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.
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.
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.
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.
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.
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.
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.
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.
Acres affected statewide in 2023: 27,000
Western balsam bark beetle (Dryocoetes 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.
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.
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.
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.
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.
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.
The annual forest health report shows the continued spread of mountain pine beetles and other forest insects during the second consecutive year of above-average temperatures and below-average precipitation.
Increased tree mortality from mountain pine beetles has the potential to alter fire behavior and wildfire risk to communities in a highly populated area of Colorado.
Semiochemicals can be important tools to monitor and control bark beetle populations as part of an integrated pest management strategy.
