Bark beetles can be destructive pests in forests and urban landscapes. Urban expansion, which generally involves forests that are cleared to make room for new housing or commercial development, can be stressful to trees that remain, thus increasing susceptibility to bark beetles. Most of the plant-damaging bark beetles that feed on stressed trees are in the family Scolytidae. There are two groups: bark beetles and ambrosia beetles. Ambrosia beetles cultivate and feed on a fungus called “ambrosia” that stains wood and reduces the value of timber. Most bark and ambrosia beetle species tend to live in recently cut, injured, or unthrifty trees that are in the process of dying. Older trees are usually not attractive to beetles. However, certain species do prefer trees that are completely dead, whereas other species normally attack and kill healthy, vigorous trees.
Bark beetles live under the bark of trees, right at the surface of the wood, and feed on phloem tissue. In general, beetles complete one generation in the tree, then abandon it, and search for another suitable tree for development. The major genera of bark beetles are Dendroctonus, Ips, and Scolytus. Certain species of Ips and Scolytus, because they etch the sapwood, are referred to as “engravers.”
Bark beetles are small, cylindrical, and less than 8 mm in length. They are typically brown to red in color. Many bark beetles are specific in the tree species they attack. Bark beetles reproduce in the thin layer of plant tissue between the bark and wood. After tunneling through the bark, the female beetle excavates a gallery in the inner face of the bark and the wood. The female then chews small niches in the sides of this gallery and lays a single egg in each. Adults protect the eggs (and larvae that emerge) by remaining in the gallery system to prevent entry by natural enemies such as parasitoids and predators into the galleries. The eggs hatch into legless white larvae that chew and create their own galleries, which are at right angles to the galleries where the eggs were laid. After several weeks to several months, depending on the species of bark beetle, the larvae reach maturity, and pupate in the tunnels. Larvae transform into adults that chew through the bark to the surface. Bark beetles normally overwinter beneath the bark.
Bark beetles tend to feed on dying trees although they may attack living trees, particularly conifers, and kill them. However, bark beetles can successfully colonize healthy conifers only when they overwhelm the tree defenses by mass attack. Adults and larvae interrupt nutrient flow by feeding in the phloem. Trees infested with bark beetles exhibit characteristics such as faded foliage at the top or middle of the tree canopy and reddish brown frass or hardened pitch tubes on the trunk.
Bark beetles are also capable of transmitting diseases. For example, Dutch elm disease, which is responsible for the decline of American elms throughout the United States, is vectored by two bark beetles: the smaller European elm bark beetle, Scolytus multistriatus, and the native elm bark beetle, Hylurgopinus rufipes. In general, the fungus is introduced by the adult beetle and spread by the larvae. The fungus spreads inward and causes the plugging of the water transport system (xylem).
Trees can differ significantly in their susceptibility to bark beetle attack, depending on location, vigor, age, and physiology. Trees growing on poor sites tend to be attacked more than trees growing on choice sites because considerably more beetles are required to kill a vigorously growing tree than a tree growing at a slower rate. Bark beetles primarily attack drought-stressed trees. They generally overwhelm a tree’s defenses by inundating it with sheer numbers of beetles. Beetles release pheromones, which attract other beetles to a site. In addition, females and males are attracted to the odors or chemicals produced by drought-stressed trees. For example, many trees that are drought-stressed emit an abundance of volatile chemicals such as terpenes, particularly alpha-pinene, which attracts bark beetles to stressed conifers. Also, drought-stressed trees have lower defenses than healthy trees. For example, the production of the defense chemical oleoresin is linked to the colonization success of bark beetles: Trees with low levels tend to be attacked by greater numbers of bark beetles than are trees that produce high quantities.
As plants dry up or dehydrate during droughty conditions, cavitations (collapse) of the water columns in the xylem tissue produce acoustical, ultrasonic, emissions that are sensed by bark beetles, which aids them in detecting drought-stressed trees.
The primary strategy to avoid dealing with bark beetles is maintaining the health of trees through proper watering, fertility, mulching, pruning, and eliminating any stress, such as mechanical injury caused by mowers or weed-whackers.