For more than a decade, Colorado homeowners have watched emerald ash borer (EAB) steadily spread across the Front Range. First detected in Boulder County back in 2013, the problematic bug’s presence was confirmed in Denver in June 2025, though it’s potentially been present for several years.
The insect’s reputation has certainly preceded its arrival. EAB has killed hundreds of millions of ash trees throughout North America since first being detected in 2002. And unfortunately, like all of the ash trees it’s already killed, our trees are at risk.
There are treatments available, but you have to move quickly. Once trees begin to visibly decline, the chances of a meaningful recovery shrink significantly. We’ll share the facts about this invasive insect below, so you can better understand the things you’ll need to do to protect your trees.
What Is Emerald Ash Borer?
EAB is a small, metallic green insect that’s historically native to Asia. The adult beetle feeds on ash tree foliage, but it doesn’t cause much serious damage; it’s the larval stage of the pest that endangers ashes.
Shortly after hatching, the beetle larvae tunnel through the tree’s outer bark, where they begin feeding on the tree’s phloem — a living layer of inner bark that makes up part of the tree’s important vascular system. While feeding, the insects damage the phloem layer and reduce the tree’s ability to transport nutrients.
While EAB is incredibly destructive in the U.S., it doesn’t seem to cause the same kind of widespread damage in its home range. Scientists are not certain the reasons this is the case, but many suspect that predators, parasites, and pathogens may keep the beetle’s population in check.
Which Ash Trees Grow in the Denver Area?
Colorado is home to three native ash species, the green ash (Fraxinus pennsylvanica), the velvet ash (Fraxinus velutina), and the singleleaf ash (Fraxinus anomala).
However, none of these species are native to the Denver area. Green ash is historically limited to the eastern portions of the state, velvet ash is restricted to the southwestern corner of Colorado, and singleleaf ash is only found in the far western reaches of the state.
In fact, no ash species are native to Denver or the surrounding communities. Nevertheless, green and white ash trees (Fraxinus americana) have been widely planted in residential, commercial, and municipal landscapes throughout the area.
Both of these species are quite susceptible to EAB attacks, and without treatment, the vast majority of these trees will die in the coming years.
Signs Your Ash Tree May Have Emerald Ash Borer
Given the threat EAB presents to Denver’s ash trees, it is critical that homeowners and property managers learn to recognize the signs of a potential infestation. A few of the most noteworthy clues include:
- Canopy Thinning: One of the earliest signs of emerald ash borer is thinning foliage in the upper canopy. As the infestation progresses, branches may begin to die back, making the canopy look sparse.
- D-Shaped Exit Holes: Adult emerald ash borers leave distinctive D-shaped exit holes, approximately one-eighth of an inch wide, as they emerge from beneath the bark.
- Epicormic Sprouting: Ash trees under stress often produce shoots along the trunk or larger branches in an attempt to compensate for damage to their vascular system.
- Bark Splitting: Vertical cracks or splits in the bark may develop as the tree responds to larval feeding beneath the surface.
- Woodpecker Activity: Increased woodpecker feeding can indicate the presence of emerald ash borer larvae. You may notice patches of bark that have been removed or more frequent woodpecker visits.
- S-Shaped Larval Galleries: If bark is removed, winding, S-shaped tunnels etched into the wood are a telltale sign of emerald ash borer larvae feeding beneath the bark.
- Progressive Branch Dieback: Without treatment, infestations typically lead to significant branch dieback. Untreated ash trees often decline rapidly once symptoms become apparent.
If you notice any of these signs, you’ll want to contact an ISA-certified arborist and have the tree assessed.
Treatment Options for Emerald Ash Borer
Fortunately, EAB can often be treated when caught early — many ash trees can survive for years (and potentially decades) with an appropriate treatment plan. The key is acting before the infestation becomes severe.
Some of the most common and effective emerald ash borer treatment options include:
- Trunk Injections: Systemic insecticides are injected directly into the tree’s vascular system, providing highly effective protection against emerald ash borer larvae. Trunk injections typically provide one to three years of control, depending on the specific product used.
- Soil Treatments: Systemic insecticides can be applied as soil drenches around the base of the tree, where they are absorbed through the roots and transported throughout the canopy. Soil treatments don’t last as long as trunk injections, and they are best suited for smaller trees.
