Drive over just about any mountain pass in northwest Colorado and you may notice stands of dead trees with a red tinge. These trees are likely subalpine fir, killed by a combination of the western balsam bark beetle (Dryocoetes confusus) and fungi that cause root decay.

Subalpine fir trees continue to be killed across high-elevation forests. Prolonged drought and dry summers prior to 2022 have weakened defenses in trees, leaving them susceptible to attack by bark beetles. It will take several years of adequate precipitation for subalpine fir to recover their defenses to fend off the western balsam bark beetle, which attacks young as well as mature trees.

Unlike other bark beetles, impacts from the western balsam bark beetle are generally less widespread within a stand of trees. Infestations of the beetle are patchy in a stand – dead trees with a reddish hue intermingle with live green trees. Along with mountain passes, groups of trees killed by western balsam bark beetle are visible near Steamboat Springs in Routt County and Aspen in Pitkin County.

Beetle-killed trees can fall and present hazards for hikers, hunters, bikers and others recreating in forests. Newly infested trees can be removed prior to the adult western balsam bark beetles emerging the following year to infest other trees. However, management is a challenge because of the low value of the wood to offset treatment costs, infestations that are spotty and unpredictable, and remote and steep terrain at high elevations that is difficult to access.

• Complicated, hazardous conditions for firefighters
• Significant amounts of smoke while burning during a wildfire
• Transition of fire to remaining green trees in the right conditions
• Increased burn severity through soil heating

Douglas-fir beetle (Dendroctonus pseudotsugae) continues to cause significant mortality of Douglas-fir trees in parts of northwest Colorado. This beetle, a close relative of spruce beetle and mountain pine beetle, is a native insect that inhabits mature Douglas-fir forests across most of the West. Levels of tree mortality from Douglas-fir beetle vary widely and occur in pockets, from scattered trees in some areas to almost the total loss of mature stands in others.

Since 2020, drought conditions have stressed trees and left them vulnerable to attack by the Douglas-fir beetle. The beetle heavily affected forests in Eagle, Garfield, Mesa and Pitkin counties last year and has increased in activity in the Roaring Fork Valley since the Lake Christine Fire in 2018.

In an attempt to slow the spread and impact of Douglas-fir mortality, the Colorado State Forest Service teamed with the City of Aspen and its partners to deploy pheromones packets on Ute and Shadow mountains. The partners plan to continue this treatment until beetle populations decline and tree defenses fortify.