Quick answer: Emerald ash borer (EAB) was confirmed in Denver on June 17, 2025. It is an invasive beetle that kills ash trees, and Denver has about 330,000 of them, roughly 1 in 6 trees in the city. An infested ash dies within about five years unless it is treated. Owners have three options: treat, remove or replace.
This guide brings the key facts together in one place. For an inspection or removal, see ash tree removal and emerald ash borer help in Denver.
How did emerald ash borer reach Denver?
| Year | What happened |
|---|---|
| 2002 | First found in the United States |
| 2013 | Found in Boulder, the first detection in Colorado |
| 2016 | Denver launches the Be A Smart Ash campaign |
| 2021 | Found in Arvada |
| 2023 | Found in Littleton |
| 2024 | Found in Lakewood |
| 2025 | Confirmed in Denver’s Congress Park neighborhood on June 17 |
The dates come from the city’s Be A Smart Ash program. The beetle spreads on its own and through the movement of infested ash wood.
How does emerald ash borer kill a tree?
Adult beetles lay eggs on the bark. The larvae tunnel underneath it and feed on the tissue that carries water and nutrients, which slowly starves the tree. According to The Park People’s Be A Smart Ash resource, it kills weak and healthy trees alike, attacks all major North American ash species and kills trees within five years unless they are treated.
What are the signs of emerald ash borer?
- Dieback of branches near the top of the tree
- D-shaped exit holes in the bark, about 1/8 inch wide
- Winding tunnels under the bark
- New sprouts on the trunk and main branches
- Vertical splits in the bark
- Heavy woodpecker activity
The first step is knowing whether you have an ash at all. See how to identify an ash tree.
What are your options?
- Treat. Insecticide protects healthy ash trees. Be A Smart Ash puts properly applied treatments at about 90% effective.
- Remove. Declining, small and badly placed ash are usually better taken down.
- Replace. A different species restores the shade without the risk.
How to choose is covered in should you treat or remove your ash tree? Replacement species are in what to plant instead of ash in Denver.
What is the city doing?
Denver treats selected large, healthy ash trees in the public right-of-way, removes small and unhealthy ones, and offers free street trees to residents with room for one. Ash trees on private property are the owner’s responsibility. See Denver’s free tree and ash replacement programs.
Emerald ash borer FAQs
When was emerald ash borer found in Denver?
It was identified in Denver’s Congress Park neighborhood on June 17, 2025, and confirmed by Colorado State University and the Office of the City Forester.
How many ash trees are in Denver?
About 330,000 in the City and County of Denver, which is roughly 1 in 6 trees. The metro area has about 1.45 million.
How long does emerald ash borer take to kill a tree?
Be A Smart Ash states that the beetle kills ash trees within five years unless they are treated. A tree can be infested for a year or more before symptoms are obvious.
