Bark Beetles

Photograph of a bark beetle resting on the tip of a match head Photograph of a bark beetle resting on a mound of pitch Photograph of a cut tree showing where larvae have exited as well as blue stain fungus Photograph of a tree trunk covered with yellowish pitch tubes

Under normal conditions, bark beetles renew the forest by killing older trees and those weakened by disease, drought, smog or physical damage. Our current prolonged drought has weakened a greater number of trees, increasing the number of successful beetle attacks. The more success the beetles have, the more beetles reproduce and spread to other areas of the forest. The bark beetle population has exploded in the Sierras.

  • Decades of fire suppression has resulted in over-crowded forests. The dense stands of mature trees compete for limited nutrients and water, reducing the health of the stands overall.
  • Long term exposure to air pollution has reduced the health of conifers in the Sierras. Excessive ozone exposure causes the trees to lose their ability to "breathe" and results in premature loss of pine needles, and the trees' ability to produce food and new tissues.
  • Healthy trees can fend off bark beetle attacks by exuding pitch into the holes, pushing the beetles out. However, drought-stressed trees have a difficult time producing enough pitch to fight off insects.
  • If the four-year drought were to end this year, it is likely the bark beetle epidemic will continue for three to five more years. The beetles will continue to spread throughout the forests until all of the vulnerable trees are consumed. Eventually the beetles will run out of trees to reproduce in and to feed upon, which will lead to a die back of the beetles and a return to normal population levels.

How to identify Bark Beetles

A photograph of pinkish colored sap exuding form a beetle exit hole A photograph depicting a penny next to a beetle exit hole exuding pinkish sap for scale Close up photo showing dead needles A photograph showing what beetle exit holes in the bark

Early signs may be difficult to interpret, but if there are signs that bark beetles have successfully attacked a tree, the tree is dead or will die soon. It often takes months for outward signs of mortality to show.

Beetle Physiology

  • One mating pair of bark beetles can reproduce more than 12 million beetles a year.
  • There are more than 600 species of bark beetles in the U.S., including 200 in California.
  • Beetles release pheromones that attract other beetles; this mass influx of beetles can quickly overwhelm a tree.
  • Bark beetles are attracted to freshly cut wood.
  • Beetles bore through tree bark and lay their eggs. Larvae feed on the trees' living tissues, cutting off the trees' natural process for transporting nutrients and water. One bark beetle infestation can create several thousand beetles and easily spread to neighboring trees.
  • Bark beetles can kill a tree in as little as two to four weeks during warmer months.




https://www.fs.usda.gov/detail/catreemortality/trees/?cid=fseprd497622