Forest Health Issues in Southwest Colorado

Trees across much of southwest Colorado had a respite from drought in the second half of 2024, but bark beetles remain a concern, with trees needing multiple wet years to build defenses.

Counties most impacted in southwest Colorado in 2024: widespread

Western spruce budworm (Choristoneura occidentalis) remains widespread in Douglas-fir and spruce-fir forests across much of southwest Colorado.

In 2024, this part of the state received a reprieve from drought in the latter half of the year. While the precipitation was good for the overall health of trees, the budworm eats the needles of spruce and fir trees, so it can benefit from periods of moisture when trees experience a spurt of new growth. Several consecutive years of adequate precipitation will be needed to alleviate the effects of drought on tree defenses, leaving many of the defoliated trees susceptible to subsequent bark beetle attack.

Defoliation in a stand of trees in Colorado caused by the western spruce budworm — Defoliation from western spruce budworm severely impacted this stand of Douglas-fir in southern Gunnison County. Photo: Ashley Baer, CSFS

This insect remains a top forest health issue across southwest Colorado. For example, staff in the Gunnison Field Office noted western spruce budworm – and subsequent attacks on budworm-weakened trees by Douglas-fir beetle – in the low-elevation transition zones in the Gunnison Basin. In some areas, the budworm has defoliated individual trees for several years and is killing trees outright, without the usual final blow from Douglas-fir beetle.

Please refer to the map of insect and disease activity with current data from last year’s aerial survey to see other places where the budworm is impacting forests in southwestern Colorado.

To manage for the budworm, Colorado State Forest Service foresters may recommend thinning susceptible trees to reduce the density of the forest and increase species diversity, or removing multiple levels of the forest canopy to hinder the spread of this insect.

Landowners with spruce or fir trees on their property should know the signs of budworm infestation and contact their local CSFS office for assistance if they suspect a tree is infested.

Counties most impacted in southwest Colorado in 2024: Chaffee, Gunnison, La Plata, Mineral, Rio Grande, Saguache

Douglas-fir beetle (Dendroctonus pseudotsugae) continues to cause tree mortality in low-elevation, mixed-conifer forests in southwest Colorado. Often the infested, over-dense stands of Douglas-fir trees have previously experienced defoliation by the western spruce budworm.

An area in Gunnison County, Colorado impacted by the Douglas-fir beetle — CSFS foresters are planning forest management treatments for 2025 in this forest in southern Gunnison County to mitigate an outbreak of Douglas-fir beetle that killed these trees. Photo: Dylan Eimer, CSFS

This is concerning because mixed-conifer forests with Douglas-fir trees make up a large component of forests on private property and in neighborhoods in the wildland-urban interface. This increases wildfire risk to homes and businesses. These mixed-conifer forests also provide excellent habitat for wildlife and are commonly used by migratory animals while they are on their winter range.

Colorado State Forest Service foresters offer science-based advice and strategies for residents and communities to manage this native bark beetle, including pheromone (semiochemical) packets to deter the beetle from attacking, assistance with removing trees killed by the beetle, and forest management approaches that increase the types of tree species and decrease the number of Douglas-fir trees with thicker trunks that are more susceptible to attack.

Landowners with Douglas-fir trees on their property should know the signs of a Douglas-fir beetle infestation and contact their local CSFS office for assistance if they suspect a tree is infested.

Counties most impacted in southwest Colorado in 2024: Gunnison, Montrose

Staff in the Gunnison Field Office continues to report damage from mountain pine beetle (Dendroctonus ponderosae) in stands of lodgepole pine in the upper Gunnison River Watershed, including the Taylor River, East River and Ohio Creek drainages. Nearly 40 percent of mountain pine beetle-caused mortality in southwest Colorado occurred in Gunnison County.

Dead lodgepole pine trees viewed from the air over Gunnison County, Colorado — Mountain pine beetle killed these lodgepole pine trees near Carbon Peak in Gunnison County. Photo: Dan West, CSFS

According to the U.S. Drought Monitor, most of Gunnison County experienced a respite from drought for much of last year. The increased moisture will help these lodgepole pines recover their defenses to ward off the mountain pine beetle, though multiple wet years are still needed for trees to fully build defenses.

