Mountain Pine Beetle
(Dendroctonus ponderosae)
Mountain pine beetle (MPB), Dendroctonus ponderosae, is an insect native to the forests of western North America. Periodic outbreaks of the insect can result in losses of millions of trees. 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 trees in the outbreak area.
MPB has a one-year life cycle in Colorado. In late summer adults leave the dying trees in which they developed. As attacked trees die; needles turn reddish to yellow, straw colored. Females seek out living, green trees that they attack by tunneling under the bark. Coordinated mass attacks by many beetles are common.
MPB larvae spend the winter under the bark. Larvae continue to feed in the spring and transform into pupae in June and July. Emergence of new adults can begin in early July and continue through September. However, the great majority of beetles exit trees during late July (lodgepole pine) and mid-August (ponderosa pine).
A key part of this cycle is the ability of MPB to transmit bluestain fungi (Ceratocystis species). Spores of these fungi contaminate the bodies of adult beetles and are introduced into the tree during attack. Fungi grow within the tree and, together with beetle feeding, weaken the tree. This mutual network of beetle galleries and bluestain fungi disrupt transport of water in the tree and rapidly kill it. The fungus gives a blue-gray appearance to the sapwood.
Natural controls of MPB include woodpeckers and insects such as clerid beetles that feed on MPB adults and larvae under bark. Extreme cold temperatures also can reduce MPB populations. However, during outbreaks these natural controls often fail to prevent additional attacks.
Perhaps the most important natural control is tree vigor. Healthy trees are less attractive to beetles than trees under stress. Vigorously growing trees also have better defenses that allow them to “pitch out” pine beetles.
The best long-term way to minimize MPB loss is to thin trees. Consult a professional forester to select the best cultural practices for your land.
Logs infested with MPB can be treated in various ways to kill developing beetles before they emerge as adults in summer. Logs may be burned to kill the larvae under bark. Intense solar radiation that dries out the cambium and raises temperatures to lethal levels (110F+) can kill MPB larvae. Beetles also die if the bark is removed by peeling or milling. Burying is another option to kill MPB in infested logs. In some cases, hauling infested logs to safe sites: a mile or more from susceptible tree hosts also is practiced.
Chemical control options for MPB have been greatly limited in recent years. (A few formulations of Lindane, usually marketed as some brand of borer spray, remain available to treat infested logs).
For more information contact: C.S.U. Extension Service for Service in Action sheet 5.528
C.S.U. Extension Service, (719) 636-8926