Forest Health Issues in Northeast Colorado

Long-term effects from drought continue to stress all species of trees in northeast Colorado, leading to a decline in forest health in some areas and increased tree mortality in others.

Counties most impacted in northeast Colorado in 2023: Clear Creek, Elbert, Gilpin, Jefferson

Brown trees killed by mountain pine dot hillsides above a highway — Brown trees killed by mountain pine beetle dot these hillsides near Black Hawk in Gilpin County. Photo: Dan West, CSFS

Pockets of mountain pine beetle (Dendroctonus ponderosae) continue to expand their footprints in localized areas within pine forests in northeast Colorado. Colorado State Forest Service foresters in the Boulder, Golden and Franktown field offices have all been tracking and helping to manage the spread of this native insect.

According to data from last year’s aerial survey, this beetle increased its impact from 70 acres of forest in 2022 to 410 acres in 2023 in Jefferson County, from 80 acres of forest in 2022 to 170 acres in 2023 in Gilpin County, and from 40 acres of forest in 2022 to 240 acres in 2023 in Clear Creek County. Foresters in Franktown have also responded to mountain pine beetle activity in the northern Black Forest in Elbert County. While it can attack various pine species, it is mostly impacting ponderosa in these counties.

More information on mountain pine beetle can be found in this fact sheet. In addition, the CSFS offers workshops for residents and access to semiochemical treatments to curb its spread and protect healthy trees.

Counties most impacted in northeast Colorado in 2023: Adams, Arapahoe, Boulder, Broomfield, Larimer

Adult emerald ash borer on leaf. — The emerald ash borer is native to Asia and a prodigious killer of ash trees in the United States. Photo: Dan West, CSFS

In 2023, Littleton joined the list of cities on the Front Range of Colorado with a detection of emerald ash borer (Agrilus planipennis). This represented a significant jump south from the prior southern-most detection in Arvada.

This non-native insect is a prodigious killer of ash trees, so residents and arborists should be on the lookout for EAB. Suspected detections should be referred to the nearest Colorado State Forest Service or CSU Extension office for confirmation from experts at CSU. Ash trees comprise roughly 15 percent of all urban trees in Colorado, making this a pest of major concern for urban forests.

The CSFS offers resources and guides to help residents and arborists detect, treat and plan for this deadly insect, including management resources for smaller communities.

Counties most impacted in northeast Colorado in 2023: Boulder, Douglas, Jefferson

Douglas-fir tussock moth caterpillar crawling on the tip of an evergreen branch — Douglas-fir tussock moth larvae consume the needles of fir and spruce trees. Photo: Dan West, CSFS

The Douglas-fir tussock moth (Orgyia pseudotsugata) could be making a comeback in areas of Douglas and Jefferson counties that were affected by an outbreak of this insect in 2014-2016. This is not totally surprising, given that outbreaks of this native moth tend to be isolated and cyclical in Colorado. The moth is also present in Boulder County during this current outbreak.

Colorado State Forest Service foresters have seen increased amounts of the moth’s cocoons on Douglas-fir trees and surrounding structures. These observations align with an uptick of Douglas-fir tussock moths found in traps set out by the CSFS and U.S. Forest Service along the Front Range. The USFS has caught up to 26 moths in a trap recently; while an uptick, it is not at alarming levels yet.

During its larval stage, the Douglas-fir tussock moth feeds on the needles of Douglas-fir, Engelmann spruce and occasionally blue spruce in urban environments. The larvae can strip an entire tree of its foliage in a single season during an outbreak. Trees that survive are weakened, making them susceptible to attack from deadly bark beetles.

The CSFS offers a quick guide and treatment options for managing the Douglas-fir tussock moth.

Counties most impacted in northeast Colorado in 2023: Jefferson, northern Larimer, Park

Western spruce budworm consumed the needles on the tips of this branch. Photo: Dan West, CSFS

The western spruce budworm (Choristoneura occidentalis) is most active in Douglas-fir trees in western and southern Jefferson County and northeast Park County. Clear Creek County could see damage as the adult moths expand their footprint in the area.

During its caterpillar stage, the western spruce budworm eats the needles of Douglas-fir, true fir and spruce trees. This defoliation weakens the trees, making them susceptible to attack from the Douglas-fir beetle, which will subsequently cause mortality to the trees. Despite subsequent years of budworm activity in these parts of northeast Colorado, however, Douglas-fir beetle has not been a significant issue so far.

Colorado State Forest Service foresters manage stands to be resilient against the budworm by increasing the diversity of both the size and spacing of trees and, in some cases, species composition. This also lowers the risk of high-severity fire in those stands.