The bark beetle is a generic name encompassing many similar species of insects that prey on coniferous trees such as pines, spruce, and fir. Under normal conditions, the bark beetle helps to rid a forest of diseased, weakened or fallen trees, making room for new, stronger growth. In adverse conditions, when trees become susceptible due to drought or other stressors, bark beetle infestation can wipe out entire stands in otherwise healthy forests.
The bark beetle is a small black, brown, or reddish beetle about .04 to .12 inches (1 to 3 mm) in length, though one species grows to nearly ½ inch (10 mm). The bark beetle gets its name from its habit of burrowing through the outer bark of the tree to the underlying cambial layer. This tender, nutritious layer lies between the underside of the tough outer bark and the inner wood. The cambial layer of the tree trunk produces nourishment and cell growth for the tree, and it is sometimes referred to as the vascular layer. It is also the layer that produces tree rings.
When a bark beetle penetrates the cambial layer, it chews out galleries, or tunneled lanes and grooves. Here the bark beetle lays eggs that hatch into larvae, which feed on the tree by gnawing out more galleries. Damage introduces fungi that can interfere with the tree’s ability to transport water. This, along with the girdling or excavation of the trunk, results in the death of the tree.
If a tree is healthy and bark beetle infestation is mild, the conifer might successfully fight off bark beetles by pitching out. Pitching out occurs when a tree oozes resin at the sight of a borrowing insect. The resin can envelop and suffocate the insect. Dark red pitch tubes often form at these sites, colored from mixing with boring dust.
The tree’s defense falls short, however, when the bark beetle engages in what is known as a “mass attack.” In this case, initial bark beetles emit pheromones to attract countless other bark beetles to the tree. The bark beetles attack in massive numbers.
Beetles not caught in pitch wait for the resin to harden to bore through again, causing more pitch out, until the bark beetles exhaust the tree’s defense system. They can then overwhelm the tree, eventually killing it. Bark beetles of different species participate in mass attacks, with each species occupying a different level of the tree. With many mass attacks in progress in a given area, bark beetles can wipe out significant areas of forest.
A tree suffering from bark beetle infestation is noticeable at a distance, as the topmost foliage loses its lush green color and fades before turning brown. While it’s natural to see a certain number of dying trees scattered throughout a large healthy forest, high numbers of brown-tipped pines usually indicate increased bark beetle activity. Heavily infested trees cannot be used for lumber, as bark beetle excavation ruins the wood.
The bark beetle has sparked much debate among environmentalists, forest management services, and even politicians. Some believe the bark beetle should be left alone to perform its natural function, even if that means losing large areas of forest. Others argue that heavy infestations should be stopped by intervention. Bark beetles are found in forests the world over.