The Mountain Pine Beetle Epidemic
Since the turn of the century, forests across the western United States and Canada have been ravaged by bark beetles, particularly the mountain pine beetle (MPB). While the MPB is native to western forests, the current outbreaks are unprecedented in extent and severity. Western lodgepole pine and ponderosa pine forests are particularly vulnerable to beetles due to historic management trends, in which widespread timber harvests to support mines and railroads at the beginning of the twentieth century – and aggressive fire suppression since then – has led to homogeneous, overstocked stands of trees that are the ideal host size. Beetle spread has likely been accelerated by warmer winters increasing beetle survivability, and drier summers weakening potential host trees.
MPB and other bark beetles infect host trees as part of their reproductive lifecycle. They swarm trees, boring through the outer bark, and then carving lateral galleries through the water-conducting inner bark in which they lay their eggs. Healthy trees combat beetles by exuding sap that physically pushes the invaders out, though this defense is overwhelmed if the tree is weak or the beetle too numerous. While some trees seem better adapted to resist beetles than others, most trees will die rapidly from a combination of loss of water conductance (girdling) and infection from symbiotic blue-stain fungus introduced by the beetles, and mortality often reaches >80% in pure pine stands. These dead trees change from green- to red -to grey-colored as their needles gradually die and drop, though the dead trunks typically stay standing for 5-10 years until rot and wind knock them over. Beetle-killed stands switch from carbon source to sink, likely subject to more sever wildfire (though not more frequent).
To date, bark beetle infestations have caused significant tree mortality over more than 60 million acres across the Rocky Mountains in the western US and Canada. MPB infestations have slowed in many areas due to the beetles running out of suitable host trees to infect. However, similar pests are reaching outbreak levels, such as the spruce beetle spreading across the higher-elevation spruce-fir forests in Colorado. Bark beetle infestations are also accelerating in Sierra Mountain ponderosa pine forests in California due to recent extreme drought there.
Other Links & Resources:
- National Geographic: The Bug That’s Eating the Woods
- Our Future Forests: Beyond Bark Beetles video series
- US Forest Service’s Rocky Mountain Bark Beetle homepage
- US Forest Service Aerial Detection surveys
- Colorado State Forest Service Mountain Pine Beetle homepage
- Colorado State Forest Service (CSFS): Colorado Forest Health Reports
- Aspen Center for Environmental Studies: Forest Health Program