Low-level populations of mountain pine beetles usually attack individual or small groups of trees that have experienced disease, lightning strikes, or other stressors. Tree mortality might be limited to small areas of lodgepole or ponderosa pine forests that often go unnoticed, or large areas can be impacted. The combination of an abundance of susceptible forest stands (such as those with high tree densities, and large trees), suitable environmental conditions (such as a drought), and an increasing mountain pine beetle population can set the stage for the development of epidemics. Colorado’s current mountain pine beetle epidemic is the largest recorded in state history.
When populations reach epidemic levels, direct control efforts are generally ineffective. Some efforts were made to control spruce beetle and mountain pine beetle outbreaks in Colorado in the 1940s and 1970s. But unfortunately, no success was documented. An effort to suppress a mountain pine beetle outbreak in Crater Lake National Park was documented to be ineffective.
Three main factors, apparently happening concurrently, have fostered a “perfect storm” resulting in the intensive mountain pine beetle outbreak in Colorado. First, extensive areas of continuous lodgepole and ponderosa pine forests are growing with characteristics that make the stands susceptible to mountain pine beetle—such as the presence of large-diameter trees and dense stands with a high proportion of host trees. Second, drought conditions began in Colorado in the late 1990s, becoming severe by 2000. This lack of moisture stresses trees, which then become more susceptible to mountain pine beetle attack. Third, cold winter temperatures are a primary mortality agent of the mountain pine beetle, and during the last decade or so, Colorado has not experienced sufficiently cold temperatures to trigger significant insect mortality.
There is no simple answer to this question. Mountain pine beetles overwinter primarily in their “worm” or “grub” stage (the larvae). During this time, they accumulate alcohols that act as an anti-freeze and provide protection from freezing. The beetles are more susceptible to cold temperatures early in the fall and late spring, when alcohol levels are low, and less susceptible to cold in mid-winter when alcohol levels are highest. Studies show that temperatures from -13 F to -31 F in mid-winter can cause mortality. However, factors such as the stage of development, duration of exposure to cold temperatures, responses to seasonal changes in temperatures, and geographical location will influence potential mortality. So what temperatures, at what time of the year, or for how long will cause extensive insect mortality is not yet well-understood.
Forest stands with high densities of trees 6 inches in diameter or larger are considered susceptible to mountain pine beetles. Initially, beetles usually do not show a preference for larger-diameter trees, but as the outbreak collapses, or ends, more larger trees are usually killed.
People have raised concerns about the movement of mountain pine beetle from lodgepole pine forests into ponderosa, limber, and Rocky Mountain bristlecone pine stands—all of which are suitable host trees for the mountain pine beetle. Little research has been conducted on this poorly understood topic. For example, the mountain pine beetle epidemics that occurred in ponderosa pine stands in the Arkansas Valley, in South Park, and around Red Feather Lakes in Colorado in the 1990s and early 2000s, and in the Front Range in the late 1960s and 1970s. Observations across the West and in southern Wyoming during the current epidemic suggest that the insects are spreading to different hosts. At present, mortality is occurring in ponderosa pine forests along the Colorado Front Range and in southern Wyoming.
The mountain pine beetle outbreak in lodgepole pine forests began in British Columbia (BC) during the mid 1990s, and by 2008 had affected approximately 35 million acres of pine forests. During the early years of the outbreak, the government of BC, together with the forest industry, worked to control beetle populations by felling and burning infested trees in inaccessible areas, and harvesting and processing trees in accessible areas. However, the increase in populations was so rapid and synchronous across the province that by 2002, control programs were abandoned, and efforts were largely redirected to salvage and recovery of impacted regions.
In 2002, and again in 2006, large numbers of beetles were carried in winds from central BC over the northern Rocky Mountains and spread across the pine forests of the Alberta Plateau, well beyond the beetle’s historic range. In an attempt to avoid further eastward expansion and potential invasion of the boreal jack pine forests that stretch across the continent, the governments of BC and Alberta implemented an aggressive control program to suppress beetle populations east of the Rocky Mountains through felling and burning infested trees. Since its inception in 2004, the program has managed to keep populations from expanding.
