir engraver, which attacks grand fir trees) over the same time period. As you can see, increased activity of this bark beetle is closely correlated with lower precipitation levels. It is typical for most other bark beetles to respond in the same manner.


With this information in mind, there are a few important points to consider.

  • Since trees killed by bark beetles in 2003 will generally not turn red until the spring of 2004, many dead trees will likely appear this spring. The trees most likely to be impacted by bark beetles in our area are grand fir and pine species.
  • Obviously, you cannot control how much precipitation your forest receives; you can, however, thin your forest, thereby increasing the amount of moisture available for each remaining individual tree. If you thin to leave the best available tree at the proper spacing, you will increase tree vigor and reduce the risk of bark beetle attack.
  • It is important to avoid thinning pine stands between December and July. Thinning pine during this time period greatly increases the chance for tree mortality caused by the pine engraver bark beetle.

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