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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 1990's on solid wood packing material.

In the U.S., EAB is a federally quarantined, invasive tree pest responsible for the death or decline of more than 100 million ash trees to date.

As of May 2018, EAB has been found in 33 states. EAB across the U.S. (PDF)

Emerald Ash Borer (EAB) Information and FAQs

How does EAB spread?

EAB moves short distances (½ mile 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 will 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 horticultural cultivars of these species such as ‘Autumn Purple’ ash. Ash trees, especially green and white ash, are popular shade trees in most Colorado communities. Ash trees are relatively fast growing and most varieties produce brilliant fall colors. 

How does EAB harm 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. By consuming the tree's vascular system, the larvae disrupt the tree's ability to move nutrients and water, eventually starving the tree to death.   

Symptoms & signs of EAB 

  • A general decline in the health of the ash tree 
  • Crown thinning 
  • Excessive sprouting 
  • 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 bark from infested trees and feed on the larvae; the damage from woodpeckers can be severe and can often be seen from the ground.

Council Study Session Sept. 8, 2015

The purpose of the study session, held on Sept. 8, 2015, was to discuss and obtain City Council’s feedback on anticipated long term impacts of the detected local emerald ash borer (EAB) infestation, city response to date, and recommendations on next steps including, but not limited to, the development of a City of Boulder Urban Forest Strategic Plan.

More information:  Sept. 8 ADDITIONAL Study Session Material - Emerald Ash Borer pdf

EAB in Boulder

Boulder is the first location where EAB has been found in Colorado. City of Boulder forestry staff identified an infected ash tree in northeast Boulder near the intersection of 30th Street and Iris Avenue, on Sept. 23, 2013. EAB will cause a significant impact on the urban tree canopy for the Colorado Front Range.

How many ash trees are in the Boulder area?

The 2013 United States Forest Service Metro Denver Urban Forest Assessment Report estimates the Denver metro area has 1.45 million ash trees and there are 616,000 trees total in Boulder with an appraised value of $1.2 billion. An estimated 15 percent or more of all urban and community trees are ash.

There are approximately 38,000 city park and public street rights-of-way trees under the jurisdiction of Boulder Parks and Recreation Urban Forestry; approximately 6,016 are ash trees (12.6 percent of the public tree population). About 78,000 ash trees are located on Boulder’s private property.

Watch Your Ash: A multimedia project about EAB in Boulder by University of Colorado students

EAB Quarantine

To prevent the spread within Colorado, the CDA imposed and has enforced quarantine on the movement of all ash tree products and hardwood firewood out of Boulder County. The quarantine took effect Nov. 12, 2013. After discussions with the City of Boulder, Boulder County and local trash haulers, CDA also included small portions of Jefferson and Weld counties to include two landfills within the quarantine area to facilitate movement of flood debris and EAB-infested material.

The quarantine prohibits the movement of all untreated plants and plant parts of the genus Fraxinus (all ash trees) out of the quarantined area and includes, but is not limited to:

  • Logs and green lumber
  • Ash nursery stock
  • Chips and mulch, either composted or uncomposted
  • Stumps, roots and branches
  • Firewood of any non-coniferous (hardwood) species

Safe Options for Handling Ash Wood in Boulder County pdf

How the City of Boulder is addressing EAB

What is the City of Boulder planning to do to address EAB in our community?

  • Tree plantings
  • Tree removals (as needed; only if removal criteria met, see below)
  • Conservative use of pesticides
  • Biocontrols
  • Wood utilization
  • Enforcement for dangerous trees
  • Ongoing public awareness
  • Continued collaboration with CU / County / State / Federal and additional partners
  • Urban Forest Strategic Plan including urban tree canopy analysis and diversification of urban canopy

EAB Treatment Options - Tree Removal (City Property)

Proactive ash removal work targets trees that meet the following criteria:

  • High risk ash trees;
  • Ash trees in poor condition, those compromised by other insect pests or exhibiting two or more known EAB signs;
  • Trees with poor placement (i.e., under power lines); 
  • Trees with poor structure that will mature into a higher maintenance tree; and/or
  • Any topped or improperly pruned ash trees. 

In 2014 and 2015, 192 declining public ash have been removed (including Pearl Street Mall removals). An additional 50 public ash are projected to be removed in winter 2015 (and replaced 2016). 

Urban forestry’s long term goal is to proactively address removal/replacement of untreated public trees (~4,500) to occur over next 7-8 years.

Why do trees have to be removed now, why can’t removal wait until the tree is dead?

Ash trees become very brittle as they decline and quickly become a public safety hazard more susceptible to storm damage. In addition, a phased removal and aggressive planting schedule will allow the department to spread required staffing and financial seasonal demand over time.  

EAB Treatment Options - Pesticides

All ash will eventually die without pesticide treatment. EAB populations expand exponentially as does ash tree mortality – in Midwest cities without use of pesticides all ash are dying in as short of a period of time as 10-12 years after initial introduction.

How many trees have been treated for EAB in Boulder?

A total of 199 public ash trees were treated for EAB in Boulder in 2014, and a total of 573 in 2015 (on public property).

