Emerald Ash Borer Overview

Emerald Ash Borer (EAB) is a metallic green beetle that feeds on ash tree species. The beetle originated from Asia and is thought to have been introduced to North America in the 1990s on solid wood packing material. It was first detected in Michigan in 2002 and has since spread to many other states and Canadian provinces.

North American species of ash have shown little resistance to EAB and an estimated 50-70 million ash trees across the U.S. have died from this pest. 

EAB was detected in the city by Boulder Forestry staff in September 2013, the first detection of this pest in Colorado. Track the spread within Colorado

10 years of EAB in Boulder

Fall 2023 marked 10 years since EAB was detected in the city by Boulder Forestry. To mark the anniversary, Boulder Forestry developed a story map with Davey Resource Group to provide community members and urban forestry colleagues insight into Boulder’s experience and lessons learned from managing EAB.

The story map explores:

  • The Urban Forest - Highlighting the importance of Boulder’s tree canopy, illustrating the extent of our tree canopy and how canopy cover has changed since EAB was detected.
  • Boulder Forestry - Boulder Forestry’s role in managing public trees.
  • Emerald Ash Borer Basics - Overview of EAB and the challenges associated with managing this invasive pest.
  • 10 Years of EAB – Chronicles the management decisions and collaborative efforts that have helped Boulder Forestry slow ash mortality following EAB's detection.
  • Outcomes of EAB's Invasion - Summarizes lessons learned and observations of EAB in Colorado.
  • What Comes Next? - How community members can help sustain Boulder’s urban tree canopy by planting and growing new trees

  • Colorado's EAB Response in Forestry: An International Journal of Forest Research
    A collaborative approach to preparing for and reacting to emerald ash borer: a case study from Colorado

    By: Kathleen Alexander, Micaela Truslove, Rob Davis, Sky Stephens and Ralph Zentz

    The study is available by email request to alexanderk@bouldercolorado.gov.

Is there a public ash tree near you?

Enter your address to find out if you have a public ash tree near you.
This map shows only public ash trees.
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Public Ash Trees

EAB Basics

EAB moves short distances (up to 6 miles annually) by flying but can move longer distances in transit on ash nursery stock, ash logs, branches and firewood. 

What trees are susceptible to EAB?

All sizes and even very healthy ash trees in Boulder will eventually be killed by EAB unless treated with an effective pesticide. Ash species attacked by emerald ash borer include green ( Fraxinus pennsylvanica ), white ( F. americana ), black ( F. nigra ), and blue ( F. quadrangulata ), as well as cultivars of these species such as ‘Autumn Purple’ white ash. Ash trees, especially green and white ash, are popular shade trees in most Colorado communities. Ash trees are relatively fast growing, and both green and white ash trees produce brilliant fall colors. 

How does EAB kill ash trees? 

Emerald ash borers experience complete metamorphosis, much like a butterfly. The adult life stage feeds exclusively on ash leaves. Adult beetles then lay tiny eggs on the bark of ash trees. These eggs develop into small larvae which bore into the trees’ vascular system. As the larvae grow and tunnel under the bark, they disrupt the tree's ability to move water and nutrients, causing a rapid decline and eventually killing the tree. 

Symptoms of EAB

Symptoms of EAB typically start at the top of the tree and progress downward over time. 

  • A general decline in the health of the ash tree 
  • Crown thinning 
  • Excessive sprouting 
  • Early fall coloration 
  • Dead branches 
  • Serpentine “S”- shaped tunnels under the bark produced by the larvae 
  • “D”- shaped adult exit holes on the bark surface 

Woodpeckers often remove the gray outer bark from infested trees and feed on the larvae; the damage from woodpeckers can be severe and the newly exposed whitish inner bark can often be visible in the upper canopy from the ground. 

Tree Removal  

Untreated public ash trees are removed when they begin to show symptoms of EAB infestation. Wherever possible, these ash trees will be replaced with a diverse mix of large maturing, non-ash tree species. 

Pesticide Treatment  

Approximately 1,300 public ash trees are included in Boulder’s EAB pesticide treatment program. Refer to the map above to see which ash trees in your neighborhood are being treated. The 1,300 trees are split into 3 groups and treated on a three-year rotation with an injectable formulation of emamectin benzoate. Boulder began treating ash trees in spring 2014 with great success.  

Pesticides are an important component of EAB management programs for several reasons as they can: 

  • Preserve ash trees long term; 
  • Reduce community-wide EAB populations and therefore slow the progression of EAB; 
  • Spread the tree removal and replacement costs over a longer time period; 
  • Lower the risk and public safety concerns associated with large numbers of dying trees; and 
  • Spread the loss of the urban tree canopy and the subsequent loss of the environmental, social, health and economic services provided by the urban tree canopy over a longer period of time. 

Adjacent property owners may be allowed to treat public street right-of-way ash trees with Boulder Forestry’s permission.  

Who Should I Hire to Work on My Ash Tree?

Only hire a licensed certified arborist to prune, remove and apply pesticides to your ash trees. Licensed Certified Arborists

Pesticide Applications

If you choose to treat your tree, pesticide treatments must continue indefinitely to preserve the tree. The frequency and method of application depend upon the product used: 

  • Emamectin benzoate – injected into the trunk every 2-4 years (Boulder public trees are on a 3-year rotation) in the spring 
  • Azadirachtin – injected into the trunk every 1-2 years (depending upon EAB populations) in the spring 
  • Dinotefuran – applied as a bark spray annually in the spring  

Pesticide applications for EAB are inexpensive though when compared to the cost of tree removal and replacement. Mature ash trees also provide huge environmental benefits to our community. 

Tree Removal 

Once infested with EAB, ash trees dry out quickly, become brittle and are more prone to fail. If you are planning to remove your ash tree, it is important to have it removed before the tree dies. It is not safe for arborists to climb trees once they become symptomatic for EAB, so removal costs may be even higher. 

More information on urban forest pests