Forest Health and Mortality
Current bark beetle outbreak
Four years of drought has led to an epidemic of bark beetles in the Central Sierra. Many ponderosa pine are dying due to western pine beetle. The pine engraver is also affecting many pines while some cedars are dying directly from drought without beetles being involved.
Keeping trees healthy
Trees weakened by drought cannot defend themselves from beetles, who burrow under the bark and lay eggs. The larvae hatch and their burrowing kills and girdles the tree. Once beetles have gained entry to the tree through the bark, there is no way to kill them and the tree will die. Keeping trees healthy so they can fight off bark beetle attack is the best way to help them. Bark Beetles of California
Plant a variety of species adapted to site: In general, native species will withstand the extremes of climate better. Also, many insects attack only a specific tree. Therefore, having a variety of trees means they won't all be attacked at the same time.
Thin / promote healthiest individuals: Many Sierra forests are overstocked after 100 years of fire suppression. Thinning out a forest allows more sun and water for the remaining trees, which will be healthier and better able to repel insect attacks.
Water: Watering trees helps them fight off beetle attacks. Usually this is only feasible for high value trees near the home that can be reached by watering systems. Use of Greywater in Urban Landscapes in California
Chemical treatments: Several pesticides have been shown to be effective at preventing western pine beetle infestation. Carbaryl provides two years of protection and pyrethroids provides one year of protection. They must be applied by licensed pesticide applicators before the trees are attacked. Using Insecticide to Protect Conifers from BarkBeetle Attack
Diagnosing tree attacks
Identify the species of insect attacking a tree:
1) Identify the species of tree (some trees have few or only one species that attack them)
2) Determine the location of insect attack on the stem. For example on large pines, engraver beetles attack near the top, red turpentine beetles attack the bottom of the trunk and others attack the middle of the stem.
3) Identify the pattern of galleries under bark which is individual to each bark beetle species
Dealing with dead trees
Dead trees pose both a safety and fire hazard depending on where they are. Dead and dying trees near a home or important infrastructure should be removed quickly. This is often a costly and difficult thing to do.
Most beetles, including the western pine beetle, feed only on live trees. Therefore they have usually left the tree before it appears completely dead. However, pine engraver beetles, which kills trees from the top down, can breed in dead or dying trees for up to 5 weeks after they have been felled. Therefore its important to avoid letting slash or green logs accumulate near living trees. Instead, logging debris should be: chipped, cut into smaller pieces (> 4“ in diameter and 3' long) and scattered (lop and scatter), piled and burned, crushed and mashed into the soil, removed from site, or wrapped in clear plastic for up to 5 weeks. Controlling Bark Beetles in Wood Residue and Firewood
Bark beetle resources
- UC Forest Research and Outreach website
- UC Forest Stewardship Series: Pests and Disease
- UC Master Gardeners - find a local program
- UC Master Gardeners - Central Sierra programs
Pest ID websites: