Emerald Ash Borer in Denver — Signs, Treatment & Removal
Emerald ash borer (EAB) has been confirmed across the Denver Metro and is killing thousands of ash trees. This guide from ISA Member Arborist Eric Petruska covers how to identify EAB, when to treat vs. remove, and what to do if your ash tree is already showing symptoms.
What Is Emerald Ash Borer?
EAB guidance reviewed July 2026
Emerald ash borer (Agrilus planipennis) is an invasive beetle from Asia first confirmed in Colorado in 2013. EAB larvae bore beneath the bark of ash trees (Fraxinus species), disrupting the vascular system that moves water and nutrients. Infested trees die within 2–5 years of initial infection — and untreated trees rarely survive once symptoms are visible in more than 30% of the canopy.
Denver status: EAB is confirmed throughout the Denver Metro including Denver, Aurora, Littleton, Englewood, Lakewood, and Arvada. An estimated 1.45 million ash trees are at risk across the Front Range.
Signs Your Ash Tree Has EAB
- Thinning or dying canopy — starting at the top of the tree and progressing downward (called “dieback”)
- D-shaped exit holes — 1/8-inch holes in the bark shaped like a capital D, left by emerging adult beetles
- S-shaped galleries — serpentine feeding tunnels visible under loose or split bark
- Epicormic sprouting — unusual shoots growing directly from the trunk or main branches (stress response)
- Woodpecker activity — heavy woodpecker feeding on the trunk exposes cream-colored larval galleries
- Bark splitting or blonding — vertical bark cracks or bleached/blond patches where woodpeckers have stripped bark
- S-pattern under bark — peel back a section of stressed bark and look for packed frass and winding galleries
Treatment vs. Removal: How to Decide
| Canopy Condition | Recommendation | Notes |
|---|---|---|
| Healthy, no symptoms yet | Preventive treatment | Emamectin benzoate injection every 2–3 years |
| Early EAB, under 30% dieback | Treatment possible | Trunk injection; must be done in spring/early summer |
| 30–50% dieback | Borderline — assess with arborist | Treatment success drops significantly above 30% |
| Over 50% dieback | Remove | Tree is unlikely to recover; structural risk increases rapidly |
| Dead or nearly dead canopy | Remove immediately | Dead ash becomes brittle — hazard timeline is short |
EAB Treatment Options
Emamectin Benzoate (Trunk Injection)
The most effective EAB treatment for Denver’s Front Range conditions. Injected directly into the trunk, emamectin benzoate (sold as TREE-äge) provides 2–3 years of protection per treatment. Must be applied when trees are actively transpiring (April–August). Cost: $150–$400 per tree depending on trunk diameter.
Imidacloprid (Soil Drench)
Applied as a soil drench around the base of the tree, imidacloprid is absorbed through the root system. Less effective than trunk injection for heavily infested trees but appropriate for preventive treatment of healthy trees. Requires annual application. Cost: $50–$150 per year.
Dinotefuran (Bark Spray)
Applied to the lower trunk bark. Fastest uptake of the systemic options — effective within days. Appropriate for early-season emergency treatment when trunk injection equipment isn’t available. Annual treatment required.
Denver EAB Resources
- Denver Parks & Recreation EAB information: denvergov.org
- Colorado State Forest Service EAB: csfs.colostate.edu
- Verify ISA arborists: the ISA at isa-arbor.com
1Can an EAB-infected ash tree be saved?
Yes — if caught early. Trees with less than 30% canopy dieback can often be saved with trunk injection treatment. Trees with over 50% dieback are unlikely to recover and should be removed before they become a structural hazard. An on-site assessment by an ISA member arborist is essential before deciding.
2How fast do ash trees die from EAB?
Most untreated ash trees die within 2–5 years of initial EAB infestation. Decline accelerates once more than 30% of the canopy is affected. Dead ash trees become structurally brittle within 1–2 seasons and should be removed promptly to avoid a hazard removal premium.
3Is there a Denver ash tree removal rebate?
Denver Parks & Recreation has periodically offered ash tree removal assistance and replanting programs. Check denvergov.org for current program availability. Some utility districts (Xcel) also offer tree replacement programs when removing trees near power lines.
How fast does emerald ash borer kill a tree?
Is it too late to treat my ash if I see woodpecker damage?
The Treat-or-Remove Decision, Made Concrete
Every metro ash owner faces the same fork, and the math is knowable. Treat when the tree is healthy (less than roughly 30% canopy decline), structurally sound, and genuinely valuable to the property — trunk injections on a two-year cycle protect it indefinitely, and a mature shade tree’s cooling, screening, and property value typically beat the treatment cost by a wide margin. Remove when decline has passed the recovery line, structure was poor anyway, or the tree’s placement never justified ongoing spend — and remove early: EAB-killed ash dries brittle fast, and dead-ash removals cost substantially more precisely because they’re more dangerous to climb and rig. The worst plan is the default one — waiting — which converts a controlled choice into an emergency price. A health assessment prices both paths for your specific tree in one visit.
Cost anchors for the decision, in Denver terms: professional trunk-injection treatment for a mature ash typically runs a few hundred dollars every two years, while removal of the same tree runs $800–$2,500+ depending on size and access — and climbs meaningfully once the tree is dead and brittle. Run the ten-year math on a tree you love and treatment usually wins; run it on a mediocre ash crowding the driveway and removal-plus-replanting usually wins. Either answer beats the untreated middle, which eventually pays the dead-tree removal premium anyway — after years of watching the decline. The assessment prices your specific tree’s version of this table in one visit, free.
First, Confirm You Actually Own an Ash
A surprising share of EAB panic calls turn out to be about maples, and a share of genuine ash trees go unwatched because owners never identified them. Ash identification in sixty seconds: opposite branching (branches and buds pair directly across from each other — only maples, dogwoods, and horsechestnuts share this among common metro trees), compound leaves with 5–9 leaflets along a central stalk, and diamond-patterned bark ridges on mature trunks. If your tree has all three, it’s on the borer’s list. Then look for the infestation signature: thinning upper crown, vigorous sprouting low on the trunk, woodpecker “blonding” where bark is stripped in patches, and the D-shaped exit holes about an eighth-inch wide — flat side down — that no other metro pest leaves. Any two of those together on a confirmed ash is an assessment call this season, not next: book the health assessment while treatment is still on the menu.
And if identification stumps you, a photo of the leaves, bark, and any suspect holes texted or emailed to us usually settles the ash question in minutes — free, like the assessment that follows if the answer is yes.
A note on scale, because it frames every individual decision: the Denver metro’s ash population runs to the hundreds of thousands of trees — roughly one in six or seven urban trees in many neighborhoods — planted heavily precisely because ash was the tough, adaptable answer after Dutch elm disease cleared the elms. That history is the warning label on monoculture, and it’s why the borer isn’t a niche gardening story here: it’s a slow-motion change to what whole streets will look like in a decade. The owners who come through it with shade intact are the ones deciding deliberately — treat the keepers, retire the marginal, and replant diverse — while the borer is still the neighbor’s emergency instead of theirs.
Questions? Call Eric Directly
ISA Member Arborist. Same-day response. Free written estimate, no obligation.