- Tree Removal and Replacement: In cases where an ash tree is heavily infested or in poor condition, removal may be the safest and most practical solution. Replacing declining ash trees with other species can help improve the long-term resilience of Denver’s urban canopy.
Not every ash tree is a good candidate for treatment. Factors such as the tree’s overall health, size, location, and the extent of any infestation should all be considered before making a decision. A professional assessment can help determine whether treatment is likely to be successful or if removal is the better long-term investment.
An Ounce of Prevention
It’s important to note that ash trees do not need to show signs of infestation to benefit from treatment. Preventive applications are actually the most effective and economical option for healthy, high-value ash trees growing in areas where emerald ash borer has been detected.
Frequently Asked Questions About Emerald Ash Borer
Given the threat EAB presents, it’s only natural to have questions about this invasive pest. Here are answers to some of the most common questions we hear from Colorado homeowners about protecting their ash trees.
Can emerald ash borer kill healthy ash trees?
Yes. Unlike some other pests that only attack stressed trees, EAB attacks healthy ash trees too. And without treatment, infestations are typically fatal.
What types of ash trees are susceptible to emerald ash borer?
All true ash trees (trees in the genus Fraxinus) are susceptible to emerald ash borer. In Colorado, green ash and white ash are among the most commonly affected landscape trees.
Should I treat my ash tree before I see signs of emerald ash borer?
In many cases, yes. Preventive treatments are often the best way to protect valuable ash trees in areas where emerald ash borer is present. Waiting until symptoms become obvious can reduce the likelihood of successful treatment.
How often do emerald ash borer treatments need to be applied?
Treatment intervals vary depending on the product and application method. Some trunk injection products provide protection for two or even three years, while soil treatments may require annual applications.
Can an ash tree recover from emerald ash borer?
Ash trees that are treated early in the infestation process can often recover and continue to thrive for many years. However, trees that have suffered extensive canopy loss or severe vascular damage may not respond well to treatment and may need to be removed.
Is emerald ash borer found throughout Colorado?
Emerald ash borer has been confirmed in numerous Colorado communities and continues to spread along the Front Range. Because infestations can be difficult to detect in their early stages, homeowners throughout the Denver metro area should assume that their ash trees may be at risk and consider proactive treatment planning.
Can emerald ash borer spread to other types of trees?
No. Emerald ash borer attacks only true ash trees (Fraxinus spp.). It does not infest maples, oaks, or elms, nor does it cause problems for unrelated trees that have “ash” in their name, such as the American mountain-ash (Sorbus aucuparia).
Protect Your Denver Ash Trees Before It’s Too Late
Having established itself in the Front Range region, the emerald ash borer is no longer a looming threat on the horizon — it’s a reality for those with ash trees on their property.
But that doesn’t mean your ash trees are doomed. Early action can help protect your trees and support their long-term health. Contact Knothead Tree & Lawn today to schedule an inspection and learn how we can help protect your valuable ash trees.
Sources and References
- Be a Smart Ash – “Be a Smart Ash: Emerald Ash Borer Prevention & Treatment”
Denver-area public awareness campaign providing homeowners with EAB identification, treatment-versus-removal guidance, and resources for protecting Colorado’s urban ash canopy.
https://beasmartash.org/ - Colorado State Forest Service – “Emerald Ash Borer: A Green Menace”
State forestry agency overview of EAB biology, signs of infestation, and management options, with current information on the pest’s advance across Colorado.
https://csfs.colostate.edu/forest-management/common-forest-insects-diseases/emerald-ash-borer/ - Rocky Mountain PBS – “The Emerald Ash Borer Is in Denver. What Does It Mean for Your Trees?”
News feature reporting the 2025 detection of EAB in Denver and explaining, with expert input, what the arrival means for local ash trees and homeowners.
https://www.rmpbs.org/news/science-environment/emerald-ash-borer-denver - Western IPM Center – “Preparing for the Invasion: Emerald Ash Borer in Colorado”
Regional IPM center story on Colorado’s proactive planning after EAB’s 2013 Boulder detection, covering urban forestry impact and coordinated response strategies.
https://westernipm.org/stories/communities/preparing-for-the-invasion-emerald-ash-borer-in-colorado/
