If you have an ash tree in your yard, this is the most important article you’ll read this year. Emerald ash borer (EAB) is no longer a distant threat creeping toward the Denver metro — it is already here. In 2025 alone, Colorado State University confirmed EAB in Aurora, Denver, Golden, Edgewater, Wheat Ridge, and Berthoud, bringing the total to more than 20 confirmed Front Range cities. Acting now is the difference between saving your tree and paying to have it removed.
At Tree Service Denver LLC, our ISA Certified Arborists have been monitoring the EAB outbreak across the metro area and are ready to help. Whether your ash tree looks perfectly healthy or is already showing signs of decline, read on — your next step matters.
What Is the Emerald Ash Borer?
The emerald ash borer (Agrilus planipennis) is a small, metallic-green beetle native to Asia that was first discovered in the United States in Michigan in 2002. Since then, it has killed hundreds of millions of ash trees across North America, devastating urban forests from coast to coast. In Colorado, it was first confirmed in Boulder in 2013 by the Colorado State Forest Service (CSFS).
EAB is dangerous not because of the adult beetle itself, but because of what its larvae do beneath the bark. After adults lay eggs on the surface of an ash tree, the hatching larvae bore into the tree and feed on the phloem and cambium layers — the tissue responsible for transporting water and nutrients throughout the tree. As those S-shaped feeding galleries multiply over the years, the tree’s vascular system is slowly destroyed.
Most ash trees die within two to four years of initial infestation, often before the homeowner realizes anything is wrong. Critically, EAB attacks both stressed and healthy ash trees — no ash is immune without treatment.
Where Has EAB Been Found in Colorado?
The spread of emerald ash borer in Colorado has accelerated dramatically in recent years. According to the Colorado State Forest Service, EAB has now been confirmed in more than 20 Front Range communities — a milestone that underscores just how rapidly this pest is moving through the region.
Here is a timeline of confirmed Colorado detections:
- 2013: Boulder — Colorado’s first confirmed EAB detection
- 2013–2022: Spread through Boulder County communities including Longmont, Lafayette, Louisville, Westminster, and Broomfield
- 2023: Carbondale — a surprising westward jump detected by CSU
- 2025: Aurora, Denver, Golden, Edgewater, Wheat Ridge, and Berthoud confirmed by CSU’s College of Agricultural Sciences
Notably, Thornton, Arvada, Littleton, and Lakewood are all within the active expansion zone. The CSFS has stated that Castle Rock and Colorado Springs should expect confirmed detections within the next five years. If you live anywhere in the Denver metro — from Highlands Ranch to Thornton, from Arvada to Aurora — your ash trees are at risk right now.
Because EAB can infest a tree for up to four years before visible symptoms appear, the actual footprint of the infestation is almost certainly wider than confirmed detections suggest. Ash trees make up an estimated 15% or more of all urban trees in Colorado, which means thousands of trees across the Front Range are vulnerable.
How to Identify EAB Damage on Your Ash Trees
Catching EAB early dramatically improves the odds of saving your tree. Here are the key signs to look for on your ash trees in Denver, Aurora, Lakewood, or anywhere across the Front Range:
D-Shaped Exit Holes
Adult beetles emerge from the bark in late spring, leaving behind distinctive D-shaped exit holes approximately 1/8 inch wide. These small, flat-sided holes are the most reliable visual indicator of EAB. Look carefully across the entire trunk and main branches — a magnifying glass helps.
S-Shaped Larval Galleries
If you peel back a section of bark on a suspected tree, you may find serpentine, S-shaped tunnels packed with sawdust-like frass. These galleries are carved by the feeding larvae as they destroy the tree’s nutrient-transport tissue. Their presence confirms active infestation.
Crown Dieback Starting at the Top
One of the earliest above-ground symptoms is thinning foliage and branch die-off beginning in the upper canopy. This happens because the larvae disrupt the flow of water and nutrients upward through the tree. If your ash tree looks thin on top while the lower branches still have leaves, take it seriously.
