Beetle Info

Mountain pine beetle (MPB) is native to the forests of western North America. Periodic outbreaks of the insect, previously called the Black Hills beetle or Rocky Mountain pine beetle, can result in losses of millions of trees. Outbreaks develop irrespective of property lines, being equally evident in wilderness areas, mountain subdivisions and back yards. Even windbreak or landscape pines many miles from the mountains can succumb to beetles.

Mountain pine beetles develop in pines, particularly Ponderosa & Lodge-pole pine. Bristlecone and Pinyon pine are less commonly attacked. During early stages of an outbreak, attacks are limited largely to trees under stress from injury, poor site conditions, fire damage, overcrowding, root disease or old age. However, as beetle populations increase, MPB attacks may involve most large trees in the outbreak area.

Mountain pine beetle has a one-year life cycle in Wyoming. In late summer, adults leave the dead, yellow- to red-needled trees in which they developed. In general, females seek out large diameter, living, green trees that they attack by tunneling under the bark. However, under epidemic or outbreak conditions, small diameter trees may also be infested.

Coordinated mass attacks by many beetles are common. If successful, each beetle pair mates, forms a vertical tunnel (egg gallery) under the bark and produces about 75 eggs. Following egg hatch, larvae (grubs) tunnel away from the egg gallery, producing a characteristic feeding pattern. MPB larvae spend the winter under the bark. Larvae are able to survive the winter by metabolizing an alcohol called glycerol that acts as an antifreeze. They continue to feed in the spring and transform into pupae in June and July. Emergence of new adults can begin in mid-June and continue through September. However, the great majority of beetles exit trees during late July (Lodge-pole pine) and mid-August (Ponderosa pine).

A key part of this cycle is the ability of MPB (and other bark beetles) to transmit blue stain fungi. Spores of these fungi contaminate the bodies of adult beetles and are introduced into the tree during attack. Fungi grow within the tree and assist the beetle in killing the tree. The fungi give a blue-gray appearance to the sapwood. However, not all blue stains are blue. Common stain shades can be blue to bluish black or gray to brown. Sometimes, the stain coloration in lumber may appear as red, yellow, orange, or purple. The blue stain has no effect on the performance and strength of lumber.

Blue stain is not a decay fungi. Blue stain fungi live on the nutrients stored in the cells of the wood, not on the cellulose fibers of the tree itself. Because blue stain does not detract from the strength properties of dimension lumber, blue-stained lumber can be used for exactly the same purpose as non-stained lumber. Blue stain is not a mold. A simple test to determine whether or not a piece of lumber is blue-stained or growing mold, lightly rub the affected surface of the wood. Mold grows on the surface and can be brushed off or smeared, where as blue stain penetrates deep into the wood and cannot be removed.