Insects and Diseases

Bark Beetle Outbreaks Persist During Warm Year

Outbreaks of bark beetles and other insects expanded their footprints in forests during hotter, dryer conditions in 2024. Following a wet and cool year in 2023, the shift back to near-record temperatures and below-average precipitation in Colorado stressed trees needing several years of mild conditions to build defenses against attack from bark beetles and other forest pests.

Western spruce budworm remains the most widespread forest pest in Colorado, increasing its impact from 202,000 acres in 2023 to 217,000 acres in 2024, according to aerial survey data from the Colorado State Forest Service and U.S. Forest Service Rocky Mountain Region.

Data from the aerial survey also indicate that mountain pine beetle continues to build populations along the Front Range, in Gunnison County and in parts of the southwestern corner of the state. Total acres affected by mountain pine beetle in Colorado are up to 5,600. In addition, acres of trees killed by Douglas-fir beetle surpassed a level not seen in nearly a decade, while western balsam bark beetle remains the deadliest forest pest in Colorado for the third year in a row, despite holding steady at 27,000 acres of affected forests. Bark beetles are deadlier than western spruce budworm because they often kill trees outright, while the budworm and other caterpillars defoliate and weaken trees but do not always kill them.

Scientists in the Skies

Aerial forest health surveys have taken place in Colorado since the 1950s. Trained aerial observers with the CSFS and USFS fly over Colorado’s forests annually to inspect and record the damage from forest pests. This thousand-foot view allows them to detect, monitor and track outbreaks of insects by viewing their impact on the tree canopy. Infested trees may appear in shades of brown, red or gray as the needles and leaves on the infested trees succumb to the pest, fade and die. Forest managers then check some of the areas recorded during the flights to confirm the insect or disease responsible and the severity of the damage.

Last year, these scientists in the skies flew over 29.6 million acres of Colorado’s forests in small aircraft and recorded their observations, which are then analyzed and presented in this report and the accompanying ESRI StoryMap as a snapshot of forest health for 2024.

The airplane used for the 2024 aerial survey in Colorado — The aerial survey crew will fly in this small airplane, parked on a tarmac in Gunnison, to survey insect and disease damage to Colorado’s forests. Photo: Dan West, CSFS

An aerial view of a lodgepole pine forest in Colorado — Stands of lodgepole pine infested with mountain pine beetles appear reddish-brown when viewed from the air. Photo: Dan West, CSFS

Dr. Dan West, collecting notes from the sky during the 2024 aerial survey in Colorado — After spotting infested stands, Dr. Dan West, entomologist with the CSFS, records his observations on a tablet from a small airplane. After the survey season ends, Dr. West and aerial observers with the USFS reconcile their observations with each other’s and on-the-ground reports to create a data set for the year’s forest health survey. These data are presented as cumulative acres for each forest pest and as polygons on the map in this report. Photo: Brandon Leeper, USFS

The 2024 aerial survey crew spots an elk herd on a ridge in Hinsdale County, Colorado — Sometimes the aerial survey crew spots more than just trees, such as this elk herd atop the Jarosa Mesa in Hinsdale County. Photo: Dan West, CSFS

Notable Forest Pests in Colorado

Explore the information and map below for data and insights on insect and disease activity in Colorado from the 2024 aerial forest health survey. More analysis, including a deeper dive into climate conditions last year, is available in the 2024 Forest Health ESRI StoryMap.

Western Spruce Budworm

Acres affected statewide in 2024: 217,000

Western spruce budworm (Choristoneura occidentalis) remains the most widespread forest pest in Colorado. This native insect is a small moth that partially consumes the needles of trees during its caterpillar stage, leaving a rust-burnt color to the damaged foliage. Trees damaged by the budworm for numerous years are at high risk of attack from Douglas-fir beetle. The budworm is widespread throughout low-elevation mixed-conifer forests and spruce-fir forests across the state, especially in central and southern Colorado.

