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 can also be stressed by air pollution, diseases, other insects, soil compaction, high tree densities, de-icing 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 (1/16th– 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.
Exotic Invasive Bark and Boring Beetles
Native bark beetles are problem enough when trees are stressed, but introduced pests from other countries can be devastating to our forests and landscape trees. Their natural enemies don’t exist here to keep them in check. Insects such as the Asian longhorned beetle, the emerald ash borer, the banded elm bark beetle and the goldspotted oak borer, to name a few, can cause widespread tree mortality with huge economic impacts and put people and property in danger.
Asian Longhorned Beetle (ALB) – The adult ALB is ¾ to 1¼ inches long with antennae 1½ to 2½ inches long. The female digs oval pits into the bark and then lays a single egg into each one, laying 35 to 90 eggs per year. The eggs hatch and the larvae feed inside the tree. The exit hole made by the adult as it leaves a tree is 3/8 inch in diameter or larger. These pests feed on boxelders and many other maples, poplars, sycamores, willows, ashes, elms and birches, to name a few.
Emerald Ash Borer (EAB) – The ash tree is a staple of the Nevada home and park landscape. The EAB could cause widespread ash decline and mortality. The adult beetles are emerald green, ¼ to ½ inch in size. Each female lays 30 to 60 eggs during the average three-week lifespan. Longer-living females may lay up to 200 eggs. Eggs are deposited individually in bark crevices. After hatching the larvae eat their way into the inner tissue and then chew galleries throughout the sapwood. A “D”-shaped exit hole in ash is a sign of EAB. EAB only attack ash trees.
What can you do to stop these invaders? Don’t move firewood, particularly when you go camping. Buy it where you burn it. Many states recommend not moving wood more than 50 miles, but the shorter the distance the better. Some won’t allow you to move wood from county to county. Many won’t allow wood from other states to be imported. Never assume wood that looks safe is safe to move or insect-free. If you see a suspected invader, get a sample and/or a picture and contact the State Entomologist, Jeff Knight at 353-3767 or firstname.lastname@example.org.