Tree mortality resulting from the drought of 2012 to 2016 has been unprecedented. Trees have died throughout the state but especially in the southern Sierra Nevada. 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
Keeping Trees Healthy
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
Replanting after Tree Removal
Replanting after dead tree removal should be carefully considered. This article about how to approach replanting may be useful. Replanting after tree removal article.
Or, consider attending a reforestation workshop.
The University of California is collaborating with Cal Fire and the Natural Resources Conservation Service to reach out to landowners affected by tree mortality through workshops and educational materials.
- Dr. Jodi Axelson/Susie Kocher, UC Berkeley Forest mortality and regeneration - Life after death (1)
- Mark Egbert, El Dorado Resource Conservation District Reforestation basics
- Zsolt Katay, Cal Fire California Forest Improvement Program
- Guy Anderson, Cal Fire Control of Sprouting Brush
- Dorus Van Goidsenhoven Selection of Nursery stock (Tuolumne County)
Saturday, February 11th, 2017 in Auberry, CA
Wednesday, March 22nd, 2017 in Mariposa, CA
- Workshop Information: 3-22-2017 Mariposa Workshop
Friday, April 7th, 2017 in Sonora, CA
- Workshop Information: Reforestation Workshop Announcement 4-7-2017.docx
Thursday, April 27th, in Oakhurst, CA
- Workshop Information: Madera Reforestation Workshop
Wednesday, May 31st in Sutter Creek, CA
- Workshop Information: Reforestation Workshop Amador county 5-31-2017
Bark beetle resources
Pest ID websites: