Emerald Ash Borer in Colorado: What Ash Tree Owners Need to Know

The emerald ash borer (EAB) is an invasive beetle that kills ash trees by feeding on the tissue beneath the bark, cutting off water and nutrient flow. First detected in Colorado in 2013, it has spread across the Front Range and now threatens millions of ash trees statewide. Once an ash tree is infested, it typically dies within 2-4 years without intervention.

Alpenglow Tree Care LLC is a licensed and insured tree service company based in Broomfield, Colorado, serving Denver, the Northern Metro, and the Front Range. We assess ash trees for EAB damage, perform treatment injections where viable, and remove trees when treatment no longer makes sense.

What Is Emerald Ash Borer?

Emerald ash borer is a metallic green beetle native to Asia. Adults are small — about half an inch long — and feed harmlessly on ash leaves. The damage comes from the larvae. After eggs hatch on the bark, larvae bore into the tree and feed on the cambium layer just under the bark, creating serpentine galleries that block the tree's vascular system.

EAB attacks all North American ash species (Fraxinus) — green ash, white ash, and black ash. It does not affect mountain ash, which is not a true ash. Colorado's most common species, green ash (Fraxinus pennsylvanica), is highly susceptible. The beetle can kill a healthy tree in as little as 2-3 years, though larger trees may take 4-5 years to decline completely.

How EAB Spreads in Colorado

EAB spreads naturally about 1-2 miles per year through adult beetle flight. The bigger threat is human-assisted movement — firewood, nursery stock, and mulch transport beetles over long distances. Colorado enacted a firewood quarantine in affected counties to slow the spread, but EAB is now established across the metro area and in mountain communities.

In Denver and Boulder, EAB has been present since the early 2010s. Infestations are now widespread. If you have an ash tree on your property along the Front Range, assume EAB is in your area and plan accordingly.

What Are the Signs of an EAB Infestation?

Early detection is difficult because symptoms don't appear until the infestation is 1-2 years old. By the time canopy thinning is obvious, the tree has already sustained significant damage. Look for these signs:

• Crown dieback — The top third of the tree thins first as upper branches die.
• Epicormic shoots — Small shoots sprouting from the trunk or main branches in response to stress.
• D-shaped exit holes — About 1/8 inch across, where adult beetles emerged from the bark.
• Bark splits — Vertical cracks in the bark, often on the trunk.
• Woodpecker damage — Patches where bark has been stripped away by birds feeding on larvae.
• Serpentine galleries — S-shaped larval feeding tracks visible when bark is peeled back.

If your ash tree has lost more than 30% of its canopy, treatment success drops significantly. At that point, removal is usually the more cost-effective decision.

Which EAB Treatments Actually Work in Colorado?

Systemic insecticides can protect ash trees from EAB if applied before infestation is severe. The most effective method is trunk injection, where the chemical is injected directly into the tree's vascular system. This delivers the insecticide where larvae feed without soil application or spray drift.

Treatment is not a cure — it's ongoing protection. Applications are repeated every 1-2 years as long as EAB pressure remains high in the area. This is a recurring cost that adds up over time, so treatment only makes sense for high-value trees in good health.

Treatment makes sense when:

• The tree is over 15 inches in diameter (larger trees are expensive to replace)
• Canopy loss is under 30%
• The tree is in a high-visibility location (front yard, park, memorial planting)
• The tree has good structural health aside from EAB

Treatment does NOT make sense when:

• Canopy loss exceeds 30-40%
• The tree has co-dominant stems, decay, or other structural defects
• The tree is young (under 10 inches diameter) — replacement is cheaper than years of treatment
• The tree is in a low-visibility or low-risk location where failure won't damage property

In the Denver metro, trunk injection treatments typically cost $8-$15 per inch of trunk diameter per application. A 20-inch diameter ash tree runs $160-$300 per treatment, repeated every 1-2 years. Over a decade, that's $1,600-$3,000+ in treatment costs. Compare that to removal ($800-$2,500 depending on size and access) plus replacement planting.

When to Remove an Ash Tree Instead of Treating

Removal makes sense when the cost of ongoing treatment exceeds the value the tree provides, or when the tree is already too damaged to save. Dead and dying ash trees become brittle and hazardous within 1-2 years — branches break easily, and the entire tree can fail in wind or snow loads.

If your ash is in decline, don't wait until it's a safety hazard. Removing a tree while it still has some structural integrity is safer and often cheaper than emergency removal after a limb falls or the tree splits.

We recommend proactive removal when:

• The tree has lost 40%+ of its canopy
• Exit holes and gallery damage are extensive
• The tree is near a house, power lines, or high-traffic area
• Treatment costs over 5-10 years will exceed removal + replanting costs

Replanting with a non-ash species — oak, maple, honeylocust, or linden — restores canopy and eliminates the EAB threat. Many Colorado communities offer free or discounted replacement trees through forestry programs to offset ash losses.

Common Questions About Emerald Ash Borer in Colorado

Last Updated: August 2026