A graph showing the increase in acres affected by the western spruce budworm — The number of acres affected by western spruce budworm continues to increase in Colorado but has not yet reached levels seen in the mid-2010s. Graph: Dan West, CSFS

Mountain Pine Beetle

Acres affected statewide in 2024: 5,600

Mountain pine beetle (Dendroctonus ponderosae) infests all native pine species in Colorado, except for piñon pines, and lives in pockets of pine forests in areas of Colorado. This native bark beetle continues to infest mature lodgepole pine in Gunnison County, limber pine in the Mosquito Range in Chaffee and Park counties, and mostly ponderosa pine along the Front Range corridor, with an uptick in activity in Jefferson, Clear Creek and Gilpin counties.

A graph showing the increase in mountain pine beetle activity in Colorado — Mountain pine beetle is making a comeback in Colorado, although tree mortality from this insect is not even close to the levels experienced during the epidemic that subsided a decade ago. Graph: Dan West, CSFS

Western Balsam Bark Beetle

Acres affected statewide in 2024: 27,000

Western balsam bark beetle (Dryocetes confusus) and associated root diseases persist in high-elevation forests. In Colorado, subalpine fir grows intermingled with Engelmann spruce and becomes increasingly susceptible to bark beetle attack and decline during prolonged drought. Infestations of this beetle are patchy within a stand, but when all these patches are added up across Colorado’s forests, the impact of this native bark beetle is more apparent. This beetle is active in nearly all areas of the state with notable infestations in Routt and Jackson counties in northern Colorado; the Grand Mesa; the Elk and West Elk Mountains in Eagle, Garfield and Pitkin counties; and the central forests of the state in Gunnison, Summit and Grand counties.

A graph showing western balsam bark beetle-affected areas in Colorado — Western balsam bark beetle remains the deadliest forest pest in Colorado, despite holding steady at 27,000 acres from 2023 to 2024. Graph: Dan West, CSFS

Douglas-fir Beetle

Acres affected statewide in 2024: 21,000

Douglas-fir beetle (Dendroctonus pseudotsugae) remains ever-present in most stands with Douglas-fir trees, despite having killed many of the largest, most susceptible trees over the past decade. Ongoing drought and defoliation from western spruce budworm play a role in leaving trees susceptible to attack from Douglas-fir beetle. This bark beetle is especially active in Colorado’s central and southern conifer forests.

A graph showing the rising acres afftected by Douglas-fir beetle in Colorado — The number of acres affected by Douglas-fir beetle is the highest it has been in nearly a decade. Graph: Dan West, CSFS

Spruce Beetle

Acres affected statewide in 2024: 11,000

Spruce beetle (Dendroctonus rufipennis) remains in decline overall statewide. This bark beetle affected 19,000 acres in 2023 but only 11,000 acres in 2024, although it continues to infest high-elevation Engelmann spruce throughout localized portions of Colorado. Intense outbreaks are ongoing in newly infested forests within eastern Gunnison and western Chaffee counties in the Sawatch Range, and beetle activity straddling Park and Chaffee counties in the Mosquito Range has intensified. Beetle outbreaks in the San Juan Mountains in Dolores, La Plata, Ouray and San Juan counties continue to expand, though the intensity has declined from years past.

A graph showing the acres affected by the spruce beetle in Colorado forests — Spruce beetle remains in decline statewide in Colorado. Graph: Dan West, CSFS

Roundheaded Pine Beetle Complex

Acres affected statewide in 2024: 7,007

Roundheaded pine beetle (Dendroctonus adjunctus) and associated native bark beetles (southwestern pine beetle, mountain pine beetle, larger Mexican pine beetle and Ips engraver beetles) continue to attack previously uninfested stands in groups of 5-10 or more trees within ponderosa pine forests in southwestern Colorado. The intensity of beetle activity continues to remain high in localized areas of the San Juan National Forest.

A graph depicting the impact of the roundheaded pine beetle complex on Colorado forests — Roundheaded pine beetle and associated native bark beetles continue to impact forests in areas of southwest Colorado as affected acres increase. Graph: Dan West, CSFS

Aspen Issues

Acres affected statewide in 2024: 9,500

Some aspen defoliation occurred across Colorado, though most is localized and non-threatening. With severe and extreme drought conditions across much of the state from 2020-2022, followed by a mild year in 2023, then warm temperatures and slightly below-average precipitation again in 2024 across much of Colorado, fungal issues in aspen stands remain low to non-existent.