Most bark beetles are brown and compact in shape. They resemble weevils, their closest relatives, except they lack the prolonged snout. Male bark beetles are attracted to females by a sex pheromone (volatile odor) and mate in or near the female's larval burrow. Females migrate to a new place on the same tree or to a new tree and start another burrow. The female lays her eggs in pockets at intervals along the tunnel. When the eggs hatch as larvae, each individual forms its own tunnel at an angle to the original burrow made by the female. The larval tunnels may radiate outward under the bark to form a gallery, the shape of which is characteristic of certain species. The larvae are white, legless grubs. After their last growth stage, they form a pupal case of the last larval skin and begin to metamorphose into adult beetles. When the beetle emerges from the pupa as an adult, it burrows directly to the surface of the tree.
Adults searching for new trees are attracted to the odors given off by damaged or diseased wood, especially those under attack by other bark beetles. Bark beetles kill trees by girdling the sapwood with their burrows, cutting off the flow of nutrients to the tree's roots. A healthy tree may resist bark beetle attack by flooding the beetle tunnels with its own thick sap, or pitch, under high pressure. Some species of bark beetle also carry plant diseases, especially fungi. The smaller European elm bark beetle, with black thorax and red wing cases, is a principle carrier of Dutch elm disease. The western pine beetle is a major pest of evergreen forests in the western United States. The damage caused by these species and others like them results in the loss of millions of dollars a year in timber resources and is a major concern of agriculture and forestry.
Scientific classification: Bark beetles make up the family Scolytidae, order Coleoptera. The smaller European elm bark beetle is Scolytus multistriatus and the western pine beetle is Dendroctonus brevicomis.