Removal of infested trees provides some local relief from the insect population if done prior to insect emergence. But removing infested trees will have no impact in a landscape saturated with mountain pine beetles and an abundance of susceptible trees. The landowner can make a better investment at this point by applying preventive insecticidal sprays on live trees prior to beetle flight. Information on this topic is available at the Colorado State Forest Service website It is important to use a commercial applicator and remove any dead trees, as they are a safety hazard.
Insecticides are safe to use when properly applied. However, they can have some harmful effects. Non-target insects may be affected, including some natural enemies of the mountain pine beetle. Homeowners should take caution when applying chemicals close to water because they will impact aquatic organisms. It is important to follow labeled instructions and avoid application during periods of elevated winds, when rain threatens, or when there is standing water or snow on the ground.
Again, the most effective method to protect uninfested trees is to use insecticidal sprays prior to insect flight. This is the only chemical-based approach that has been rigorously examined by scientists and proven to be effective.
Solar treatments for reducing mountain pine beetle survival are effective, but they are laborious and only practical on a small scale. They also are more suitable for use under the more open canopy and lower elevation of ponderosa pine forests because the solar radiation is more intense. Solar treatments can be used in lodgepole pine forests, but site selection is important to ensure strong solar radiation. For solar treatments, debarking of trees is not necessary. Guidelines for the use of solar treatments for ponderosa and lodgepole pine can be obtained from the Colorado State Forest Service website. However, debarking of trees alone will kill mountain pine beetles by exposing them to unprotected environments. Debarking can be done with special chainsaw attachments, but is also quite laborious. Still, the abundance of mountain pine beetles and the extent of the current epidemic make the use of these approaches questionable.
Wrapping of standing trees should not be done because there is no information to support its use. There are no proven or rigorously tested methods for killing beetles once they are in the trees. Landowners are encouraged to make sure adequate research has demonstrated the efficacy of products offered on the market.
Studies continue on the use of preventive insecticide sprays and insecticides injected into the trees (systemic insecticides). Studies also continue to develop approaches using anti-aggregation chemicals, which simulate the chemicals that beetles produce to keep other beetles away. These anti-aggregation chemicals are intended to interfere with the ability of the mountain pine beetle to infest trees.
Mountain pine beetles have several natural enemies such as parasitic and predatory insects, birds that utilize insects as food, and diseases. These organisms impact mountain pine beetle populations under low-level populations but, in most cases, have little impact on beetle populations at epidemic population levels.
Research is currently underway by various agencies to better understand the historical conditions of Colorado’s lodgepole, ponderosa pine, and mixed conifer forests and how this relates to bark beetle ecology.
The use of vegetation management through silvicultural thinnings can help manage mountain pine beetle. The appropriate time to treat stands is when the current epidemic collapses and populations return to endemic levels. Although we know the conditions that make stands susceptible to mountain pine beetle attack, and treatments that can be effective in reducing susceptibility, we do not know what the optimal treatment size is or what level of insect populations can still overcome treated stands. Treating entire landscapes would also be difficult, costly, and ecologically undesirable, so there will always be susceptible stands in which insect populations can increase and spill into treated areas. Despite any and all efforts, there will never be a “beetle-proof” stand.
Mountain pine beetles are helping to create multi-aged forest stands by removing large-diameter trees, reducing stand densities, and providing growing space for other canopy trees and below the main canopy. Future stand development will vary across the landscape and will be affected by current stand composition and further disturbance. Future dominance of species will vary from site to site depending on factors such as site quality, established regeneration, moisture, elevation, and cardinal direction of stands.
Bark beetles: A natural and dramatic forest disturbance (summary of ongoing research)
Colorado State Forest Service information on mountain pine beetle (website)
Solar treatments for reducing survival of mountain pine beetle in infested ponderosa and lodgepole pine logs (publication)
Mountain pine beetle (publication)
Natural falling of beetle-killed ponderosa pine (publication)
Ponderosa pine mortality resulting from a mountain pine beetle outbreak (publication)