Bark beetles are one of the most destructive pests of pines in forests and urban landscapes. The beetles typically attack trees weakened by lack of water, root damage, close spacing between trees, or other conditions which cause stress. Healthy trees growing near beetle-infested trees may also be attacked, especially during periods of drought when bark beetle populations increase.
Pine bark beetles are small insects about the size of rice grains, and they vary in color from dark red to brown or black. Adult beetles bore into a tree and make tunnels in the live tissue under the bark. Eggs laid in the tunnels hatch into cream or white grub-like larvae that create new tunnels as they feed. After several weeks or months when the larvae mature they transform into adult beetles and tunnel out through the bark to begin the cycle again. Up to four complete generations are produced each year.
Feeding by adult bark beetles and larvae damages live tissues in the tree which produce new growth and transport water and food between the roots and foliage. Adult beetles also introduce fungal spores into the tree which grow and plug water-conducting tissue. This damage causes the needles on infested pines to fade from a dark-green color to yellowish-green, then red or brown. The discoloration usually starts at the top of a tree and spreads downward. By the time this symptom is observed it is usually too late to save a tree.
Adequately irrigating pines during dry weather and good cultural practices is the best way to protect trees from bark beetles. Healthy trees that have adequate soil moisture are less attractive to bark beetles and are more capable of resisting attack. Adult beetles often drown in pitch when they try to bore into the bark of a healthy tree.
Pines need a steady supply of soil moisture and should be irrigated every 2 to 4 weeks during dry weather. Apply enough water to wet the soil at least two feet deep near the tree’s outer canopy. Also apply a layer of ground bark, wood chips, or other mulch on soil around trees to conserve moisture and control weeds.
Pines need little fertilization or pruning. If trees are not growing well a small amount of nitrogen applied during winter will stimulate vegetative growth. Healthy trees should not be severely pruned or topped. Trees can be pruned lightly if desired, but do not prune from February to October when most adult beetles are flying. Unseasoned, freshly cut wood can harbor beetles and should not be piled near susceptible trees.
Pines that exhibit fading foliage due to bark beetle infestation should be removed promptly to protect nearby healthy pines from attack. Take the cut up tree to a local landfill for disposal. If logs are kept for firewood, cut them into pieces and strip off the bark to keep beetle larvae from developing in the wood. For more information see UC Pest Note 7421 Bark Beetles. http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7421.html .
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension,