Mountain Pine Beetle

Background

The mountain pine beetle (Dendroctonus ponderosae) is a beetle whose native range spans most of western North America.  This includes most of the western United States: Washington, Oregon, Idaho, Montana, Wyoming, Colorado, Utah, South Dakota, California, Nevada as well as the Canadian provinces of British Columbia and Alberta and northern Mexico (Furniss and Carolin 1977).  The beetles have a varied vertical range as well, which has been observed as low as sea level to as high as 10,000 feet (Nat'l Park Service, Sambaraju et al. 2011).  Adult beetles range from 4 to 7.5 mm in length and have black, cylindrical bodies (Furniss and Carolin 1977).

Estimated mountain pine beetle range
(Courtesy of the Nat'l Park Service)

The beetles inhabit several types of pine trees which include: lodgepole, ponderosa, whitebark, limber, and white pine.  The beetles have been documented to inhabit other pine species as well.  Other species of beetle have been known to nest in other types of trees such as Douglas fir and assorted spruce species (Colorado State University; Dept. of Natural Resources, Montana).

Life History

During the summer, adult beetles emerge from pine trees and begin their search for a new host tree.  Females are searching for trees that match specific requirements.  Preferred tree characteristics are: large trees, possibly damaged, stressed, crowded, and/or diseased.  It has been hypothesized that during large beetle infestations, these preferred characteristics may not be important.  When females find the desired tree, they burrow in the tree trunk.  A response to the burrowing is a distinctive resin at the entrance produced by the tree as a defense mechanism (Colorado State University).

Resin at the entrance of tubes
(Courtesy of the DNRC)

After females have finished burrowing, adult males mate with the females and the females lay 75 eggs (Colorado State University).  The individual eggs hatch and produce larvae.  The larvae feed on the phloem of the tree during the course of the winter (Dept. of Natural Resources, Montana).  By the time summer arrives, about July and August, the larvae have matured and exit the trees to find a host tree to mate in (Colorado State University).  

Pheromones

Females searching for trees in the summer receive and emit signals to other beetles.  Females emit the compound, trans-verbenol, that attracts males and females to the tree.  The males that make their way to the tree, mate with the females and produce the compound exo-brevicomin.  This compound targets female beetles.  The combination of these two compounds signals to others that the tree is suited for burrowing.  After many individuals burrow into the tree, trans-verbenol and exo-brevicomin excretion decreases.  Males proceed in emitting frontalin and females produce verbenone, a combination that indicates that the tree is fully occupied (Erbligin et al. 2013).