Mountain pine beetle is a tiny black insect that is a major killer of pine trees in North America. An outbreak of mountain pine beetle can kill millions of trees over large regions. Since the 1990’s, outbreaks of mountain pine beetle have caused 60 million acres (25 million hectares) of tree deaths from northern New Mexico to British Columbia. Most of the deaths occurred among lodgepole and ponderosa pines. But mountain pine beetles also feed upon jack, sugar, western white, and whitebark pines. Mountain pine beetles are native to North America and have long played an important role in forest ecosystems by clearing areas for new growth. However, recent warm and dry weather, along with an abundance of susceptible trees, have led outbreaks of mountain pine beetle to spread beyond the control of such natural predators as other insects and birds. A large outbreak of pine beetles can dramatically change forest ecosystems.
The mountain pine beetle lives in pine forests from northern Mexico to the province of British Columbia, in western Canada. The insect measures about 1/5 inch (5 millimeters) in length, about the size of a grain of rice. Across most of its range, the beetle usually completes its life cycle in one year. In late summer, the adult beetle bores through the bark of a pine tree and into the layer between the wood and the bark. A structure on the beetle’s mouthparts carries the disease-causing blue stain fungus. The beetle’s entry introduces the fungus. The fungus grows rapidly, slowing the movement of fluids and disrupting the tree’s defenses against the invading beetles. Eventually, a lack of water and nutrients kills the tree. The female beetle lays many eggs beneath the bark. The eggs develop into larvae (young) that spend winter inside the tree, developing into adult beetles the following summer. These beetles then emerge from the bark and fly in search of a new host tree.
Signs of mountain pine beetle infestation include the presence of pitch tubes, discoloration of tree needles, and an accumulation of frass, a sawdustlike waste. Pitch tubes are small masses of resin that form where the beetle has entered the bark.
Trees that are weakened by old age, crowding, drought, disease, or fire are the most susceptible to mountain pine beetle attacks. Severe outbreaks usually occur where there is a large population of mature trees. The outbreaks may be driven by circumstances that weaken a tree’s defenses, including drought or disease. Outbreaks may also be driven by circumstances beneficial to the beetle. For example, warm winters enable the beetles to extend their range to areas that are normally too cold. Warm winters can also enable more larvae to survive to maturity. The beetles may also benefit from decreased predator activity by birds and wasps and from windthrow (the uprooting or breaking of trees by the wind).
