Emerald Ash Borer in Boulder: Treat It or Remove It?

Because Boulder is the origin point. According to the City of Boulder, emerald ash borer was detected in the city by Boulder Forestry staff in September 2013, and that was the first detection of the pest anywhere in Colorado. Boulder has been managing this longer than any other community in the state.

The City is unambiguous about the prognosis for untreated trees: all sizes of ash in Boulder, including trees that currently look completely healthy, will eventually be killed by EAB unless they are treated with an effective pesticide. Nationally, the City cites an estimated 50 to 70 million ash trees lost to this pest across the United States.

The species at risk are the ones that were planted most heavily as Front Range shade trees. The City lists green, white, black, and blue ash as susceptible, along with cultivars such as Autumn Purple white ash. Green and white ash in particular were popular because they grow quickly and color well in fall, which is exactly why so many Boulder streets ended up with so many of them.

Symptoms generally start high in the tree and progress downward, which is unhelpful, because the early stage is the part you cannot see well from the ground. The City of Boulder lists the following signs:

It is frequently the first thing a homeowner actually notices. Woodpeckers feed on EAB larvae under the bark, and to reach them they strip off the gray outer bark. The City notes that this damage can be severe, and that the newly exposed whitish inner bark is often visible from the ground in the upper canopy.

So if you are looking up at an ash and seeing pale patches high in the crown that were not there last year, that is worth an inspection. The woodpeckers found the larvae before you did.

No, and this is a point where a lot of older advice is now out of date. Colorado did establish a quarantine around Boulder County to prevent and reduce the spread of ash wood after the detection. According to the Colorado State Forest Service 2025 Forest Health Report, that quarantine was repealed effective December 30, 2019.

What has not changed is the pest. EAB is still present and active in Boulder. Boulder County still advises that the best window for tree removals runs from early September through April, outside adult flight season, and still asks people to be careful with logs, green lumber, nursery stock, wood chips, and mulch. The regulation went away. The biology did not.

It is a useful benchmark for what actually works, because Boulder has been doing it at scale since 2014. Per the City, approximately 1,300 public ash trees are enrolled in its EAB pesticide treatment program. Those trees are split into three groups and treated on a three-year rotation with an injectable formulation of emamectin benzoate, a program that began in spring 2014, and the City describes it as having had great success.

The City is explicit about why it treats rather than simply removing everything: treatment preserves ash long term, reduces community-wide EAB populations and slows the pest's progression, spreads removal and replacement costs over a longer period, lowers the safety risk of large numbers of dying trees at once, and spreads out the canopy loss.

For untreated public ash, the approach is different. The City removes them once they begin showing symptoms of infestation, and replaces them where possible with a diverse mix of large-maturing, non-ash species.

The product determines the schedule. The City of Boulder lists three approaches and their intervals:

For a tree worth keeping, generally yes. The City makes the economic case plainly: pesticide applications for EAB are inexpensive when compared with the cost of tree removal, and mature ash provide substantial environmental benefit to the community.

The commitment is the part to understand before you start. The City is direct that if you choose to treat your tree, treatments must continue indefinitely to preserve it. This is not a one-time cure. It is an ongoing maintenance decision, like any other recurring cost of owning a mature tree.

Which is why the honest answer for some trees is removal. An ash that is already in significant decline, poorly placed, or structurally compromised is not a good candidate for a permanent treatment subscription. Replacing it with a well-selected non-ash species is the better long-term investment.

Because dead ash is dangerous to work on. The City of Boulder puts it directly: once infested with EAB, ash trees dry out quickly, become brittle, and are more prone to fail. It is not safe for arborists to climb trees once they become symptomatic for EAB, so removal costs may be even higher.

That changes the entire job. A removal that could have been done by climbing has to shift to equipment access or more elaborate rigging, which takes longer, needs more gear, and costs more. The City's recommendation is straightforward: if you are planning to remove your ash tree, it is important to have it removed before the tree dies.

This is the whole argument for making the decision now rather than next year. The cost of your ash tree only goes one direction while you think about it.

The City of Boulder's guidance is to only hire a licensed certified arborist to prune, remove, and apply pesticides to your ash trees. Boulder maintains two license categories, and the distinction matters specifically for ash. A Certified Arborist License holder may perform work on trees in the public rights-of-way and on public and private trees that are infested with a major insect or disease problem such as EAB. A Tree Contractor licensee may not do either of those things.

There is also a process for the ash tree on the city strip in front of your house. Those are public trees, but the City notes that adjacent property owners may be allowed to treat public street right-of-way ash trees with Boulder Forestry's permission. Start with Boulder Forestry, not with a saw.