Bark beetles are a major cause of tree mortality in California, and are significantly impacting Del Mar's Torrey Pines and other pine trees due to the drought. Usually trees can protect themselves by pushing the beetles out with sap, especially if the group of beetles is small. But, trees that are dealing with severe and prolonged drought conditions are very vulnerable to the threats posed by the bark beetles. The beetles can succeed in killing large numbers of trees, because they cannot use sap to protect themselves. The beetles use the dying trees as breeding colonies which can spread the pests to nearby trees.
What is a Bark Beetle?
Bark beetles are small hard bodied beetles that bore through the protective bark and lay eggs in the softer inner bark. The mother beetle and the larvae eat the moist Phloem. The bark beetles kill trees by cutting off the tree’s ability to distribute nutrients. The gallery found on the underside of the bark is typically the result of the mother beetle burrowing into the bark and laying eggs on the wall of the gallery. The eggs hatch and the larvae create ‘larval galleries’ that tend to increase in width as the larvae increase in size.
Identify Bark Beetle Damage
- pitch tubes
- reddish boring dust
- adult exit holes
- yellowing foliage
Effectiveness of any treatment is dependent on the severity of the attack. However, there are several ways to stop initial attacks from becoming more severe and discourage the beetles.
- If the beetles have already infested the tree, it is recommended to water the tree twice a month with a soaker hose -soaker hoses are made of porous material that “leak” or seep water all along their length - or drip line to allow water to get to deep roots. A general rule of thumb is to use 10 gallons per inch of trunk diameter at shoulder height.
- There is also a bagged anti-aggregate pheromone called verbenone that can be hung on a tree. The pheromone imitates the scent of a ‘fully occupied’ tree that discourages other bark beetles from attacking the tree.
- Remove any limbs that are infested so those beetles do not attack other parts of that tree or other trees.
- The injection and pheromone traps are also helpful to remove beetles from trees.
The beetles attack the drought stressed trees. Healthy trees are often able to resist the beetles by pushing the burrowing beetles out with the help of resin. But in drought-prone areas the trees produce less amount of resin that leaves the trees more vulnerable.
- Watering your trees during severe or protracted drought periods can also give your trees a boost. Watering should occur early in the growing season by saturating the soil down to two feet near the outer edge of the tree branches. Be careful not to over water!
- Remove all trees that currently contain beetles. The effectiveness of this treatment greatly depends on the bark beetle species, however, removing trees promptly when they have been successfully attacked will reduce the pheromone (chemicals beetles use to communicate) source associated with attacked trees. All infested green material >3 inches in diameter should be removed from the site, chipped, buried or burned.
- Barrier Treatments: There are two ways to administer the barrier treatment to trees - sprays and injections. It would be inadvisable to use sprays on trees in Del Mar because of the proximity of trees to houses and waterways. Chemical injections like Tree-age with Arbor Jet (PDF) - a system in which a chemical is injected into the cambium layer every inch or two around the trunk 4.5 feet above the ground. The tree then takes the chemical up through the xylem (where water is also transported throughout trees) and distributes it throughout the whole tree. The chemical should provide 2-3 years of protection for the tree.
- Avoid causing injuries to trees, such as knocking off bark, compacting/excavating soil near trees or disturbing the root system.
- Keep trees vigorous to reduce attacks.
- Always clean up any recently blown down trees or fresh slash to avoid creating more habitat for beetle development.
- Long Term: Thinning trees to a wide spacing is the best long-term solution to increase tree health and vigor and reduce the likelihood of bark beetle attacks.