Insects and Diseases

Forest Pests Continue to Spread in Colorado Despite Milder Year

Colorado experienced wetter, cooler conditions in 2023 compared to recent years. This was good for many species of trees in areas of Colorado suffering from prolonged drought, but trees will need several years of adequate moisture and lower temperatures to recover, regain their health and ward off attack from bark beetles and other forest pests. Populations of bark beetles and western spruce budworm remain high in forests and a milder year is not enough to reverse recent trends.

Western spruce budworm remains the most widespread forest pest in Colorado, 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 western balsam bark beetle remains the deadliest forest pest for the second year in a row, despite it impacting fewer acres of Colorado’s forests in 2023.

Thousand-Foot View of Forest Health

Since the 1950s, forest managers have flown over Colorado to detect and monitor disturbances to forests from insects and diseases, collecting data that provide an annual snapshot of forest health. With this information, managers can see how and where bark beetles and other pests are moving across the landscape and proactively manage these forests to lessen their impacts.

The aerial survey crew fuels its aircraft in Telluride before surveying insect and disease damage to forests. — The aerial survey crew fuels its aircraft on an airstrip in Telluride before taking to the skies to survey insect and disease damage to forests. Photo: Dan West, CSFS

Last year, trained aerial observers with the CSFS and USFS flew over 36.6 million acres of Colorado’s forests in small aircraft. Forest managers then ground-checked some of the areas spotted during flights to verify the insect or disease responsible and the severity of the damage.

Aerial observations and on-the-ground assessments showed that, overall, bark beetles and other forest pests continue to expand their footprint into new forests as outbreaks persist.

Wetter, Cooler Conditions Overall

Weather plays an important role in insect and disease activity. During periods of drought, trees are unable to produce enough resin to fight off insects trying to bore through the bark. At the same time, warmer temperatures place more demand on trees for water to offset evapotranspiration, the process in trees of “breathing” where trees release water vapor into the air. Ongoing drought and above-average temperatures are a recipe for stressing trees and leaving them susceptible to attack from bark beetles and other forest health issues.

While needing several years of adequate precipitation to build defenses against pests, trees in parts of Colorado did receive a respite from drought conditions last year. Data from the National Oceanic and Atmospheric Administration show 20.02 inches of precipitation pouring into Colorado during the 2023 water year (Oct. 1, 2022, to Sept. 30, 2023), compared to the 100-year average of 18.10 inches.

For the 2023 water year (Oct. 1, 2022, to Sept. 30, 2023), Colorado experienced its coolest since 2010, according to NOAA. Temperatures, on average, were the coolest in 13 years, though still above the 100-year average. Fall temperatures warmed up statewide with above or much-above the 100-year average, while precipitation was below to near average in the fall, according to NOAA.

Slightly Tardy, But Still Active

A cool, wet spring delayed most species of bark beetles by 2-3 weeks from emerging out of infested trees to seek new ones to attack. As summer temperatures warmed, adult beetles emerged from trees in July through August, slightly tardy but with sufficient time to fly to new host trees.

Bark Beetles on the Move

Both the Douglas-fir beetle and mountain pine beetle impacted more acres of forests in 2023 than the prior year, according to aerial survey data. Localized drought conditions continue to leave forests riddled with water-stressed trees ripe for attack. As long as populations of these native bark beetles remain high in forests, the threat persists to Douglas-fir and pine trees, particularly ponderosa.

This forest in the west-central San Juan Mountains in eastern Dolores County is riddled with dead and dying trees attacked by Douglas-fir beetles. Photo: Dan West, CSFS

Aerial observations from the CSFS and USFS show a decrease in acres impacted by western balsam bark beetle, down from 35,000 acres statewide in 2022 to 27,000 in 2023. It is still the deadliest forest pest in Colorado, however, as it affected more acres than any other bark beetle last year.

Western spruce budworm, which defoliates trees but does not always kill them, continues to be the most widespread forest pest in Colorado. The budworm impacted 202,000 acres statewide last year, up from 112,000 acres of forests in 2022. This large increase in acres is likely due to the ability of aerial observers to detect damaged tree canopies from the air. In other words, warmer and drier conditions in recent years spurred budworm populations, and observers are now seeing the full impact of these increased populations as more trees succumb to the budworm.

Notable Forest Pests in Colorado

Western Spruce Budworm

Acres affected statewide in 2023: 202,000

Western spruce budworm (Choristoneura occidentalis) remains the most widespread forest pest in Colorado. This 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 the deadly Douglas-fir beetle. The budworm continues to infest forests with Douglas-fir, though it occasionally consumes the needles of white fir, Engelmann and blue spruce. It is a problem in forests across Colorado, especially in the south-central part of the state.

Forests in Park County experienced 27,000 acres in damage from the budworm in 2023, holding steady on the number of affected acres from 2022.
Gunnison County continues to lose more of its Douglas-fir to the budworm because of repeated consumption of foliage for more than a decade there.
The northern Elk Mountains in Pitkin County saw an increase of 12,200 acres damaged by the budworm in 2023.
The Grand Mesa in Delta and Mesa counties experienced noteworthy upticks of 17,000 acres and 16,000 acres, respectively, in damage by the budworm in 2023.

