Though mountain pine beetle numbers are declining, drought conditions from the summer have some officials worrying about a resurgence of the epidemic in Summit County.
In 2011, foresters observed an overall area of 752,000 acres of lodgepole, timber and ponderosa pine forest affected by the bark-beetle epidemic.
Bark beetles require ample sources of lodgepole pine to feed on and occupy, so the decreased density of Summit County forest lands leaves little food for bark-beetle populations to thrive. But the availability of lodgepole pine, large enough and old enough to support beetle populations are still present in certain areas, namely Breckenridge.
When the epidemic surged through Summit, the lodgepole pine of Breckenridge remained largely unscathed - leaving a small, but not insignificant place for bark beetles to thrive, giving a resurgence to the epidemic.
Officials like Jim Witcosky, an entomologist for the U.S. Forest Service, say a second wave of the epidemic is possible but unlikely in Summit County.
"Our forests have been depleted and will no longer serve as thriving habitats for the bark beetle," Witcosky said. "There may be a chance of trees becoming infected, but the expanse of the last epidemic will not be possible because there simply are not enough trees that are healthy enough, large enough and old enough to support a pine-beetle infestation."
For Summit, it all begin in the late 1990s when researchers in forest health noticed signs of infection of lodgepole pine throughout the county.
Beetles began to thrive during a period of drought and warm spring and summer temperatures that allowed the population of the beetles to increase.
Bark beetles thrive after drought years because trees have less defense capabilities and are more prone to infestation. Sap in trees' trunks becomes depleted after drought years because the lack of water reaching the trees makes the sap less useful and pressurized.
"A healthy tree would easily fend off the bark beetle when the sap oozes out and suffocates them," Witcosky said. "Following drought years trees can be taken over by multiple beetles - it usually takes thousands of beetles to kill a tree, though it can be considered infected with just one."
Infestations in lodgepole pine in Summit County have declined, largely due to loss of mature host trees. Few trees remain that are large enough to support populations of the beetle.
Some activity still is present in portions of the Colorado forests and along the eastern slopes of the Rawah Range in lodgepole pine stands less than 60 years old. Aerial surveyors also noted a decline along the Interstate 70 corridor, from Empire west to Berthoud Pass and the Eisenhower Tunnel, attributed to the loss of mature host trees.
A decline in activity was observed in lodgepole pine forests south of I-70, including the area from Georgetown south to Guanella Pass and in the Chicago Creek Basin north of Mount Evans.
Reduced activity also was noted in the Geneva Creek Basin south of Guanella Pass, where heavy damage occurred from 2008-2010. Infestations in lodgepole pine forests on the west side of South Park, from Kenosha Pass south to Fairplay, which also have been observed for approximately three years, decreased significantly in 2011. These areas still contain a high percentage of mature lodgepole pines, so the decline in mountain pine beetle activity in these areas does not appear to be due to host depletion.
Numerous activities are under way to prevent future attacks on high-value trees, making use of wood from dead trees and reducing the hazards presented by falling trees.
Individual high-value pines can be protected from bark-beetle attacks by applying preventive sprays. Three chemical insecticides - carbaryl, permethrin and bifenthrin - are registered for use by the Colorado Department of Agriculture as preventive sprays, which can be up to 95 percent effective when used properly.
Private landowners in Colorado's forested and urban communities have been treating individual pines with high rates of success. Preventive sprays also have been used extensively by the Forest Service to protect valuable trees in campgrounds and other high-use recreation areas.
The use of pouches containing the anti-aggregant chemical verbenone, a pheromone produced by attacking beetles, communicates to other beetles that a tree already has been attacked and is unavailable.
Verbenone, though proven less effective than preventative sprays, provides an alternative to spraying trees when chemical use is prohibited in places near bodies of water.
Forest thinning, especially in ponderosa pine forests, also tends to make trees less susceptible to beetle attack. Thinning reduces fuels available for wildfire and can increase growth rates of standing trees.
Dead and dying trees also are changing the characteristics of wildland fuels in Colorado's forests.
As fuel characteristics change over time, and as trees fall, the risk of high-intensity, long-duration fires may increase. Fires of this nature can pose additional threats to public infrastructure and private property. For example, intense fires could drastically alter soil characteristics and negatively impact watersheds.
Many landowners and forested communities have begun to remove dead trees from surrounding forests to mitigate fuels and provide for defensible space around structures, should a wildfire occur.