Posted on 13/09/03 in Environment
Over 600 beetles in the family Scolytidae are commonly referred to as bark beetles. Species identification is difficult because nearly all bark beetles are black or brown, cylindrical, hard- shelled, and between 1/8 and 1/3 inch long. Luckily, similarities in their life cycles and in the injury they cause usually make species determination unnecessary for making control decisions.
Adult bark beetles bore through the bark to the cambium layer of suitable host trees. Females excavate a tunnel between the bark and wood along which they lay their eggs. Upon hatching, each grub burrows away from the egg tunnel and feeds on the live bark tissue (phloem) and outer cell layers of wood (xylem). The resulting network of egg and larval tunnels beneath the bark is called a gallery. The “shot hole” appearance of the bark in infested trees indicates that numerous beetles have matured, chewed exit holes, and flown off to find new breeding sites.
From one to six generations per year are typical depending on the species.
As a general rule, bark beetles attack trees that are weakened or dying due to stress factors such as drought, disease, smog, mechanical injury, or alteration of the water table and root damage due to nearby construction. They are also attracted to recently cut wood which still has bark. In pines, resin often oozes from the bark where beetles first attack, producing conspicuous pitch tubes. Some beetles become trapped in the pitch and die. A healthy tree produces enough pitch to prevent successful attack by many beetles, but sometimes by sheer numbers alone bark beetles are able to overwhelm and kill healthy trees. This may happen to trees which are near heavily infested breeding sites. Once a bark beetle is successfully established in a tree, it emits a chemical called a pheromone which attracts other beetles to the same tree. Once infested, trees almost never recover and control efforts are usually futile. Bark beetles do not attack trees and wood that are dead and dried, nor dying or recently cut wood if the bark is removed.
Several of our most common types of bark beetles are listed below, along with characters which should help you identify them. Remember that there are many other species which may be encountered in shade trees and wooded areas from time to time.
Ips Beetles. Bark beetles in the genus Ips are commonly called engraver beetles or simply Ips beetles. They can be distinguished from other bark beetles by their scooped-out hind ends. Ips galleries, found in pines, have egg tunnels in the form of an H or a Y. Though capable of attacking the entire tree, Ips beetles are usually confined to the crown. Southern Pine Beetle.
One of our smaller bark beetles, the southern pine beetle is barely 3/16 inch long. Following long, dry spells or poor forest management, outbreaks occur that rapidly kill large areas of pine forests. Southern pine beetles attack mainly the middle or upper part of the tree trunk. All ages and sizes of pine trees are potential hosts. Their egg and larval tunnels wind around in an unorganized pattern. Healthy, vigorous trees and proper forest management practices reduce the likelihood of outbreaks and tree losses.
Black Turpentine Beetle
This beetle is large for a bark beetle, about 1/3 inch long. It attacks pine trees at the base of the trunk, and may also breed in stumps. Black turpentine beetle grubs feed together and excavate large patches under the bark. A common characteristic of this beetle’s attack is the presence of a glob of pitch, about 1/2 inch in diameter, at the exit hole. Sometimes there will be large numbers of white pitch globs on the dark bark.
Elm Bark Beetles. There are two species of bark beetles which attack elms. Both of them are capable of transmitting Dutch elm disease when they feed on healthy trees. The European elm bark beetle feeds in the crotches of one- to three-year-old-twigs; the native elm bark beetle feeds in the thick bark of trunks and limbs. Native elm bark beetles construct egg tunnels across the wood grain. Egg tunnels of the European elm bark beetle are parallel to the grain. Both make galleries and breed only in recently killed or dying elm wood three inches or larger in diameter.
Other common bark beetles include: Shothole Borer which attacks fruit trees, wild cherry, serviceberry, and occasionally elm; Peach Bark Beetle in stone fruits, mountain ash, elm, and mulberry; Pityogenes spp. and Pityophthorus spp. in pines; Phloeosinus spp. in cypress and junipers; Ash Bark Beetle in ash; Birch Bark Beetle in birch, beech, wild cherry and red gum; and Hickory Bark Beetle in hickory.
On healthy trees bark beetles may attack individual twigs and branches that are dying from shading out or other causes. For example, some species breed only in the dead or dying twigs, branches, and limbs of pines. These bark beetles will not breed in live branches, and thus are not a progressive destructive threat to healthy parts of trees.
The old adage about an ounce of prevention being worth a pound of cure is especially appropriate here. Once a tree becomes infested with bark beetles, it usually dies rapidly. Bark beetles attack weakened, stressed, or dying trees. Preventative measures include: 1.) maintaining healthy, vigorous trees; 2.) eliminating beetle breeding sites, such as recently dead or cut trees, limbs, slash, and firewood with bark; and 3.) applying residual insecticides to susceptible but as yet uninfested trees, especially those under stress and therefore attractive to bark beetles. Treating infested materials before bark beetles emerge will kill them as they chew their exit holes. Check the Virginia Pest Management Guides for current insecticide recommendations. Always read and follow the instructions on the pesticide label.