Gunnison County is not alone when it comes to this beetle. Colorado State Forest Service foresters and aerial survey data also note mountain pine beetle activity in Montrose County in the northern reaches of the Uncompahgre Plateau. A complex of native bark beetles, including mountain pine beetle, is affecting the southern portions of the plateau.

Early detection is key to managing the mountain pine beetle. It is mostly impacting lodgepole pines in this part of the state, so residents with lodgepole pines on their property should keep an eye out for signs and symptoms of infestation.

CSFS foresters in Gunnison have observed a reduction in lodgepole pine losses when the sanitation of mountain pine beetle trees is completed in conjunction with the use of repellent pheromones. This integrated pest management practice appears to be most effective during the early stages of a mountain pine beetle outbreak. For landowners affected by stand level losses, even-aged management practices may be successful at regenerating a healthy new cohort of lodgepole pine.

Education is also an important strategy for managing the mountain pine beetle. The CSFS offers a fact sheet, poster and storybook available to order online.

Counties most impacted in southwest Colorado in 2024: Dolores, Montezuma, Montrose, San Miguel

Colorado State Forest Service foresters continue to see impacts from roundheaded pine beetle (Dendroctonus adjunctus) and other associated native bark beetles in the Glade area of the San Juan National Forest northwest of Dolores. As they cruise this area for Good Neighbor Authority projects with the U.S. Forest Service, they have noticed up to 75 percent mortality in some sites.

A forested area in Dolores County, Colorado impacted by the pine beetle complex — This outbreak of the pine beetle complex is in Dolores County. Photo: Dan West, CSFS

Despite having fewer suitable trees to attack, the beetle continues to intensify and expand its footprint in this area. New infestations have occurred in San Miguel County as these populations build in the Naturita Canyon and Iron Spring Mesa areas.

Please refer to the map of insect and disease activity with current data from last year’s aerial survey for other specific locations where the roundheaded pine beetle complex is impacting forests.

Outbreaks of the roundheaded pine beetle have been present in parts of southwest Colorado for more than a decade. This native bark beetle has thrived in southwest Colorado due to drought conditions over much of that time that have weakened trees. It often works with other bark beetles, including the mountain pine beetle, to injure and kill ponderosa pines. The CSFS is working to identify the most prevalent species of bark beetles driving these outbreaks to marry management objectives with the biology of the pests.

Contact your local CSFS office for assistance in managing for roundheaded pine beetle and other bark beetles.

Counties most impacted in southwest Colorado in 2024: Chaffee, Dolores, Gunnison, Hinsdale, La Plata, San Juan

Spruce beetle (Dendroctonus rufipennis) remains active in forests in southwest Colorado despite a statewide decline. This bark beetle has depleted susceptible Engelmann spruce and in areas has attacked lodgepole pine, often killing the tree, though few beetles emerge from these attacks. It prefers large-diameter Engelmann spruce trees but attacks and kills trees as small as 4 inches in diameter.

A forest in northwest Mineral County, Colorado with spruce beetle damage — Spruce beetles have affected nearly every tree in this forest in northwest Mineral County. Photo: Dan West, CSFS

Gunnison County was the county most impacted statewide by the spruce beetle in 2024. Elsewhere in this part of the state, Colorado State Forest Service foresters in the Alamosa office have noticed impacts from the spruce beetle throughout the Culebra Range in Costilla County south to the New Mexico border. In Chaffee County, spruce beetles continue to move north of the Chalk Creek drainage to Mount Columbia, northwest of Buena Vista. Beetles are also moving north through the Mosquito Range in Park County toward Leadville.

Spruce beetle-caused mortality in forests north of Durango and south of Silverton in the Needle and San Juan mountains within Hinsdale, La Plata and San Juan counties is expanding to previously uninfested areas, indicating the beetle is still on the move and will likely cause damage in the coming year.

CSFS foresters manage spruce beetles by removing susceptible and affected trees before an outbreak.

A way to manage for spruce beetle on smaller acreage is by placing pheromone packets on trees. These packets release a chemical that tricks beetles into thinking an otherwise healthy tree is already infested with beetles. These treatments will be available through the CSFS in early 2025. Contact your local CSFS office for more information.