Bark Beetles Strike Mostly in Weakened Trees

Bark beetles are some of the most destructive forest insect pests found in California and other western states. There are many different species of bark beetles, and normal population levels are usually found in most mature forests. However, under certain conditions favorable to the insects, serious epidemics periodically develop from these normal infestations. During these outbreaks, large numbers of trees may be destroyed over a wide area.

Generally, healthy, vigorously growing trees are able to withstand normal levels of bark beetle infestation. However, when beetle populations multiply or when tree vigor declines, the potential for damage is increased greatly.

Many factors lead to decreased tree vigor. Some trees decline normally as they reach maturity and old age. Drought, root diseases, overcrowding and dwarf mistletoe infestations are other natural causes of loss of tree vigor. Human factors such as construction activities and air pollution lead to tree decline also.

Bark beetles typically begin their attacks on trees already weakened by stress and other factors. The insects bore entrance holes through the bark into the cambium layer. Healthy trees normally are able to fight off beetle infestations because the insects are not able to overcome the flow of pitch produced by the trees. lf a tree is weak, however, the adult beetles may successfully reach the surface of the wood under the bark, where they excavate egg-laying chambers and deposit their eggs.

When bark beetles do invade a tree, the first ones emit odors into the air that attract other beetles to the tree, thus intensifying the attack. Large numbers of beetles then concentrate their attack on this tree and others nearby. Because of the massive numbers of beetles involved, healthy trees are sometimes overwhelmed as well as weaker ones.

Trees are killed by bark beetles in two ways. The galleries excavated by adults prior to egg laying, and those formed by feeding larvae, may girdle a tree when present in large numbers. Also, bark beetles carry microscopic fungi that are deposited in the tree’s vascular system. These fungi multiply quickly and clog the water conducting tissue of the tree, thus hastening its death.

The first signs of beetle attack on individual trees is the boring dust that comes from the entrance holes and collects in the bark crevices. Sometimes this boring dust is mixed with pitch, and a sticky, waxy material may be found on the ground at the base of the tree and in bark cracks. Masses of pitch sometime emerge from the entrance holes and flow down the trunk of the tree as well. Depending on the particular species of beetle involved, the attacks may be concentrated near the top of the tree or farther down along the trunk.

As the beetle attack progresses, more visible signs begin to appear. The foliage at and above the area of attack begins to fade, passing from a healthy green color to light green, then to straw yellow and finally, to red. Depending on the type of beetle, only the top will be killed or the entire tree may die. Woodpeckers soon discover the beetle populations, and they tear at the bark in order to reach the insect forms found underneath.

The best way to control bark beetles is to keep trees healthy and growing vigorously, thus eliminating possible reservoirs for these insects. Not much can be done for the large numbers of conifers growing in private or national commercial forests except to follow sound silvicultural techniques that include periodic thinning and logging and the proper disposal of slash and debris after a timber sale. Removal of individual beetle-infested trees in commercial forests usually is not practical nor economical.

On small parcels and around homes, trees can be protected from beetle attacks by preventing soil compaction around roots, minimizing the cutting of roots during excavations, keeping beetle-infested firewood away from the premises and removing beetle-infested trees. Although there are chemicals registered for control of bark beetles in California, it is normally impractical to spray most trees because of their large numbers and sizes. Chemicals can be used to spray infested trees that have been cut down, although it is better to destroy them by burning.

Trees that are already infested with bark beetles should be promptly cut down and burned, debarked, sprayed with Lindane or removed from the premises. These actions may help to prevent or reduce the chance of attacks on adjacent trees. To be completely successful, however, all infested trees in an entire area or neighborhood need to be removed.

UC Farm Advisor Don Appleton column