Vertical Bark Splits
As EAB damage accumulates, the bark may develop vertical cracks and splits along the trunk. This is a sign of advanced infestation and indicates that the tree’s cambium layer has been severely compromised.
Increased Woodpecker Activity
Woodpeckers love EAB larvae and will hammer into infested trees in search of food. Unusual or intensified woodpecker activity on an ash tree — especially blonding (large patches of outer bark stripped away) — is a strong indicator that EAB larvae are present under the bark.
Epicormic Sprouting (Water Sprouts)
As an ash tree under stress tries to compensate for canopy loss, it often sends up clusters of small, leafy shoots at the base of the trunk or along main branches. These water sprouts are a distress signal and warrant an immediate professional arborist assessment.
If your ash tree is showing any combination of these signs, do not wait. Schedule a tree health assessment as soon as possible.
When to Treat for Emerald Ash Borer in Denver
Timing is everything with EAB treatment in Denver. The most effective treatment window aligns with the beetle’s biology — you want insecticides moving through the tree when larvae are actively feeding and adults are beginning to emerge. According to Colorado State University Extension, the optimal application window is mid-May through late June, shortly after bud break when the tree is actively moving water.
Trunk Injection: The Gold Standard
Trunk injection with emamectin benzoate is the most effective method for controlling EAB and is the approach favored by professional arborists for medium and large trees. This treatment delivers insecticide directly into the tree’s vascular system, where it kills feeding larvae and adult beetles. When applied by a licensed professional, a single trunk injection typically provides two years of protection, and in some cases up to three years.
Soil-applied treatments using imidacloprid or dinotefuran are also options, particularly for smaller trees, but are generally less reliable in Colorado’s dry, clay-heavy soils without consistent supplemental irrigation.
Don’t Wait for Symptoms to Start
Because EAB can be present in a tree for years before visible decline begins, preventive treatment is strongly recommended for any ash tree within a confirmed EAB zone — which now includes all of the Denver metro area. If your tree is currently healthy and you live in Denver, Aurora, Golden, Lakewood, Thornton, or any surrounding community, now is the time to act.
Treatment Is Far Cheaper Than Removal
Professional EAB trunk injection treatment typically costs $200–$400 per treatment cycle, depending on tree size. Compare that to the cost of ash tree removal — which commonly runs $2,000 or more for a large, mature specimen — and the math is clear. Treating proactively is not just the best choice for your tree, it’s the smartest financial decision you can make as a homeowner.
Should You Treat or Remove Your Ash Tree?
Not every ash tree is a candidate for treatment. When our ISA Certified Arborists evaluate an infested ash tree, we look at several key factors to help you make the right decision.
The 30% Rule: When Treatment Makes Sense
Research supported by CSU Extension indicates that ash trees with less than 30% canopy loss have a strong likelihood of recovery when treated with professional-grade trunk injections of emamectin benzoate. If your tree still has most of its canopy and you’ve caught the infestation early, treatment is almost certainly the right call.
When Removal Is the Better Choice
Once an ash tree has lost more than 50% of its canopy, or shows extensive crown dieback, major trunk damage, and advanced larval galleries, removal becomes the safer and more cost-effective option. A severely infested tree becomes structurally unstable — large branches can fail without warning, posing a serious safety hazard to your family, neighbors, and property.
If removal is necessary, our team handles it safely and completely, per all applicable Denver and Colorado ordinances. Learn more about our tree removal services.
Other Factors Our Arborists Consider
- Tree size and species: Larger, healthier trees generally respond better to treatment and are worth the investment
- Location: Trees near structures, power lines, or high-traffic areas may need removal regardless of infestation level
- Overall tree health: Pre-existing disease, root damage, or drought stress reduces treatment success
- Proximity to other ash trees: A heavily infested neighbor tree can reinfest a treated tree more rapidly
The bottom line: there is no one-size-fits-all answer, and you shouldn’t have to figure this out alone. Our arborists will give you an honest, expert assessment — not just a sales pitch.
How Tree Service Denver LLC Can Help
Tree Service Denver LLC has been serving homeowners across the Denver metro for 15+ years. Our team of ISA Certified Arborists is specifically trained in EAB identification, damage assessment, and treatment protocols — and we stay current with the latest guidance from the Colorado State Forest Service and CSU Extension.
