Insects and Diseases

Western Balsam Bark Beetle Overtakes Spruce Beetle for Most Forested Acres Affected

Spruce beetle is no longer the most prolific bark beetle in Colorado after a decade at the top of the list. Last year, western balsam bark beetle impacted more acres of forests statewide than spruce beetle, according to aerial survey data collected by the Colorado State Forest Service and U.S. Forest Service Rocky Mountain Region.

The aerial survey data also showed that western spruce budworm is once again the most widespread forest pest in Colorado, affecting 112,000 acres statewide in 2022. This insect partially eats the needles of trees. It can directly kill a tree after numerous years of defoliation. More often, however, Douglas-fir beetles finish the job and kill the tree weakened by western spruce budworm. Despite the budworm impacting many more acres across Colorado, bark beetles kill more trees.

Eyes in the Skies for Forest Health

Since the 1950s, forest managers have taken to the skies to detect and monitor disturbances to forests from insects and diseases, collecting data that provide an annual look at forest health in Colorado. 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.

Flying over forests allows managers to detect disturbances from insects and diseases. — Flying over forests allows managers to detect disturbances from insects and diseases, such as this roundheaded pine beetle infestation in a ponderosa pine forest in the San Juan National Forest in Dolores County. Photo: Dan West, CSFS

Last year, trained aerial observers with the CSFS and USFS flew over 29.9 million acres of forests in small aircraft. Some of the areas spotted during flights were then ground-checked 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 remain on the move in Colorado’s forests, expanding into new areas as susceptible host trees are killed and new stands offer drought-stressed trees ripe for attack.

Small Infestations, Large Impact

Prolonged drought and warm temperatures in Colorado’s high-elevation forests have left subalpine fir trees susceptible to attack by western balsam bark beetle. Infestations of this beetle are patchy within a stand of trees – dead trees with a reddish hue intermingle with live green trees. When all of these small patches of infested trees are added up across forests statewide, the impact of this native bark beetle is more apparent. Western balsam bark beetle affected 35,000 acres statewide in 2022.

While no longer holding the top spot among bark beetles, spruce beetle remains impactful despite its overall decline statewide. This native bark beetle affected more than 8,000 new acres across Colorado in 2022 and has the potential to kill a large number of trees weakened by drought in areas experiencing outbreaks, leaving hillsides that may appear much more gray than green over time.

Spruce beetle progression map 2021 to 2022 — This progression of maps shows the spruce beetle’s spread across forests in Colorado from 2001 to 2022. The maroon areas show locations where it was active that year, while the gray areas show where it had already affected trees. Since 2000, spruce beetle has affected at least 1.9 million cumulative acres in Colorado, a little more than 40 percent of the state’s spruce-fir forests. Maps: Dan West, CSFS. Graphic: Brian Sathe, CSFS

Other bark beetles such as Douglas-fir, piñon Ips and the native bark beetle complex associated with roundheaded pine beetle are all expanding into new forests in Colorado as well, as a result of drought and warmer temperatures linked to climate change.

Western spruce budworm, which is a small moth that partially consumes the needles of Douglas-fir trees during its caterpillar stage, continues to be the most widespread forest pest in Colorado. The number of acres impacted statewide is more than the acres impacted by the seven most prolific bark beetles combined.

Western spruce budworm is the most widespread forest pest in Colorado. This insect affected more acres statewide in 2022 than the seven most prolific bark beetles combined. Graph: Dan West, CSFS

Warm, Dry Weather Continues to Stress Trees

Warm temperatures and below-average precipitation continue to leave trees susceptible to bark beetle attacks and other forest health issues. During periods of drought, trees are not able to produce enough resin to ward off insects trying to enter through the bark. Despite monsoonal rains arriving last year in parts of Colorado, which relieved some of the drought, several years of adequate precipitation will be necessary for trees to recover their defenses.

The water year for Colorado for 2022 (Oct. 1, 2021, to Sept. 30, 2022) was the 35th driest on records dating back to 1895. Graph: Colorado Climate Center, Colorado State University. Data: National Oceanic and Atmospheric Administration

Leading into the winter of 2021-22, October to December 2021 was the record warmest, according to the National Oceanic and Atmospheric Administration. The snow season started late but was active through mid-winter. Although 2022 snowpack led to the most runoff in Colorado since 2019, it was still below historical averages. May snowfall ended the season later than usual, causing frost damage on trees throughout areas of the Front Range and Palmer Divide.

