People sometimes wonder how something as small as a bark beetle can kill something as big as a tree. Many species of bark beetles are natural inhabitants of any forest, including urban ones. Usually, they are present in small numbers, occasionally killing weakened trees. Populations are held in check by natural predators. When populations are at their usual low levels, the beetles play a useful role in removing clusters of trees, providing dead trees for wildlife use and helping to recycle valuable nutrients from diseased and dying trees. Bark beetles also provide food for woodpeckers and other insects.
During droughts, floods and fires, trees become stressed and more susceptible to attack by bark beetles. Trees also can be stressed by air pollution, diseases, other insects, soil compaction, high tree densities, deicing salts, improper fertilization and herbicide damage from weed killer/fertilizer combinations. Bark beetle populations rise dramatically when there are large numbers of stressed trees.
Bark beetles are usually not much bigger than a grain of rice. They feed and reproduce in the inner bark layer, found between the outer bark and the wood of the tree. For some species, such as the Jeffrey pine beetle, when an adult female attacks a tree, she bores through the bark and begins to excavate a tunnel, called a “gallery,” into the inner tissue and on the wood surface. She then chews pockets in the sides of the gallery, laying one egg in each pocket, until she’s laid several dozen eggs.
If you see wood from a bark beetle-attacked tree, it has a characteristic gallery pattern on its surface. Each beetle produces its own gallery pattern. After the eggs hatch, the larvae feed on the inner bark and dig out their own galleries, often perpendicular to the parent’s galleries. As the larvae feed, their ever-widening galleries, in addition to the adult galleries, will cut the inner tissue and outer sapwood all the way around the tree, damaging or killing it. This is called “girdling.” Some beetles also carry a fungus on their body surface or in special pouches on their body. The fungus infects the tree, and, in pines, causes a blue/gray staining of the wood. The fungus clogs the tree’s water-conducting ability. The physical damage caused by chewing, together with the action of the fungus, may kill the tree.
Often, the first noticeable sign of an attack by bark beetles is the appearance of “pitch tubes” of sap in the bark crevices. Sometimes these are filled with sawdust called “frass.”
As the beetles bore small (1/16thto to 1/8th-inch) holes through the bark, the tree secretes sap or resin through the hole, which often “pitches” the beetle out of the tree, unless it is stressed and unable to produce the pitch. Damage can occur so quickly that often you will not see a decline in color until the tree is completely dead.
Trees under attack often must be removed, because chemical treatments rarely work. The best option is prevention — keeping the trees healthy in the first place. This means ensuring trees have sufficient water, including during fall and winter. Avoid pruning pines during the growing seasons. Don’t damage trees with mowers, weed cutters, vehicles or other objects. Fertilize appropriately and avoid herbicide and fertilizer combination products near trees.