Free EAB Inspections
Not sure if your tree is an ash, or if it’s showing symptoms? We offer free EAB inspections for homeowners across our service area. One of our certified arborists will walk your property, examine your trees, and give you a clear, honest answer about what you’re dealing with and what your options are.
Customized Treatment Plans
If treatment is the right path, we develop a customized EAB management plan tailored to your tree’s size, health status, and location. We use professional-grade trunk injection treatments proven effective by CSU research, applied by licensed commercial pesticide applicators as required by the Colorado Department of Agriculture.
Safe, Licensed Tree Removal
When removal is the best option, our crew handles the job safely and thoroughly. We carry $2 million in liability insurance, work in full compliance with Denver ordinances, and clean up completely when the job is done. Learn more about our tree removal services.
Service Areas
We serve homeowners throughout the Denver metro, including Denver, Aurora, Arvada, Boulder, Lakewood, Littleton, Westminster, Thornton, Golden, Castle Rock, and Highlands Ranch. If you’re unsure whether we cover your neighborhood, just give us a call — we’re happy to help.
Ready to protect your ash trees? Contact us today to schedule your free inspection or get a quote.
Frequently Asked Questions About EAB in Colorado
How do I know if my tree is an ash tree?
Ash trees (Fraxinus species) have several distinctive features. Look for compound leaves with 5–11 leaflets arranged in opposite pairs along a central stem, with a single leaflet at the tip. The bark of mature ash trees has a distinctive diamond-shaped ridged pattern. Ash also have paddle-shaped seeds (samaras) that hang in clusters. If you’re not sure, send us a photo or schedule a free inspection — our arborists can identify your tree species in minutes.
Can a tree survive emerald ash borer?
Yes — but only with professional treatment, and only if the infestation hasn’t progressed too far. According to CSU Extension research, ash trees with less than 30% canopy thinning have a strong chance of recovery when treated with trunk injections of emamectin benzoate. Trees caught early and treated consistently can survive indefinitely with ongoing management. Trees that have lost more than 50% of their canopy are generally past the point where treatment is practical.
How much does EAB treatment cost in Denver?
Professional EAB trunk injection treatment in Denver typically runs $200–$400 per treatment, depending on the size of the tree. Because the most effective treatment (emamectin benzoate) lasts two years per application, your annual cost averages $100–$200 per year — a fraction of what removal costs. For a free, no-obligation quote specific to your tree, call us at (720) 807-2785.
Is EAB in my Denver neighborhood?
Almost certainly yes, or very close. In 2025, CSU confirmed EAB in Denver, Aurora, Golden, Edgewater, and Wheat Ridge, and the Colorado State Forest Service now lists more than 20 confirmed Front Range communities. Because EAB spreads naturally through adult beetle flight and can travel several miles from known infestations, any ash tree in the greater Denver metro should be treated as at-risk. The CSFS maintains a public detection map — and the trend is clear: EAB is spreading faster than ever.
When is the best time to treat for EAB in Colorado?
The optimal treatment window in Colorado is mid-May through late June, aligned with bud break and the period when adult beetles begin to emerge. This timing ensures the insecticide is actively moving through the tree when it’s needed most. That said, if you’ve missed this window, don’t wait a full year — our arborists can advise you on the best approach year-round, and some products offer more flexibility in application timing. Contact us to find out what makes sense for your specific tree.
Protect Your Ash Tree Before It’s Too Late
Emerald ash borer is now confirmed across the Denver metro. Every season you wait makes treatment less effective and removal more likely. Our ISA Certified Arborists are ready to inspect your trees, give you an honest assessment, and help you protect one of your property’s most valuable assets.
Free EAB inspections are available now for homeowners in Denver, Aurora, Arvada, Lakewood, Littleton, Westminster, Thornton, Golden, Castle Rock, Highlands Ranch, and surrounding communities.
Call us today: (720) 807-2785
Or schedule your free inspection online — no obligation, no pressure, just expert answers.