The summer of 2022 was warm and dry for most of the state. A heat wave in early July broke records for a 10-day period and again in September, according to NOAA. Record heat last year added to stress on trees already weakened by prolonged drought.

The water year for Colorado for 2022 (Oct. 1, 2021, to Sept. 30, 2022) was the 6th warmest on records dating back to 1895. Graph: Colorado Climate Center, Colorado State University. Data: National Oceanic and Atmospheric Administration

Notable Forest Pests in Colorado

Spruce Beetle

Acres affected statewide in 2022: 29,000

Spruce beetle (Dendroctonus rufipennis) remains in decline overall statewide. This bark beetle affected 53,400 acres in 2021 but only 29,000 acres in 2022. While the number of trees to infest is declining in high-elevation Engelmann spruce forests with outbreaks, the spruce beetle is moving into new areas:

The Sawatch and Mosquito ranges, primarily in Gunnison, Chaffee and Park counties, continue to experience new infestations.
Forests in the Needles and San Juan Mountains within Hinsdale, La Plata and San Juan counties also have new acres affected, indicating the expansion of beetle populations.
The intensity of spruce beetle infestation in Grand and Gunnison counties has progressively declined as fewer large-diameter Engelmann spruce remain in the forest. However, new acres were recorded in Grand County as the beetle’s footprint continues to spread.
Spruce beetle continues to kill trees in Rocky Mountain National Park, particularly around the headwaters of the Colorado River in the Kawuneeche Valley, and along the southern park boundary, on the west side of the Continental Divide. Despite infesting new areas, the spruce beetle has declined in intensity overall in and around the national park for the second year in a row.

Since 2000, spruce beetle has affected at least 1.9 million cumulative acres in Colorado. — Since 2000, spruce beetle has affected at least 1.9 million cumulative acres in Colorado, a little more than 40 percent of the spruce-fir forests in the state. Graph: Dan West, CSFS

Douglas-fir Beetle

Acres affected statewide in 2022: 9,700

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. This bark beetle is especially active in Colorado’s central and southern conifer forests. Custer, Eagle, Freemont, Garfield, Gunnison, Hinsdale, Huerfano, La Plata, Mesa, Pitkin and Saguache counties continue to see severely affected Douglas-fir stands.

Western Spruce Budworm

Acres affected statewide in 2022: 112,000

Western spruce budworm (Choristoneura freemani) continues to infest forests with Douglas-fir, white fir, and Engelmann and blue spruce, especially in the south-central forests of Colorado. Delta, Saguache, Gunnison, Chaffee, Mesa, Mineral, Park, Pitkin, Teller and Fremont counties were most affected by budworm in 2022. Tree mortality resulting from numerous years of defoliation is becoming more frequent throughout the southern half of the state. Trees damaged by the budworm for numerous years are at high risk of attack from the deadly Douglas-fir beetle.

Piñon Ips

Acres affected statewide in 2022: 4,600

The piñon Ips beetle (Ips confusus) is increasing populations in lowland piñon-juniper forests spurred by prolonged drought conditions. Delta, Garfield, La Plata, Montrose, Mesa and Pitkin counties have significant infestations through 2022 of this Ips beetle, with piñon-juniper forests in other parts of Colorado experiencing pockets of mortality that have progressively increased in recent years.

Western Balsam Bark Beetle

Acres affected statewide in 2022: 35,000

Over the last several years, western balsam bark beetle (Dryocetes confusus) and associated root diseases have increased in occurrence and intensity across subalpine fir trees. This beetle usually persists at normal levels in high-elevation forests in balance with associated fungi, but has built populations as drought has left subalpine fir vulnerable to attack. Clear Creek, Boulder, Eagle, Garfield, Grand, Gunnison, Jackson, Mesa, Moffat, Rio Blanco, Routt and Summit counties are experiencing the highest levels of subalpine fir decline.

Mountain Pine Beetle

Acres affected statewide in 2022: 2,400

Mountain pine beetle (Dendroctonus ponderosae) persists in pockets of pine trees in areas of Colorado. Localized infestations in ponderosa, limber and bristlecone pines in Chaffee County and in lodgepole pines in Gunnison County continue to expand. Localized activity in the Black Forest in El Paso County persists as well. A developing population of mountain pine beetle in Gilpin County continues to grow in scale and intensity.