The number of acres affected by western spruce budworm is the highest it has been in Colorado in the last six years: Graph: Dan West, CSFS — The number of acres affected by western spruce budworm is the highest it has been in Colorado in the last six years. Graph: Dan West, CSFS

Western Balsam Bark Beetle

Acres affected statewide in 2023: 27,000

Western balsam bark beetle (Dryocetes confusus) and associated root diseases have increased in high-elevation forests, spurred by drought and warm temperatures that make subalpine fir trees vulnerable to attack. Infestations of this beetle are patchy within a stand, but when all of these patches are added up across Colorado’s forests, the impact of this native bark beetle is more apparent.

Routt County saw a decrease in 3,700 acres affected by this beetle compared to 2022, but the county still has 4,300 acres of forests actively infested and mortality of subalpine fir is ever present in high-elevation forests there.
Jackson County saw a decrease in acres affected by this beetle, from 2,300 acres in 2022 to 1,300 acres in 2023.
Infestation by this beetle on the Grand Mesa continues to expand with 3,800 acres affected, up 1,400 acres from 2022.
Eagle, Garfield and Pitkin counties continue to see groups of subalpine fir lost to this beetle across high-elevation forests.
Gunnison and Rio Blanco counties in the central forests of the state are also notable areas of western balsam bark beetle activity again in 2023.

Western balsam bark beetle remains the deadliest forest pest in Colorado, despite it impacting fewer acres of forests than the previous year. Graph: Dan West, CSFS

Douglas-fir Beetle

Acres affected statewide in 2023: 17,000

Douglas-fir beetle (Dendroctonus pseudotsugae) remains a significant killer of Douglas-fir trees in the state’s central and southern mixed-conifer forests. Ongoing drought and defoliation from western spruce budworm play a role in leaving trees susceptible to attack from Douglas-fir beetle. Many of the largest, susceptible-sized trees in affected areas have been depleted over the past decade.

Heavy infestations continue in Gunnison, Mineral, Rio Grande and Saguache counties.
Intense infestations continue in the northern Sangre de Cristo Mountains of Freemont County and southern portions of the Sawatch Range within Chaffee County.
Northern areas within the La Plata Mountains and southern reaches of the West Central San Juan mountains have nearly doubled in affected acreage this past year.
Areas within the Roaring Fork Valley through Smuggler Mountain and around the Frying Pan and Ruedi Reservoir continue to see losses of severely affected Douglas-fir stands.

Graph of forested acres affected by Douglas-fir beetle in Colorado from 2015 to 2023 — Douglas-fir beetle continues to impact more acres of forests in Colorado, reaching a level not seen since 2016. Graph: Dan West, CSFS

Mountain Pine Beetle

Acres affected statewide in 2023: 3,100

Mountain pine beetle (Dendroctonus ponderosae) persists in pockets of pine trees in areas of Colorado. This native beetle is attacking mostly ponderosa pine, but also five-needle limber pines in the Mosquito Range. This beetle is active in Gilpin, Jefferson, Clear Creek, Elbert, El Paso, Park and Teller counties, with some areas seeing four times the impact in 2023 from the previous year. An active population of this beetle continues to kill lodgepole pines in the Taylor River drainage in Gunnison County.

Mountain beetle pine continues to impact more acres in Colorado, although tree mortality from this insect is not even close to the levels experienced during the epidemic that subsided in 2014. Graph: Dan West, CSFS — Mountain pine beetle continues to impact more acres in Colorado, although tree mortality from this insect is not even close to the levels experienced during the epidemic that subsided in 2014. Graph: Dan West, CSFS

Spruce Beetle

Acres affected statewide in 2023: 19,000

Spruce beetle (Dendroctonus rufipennis) was the deadliest forest pest in Colorado until 2022 and is in decline statewide. This beetle has depleted many of the largest, most susceptible spruce trees, although it continues to expand its footprint in high-elevation Engelmann spruce forests in Chaffee, Park, La Plata, San Juan and Costilla counties, while infestations in Grand County continue to decline.

Spruce beetle remains in decline statewide in Colorado. Graph: Dan West, CSFS

Piñon Ips Beetle

Acres affected statewide in 2023: 1,200

Piñon Ips beetle (Ips confusus) declined statewide in acres impacted, from 4,600 acres of forests in 2022 to 1,200 acres in 2023. Despite the decline statewide, prolonged drought continues to spur this beetle and it is killing new acres of lowland piñon-juniper forests, primarily in southwest Colorado. La Plata, San Miguel, Montrose and Mesa counties all still have significant infestations of this beetle, while other counties across the state experienced localized pockets of mortality, as has been the case in recent years with extensive precipitation and temperature fluctuations.

Roundheaded Pine Beetle Complex

Acres affected statewide in 2023: 5,280

Roundheaded pine beetle (Dendroctonus adjunctus) and associated native bark beetles (western pine beetle, mountain pine beetle, Mexican pine beetle and Ips engraver pine beetles) continue to increase their footprint of killed trees, attacking areas adjacent to infestations and previously uninfested stands. The bark beetle complex is killing groups of 5-10 or more trees within ponderosa pine forests in Dolores, Montrose and San Miguel counties. The intensity of beetle activity continues to remain high in localized areas of the San Juan National Forest and is moving into previously uninfested forests at a rapid pace. For the past three years, affected areas within San Miguel County, south of Norwood, are seeing pockets of tree mortality expand, more than doubling its scale across the county in 2023.