Why pesticides?

Safe pesticides are available, when properly used. Pesticides are an important component of EAB management programs for a number of 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 and economic services provided by the urban tree canopy over a longer period of time.

Pesticide treatment goals

  • Slow the spread of EAB within Boulder and to other communities
  • Stage removals  to spread out costs
  • Maintain urban tree canopy
  • Preserve significant tree canopy

Long term pesticide treatment plan

  • Treat primarily with TREE-äge, some trees with TreeAzin
  • Treat 25 percent of public ash trees on a 3 year rotation (1500 trees total; approx. 500 trees/year)
  • Adjacent property owners may opt out of treatment

EAB Treatment Options - EAB Biocontrol

The USDA Animal and Plant Health Inspection Service (APHIS) rears the EAB biocontrols at their Michigan facility and provides the biocontrols at no cost to cities. Local APHIS staff applied for the permits necessary to release four biocontrols in Boulder. The tiny, non-stinging parasitic wasp species (Tetrastichus planipennisi) was released in September 2014, the egg parasitoid, Oobius agrili, was released over a six week period in spring, 2015, and the two remaining species, Spathius agrili and Spathius galinae were released in August, 2015.

The release site for all four biocontrol wasp species was along Boulder Creek Path, on CU Boulder property near the CU East Campus, and was approved by APHIS through the permitting process. The biocontrol releases are a cooperative project including staff from city forestry, CU, CDA and APHIS.

Sustaining a Healthy Tree Canopy in Boulder

Tree Plantings

In 2014 and 2015, Parks and Recreation urban forestry staff planted 968 trees including 586 in known infested areas or Parks and Recreation parks/facilities. The long term tree planting goal is to plant 500 new trees annually with a focus on long term maintenance.

Wood Use

In 2014, for example, the City of Boulder entered into an agreement with Boulder County to use wood chips from whole trees as a quality heating fuel. The city continues to explore safe, creative, and environmentally responsible means of handling wood debris. Use of the wood for our park furnishings and the like is also being explored.

Forestry staff has also been working collaboratively with the Colorado State Forest Service to identify markets for all urban wood, not just for ash. Potential markets include:

  • Mulch
  • Chips for biomass fueled boilers for heating facilities
  • BioChar
  • Slabs
  • Rough cut lumber
  • Hardwood flooring
  • Landscape timbers
  • Blanks
  • Wood stakes
  • Wood pellets

The feasibility for each option will be explored in more depth during the process.

EAB on Private Property

Who Should I Hire to Work on My Ash Tree?

City of Boulder Forestry Division wants to remind residents to only hire licensed tree care or landscape companies to perform work to their ash trees - see contractor listings below.

Safe Options for Handling Ash Wood in Boulder County pdf

Pesticide Applications

If you choose to treat your tree, pesticide treatments must continue indefinitely to save the tree, which is costly and can have environmental impacts. Information about pesticide options for control of EAB pdf  can help inform you about some of the options available. If you are considering treating a tree in the public right-of-way, per ordinance, property owners must get preapproval from the city. Please fill out a quick online form to report EAB treatment of a public tree.

Before hiring a commercial pesticide company, consumers should know that the Colorado Pesticide Applicators’ Act requires any person or company that applies pesticides for hire to be licensed as a commercial applicator through the Colorado Department of Agriculture (CDA) including companies applying only organic or natural control products.

Licensing ensures commercial applicators have the necessary training, expertise and experience to apply chemicals to protect the environment and public safety. Every licensed commercial applicator in Colorado has to meet strict testing requirements that are enforced by the CDA.

A person or company that performs applications of pesticides for hire without holding the appropriate license may receive a maximum fine of $1,000 per violation. CDA officials conduct routine office inspections to review records of applications, equipment maintenance and storage and disposal of pesticides in order determine that each pesticide applicator business is performing their work in accordance the pesticide use laws.

The CDA also investigates pesticide use and application complaints. To file a complaint, contact CDA at  303-239-4151  . With additional questions or to determine if a pesticide company is licensed, call  303-239-4146  or visit

Tree Pruning and Removal

Any individual or company pruning or removing trees on either public or private property within the city must hold a current City of Boulder Tree Contractor License.

It is a two tier system:

TIER 1: Arborist License

Licensees in this category have demonstrated a higher level of knowledge and professionalism and are allowed to perform tree work to trees in the public rights-of-way and to both public and private trees that are infested/infected with a major insect/disease problem (i.e. emerald ash borer, Thousand cankers disease of black walnut, Dutch elm disease, mountain pine beetle, etc).

Licensed Certified Arborist Contractors pdf

TIER 2: Licensed Tree Contractor

Licensees under this category are not allowed to perform tree work to trees in the public rights-of-way or to both public and private property trees that are infested/infected with a major insect/disease problem. Tree care companies performing work to public or private ash trees must have a City of Boulder Arborist license.

Licensed Tree Contractor Contractors pdf

Please contact the City of Boulder Forestry Division at  303-441-4406  with any questions about the
arborist licensing program.

More information: Arborist Licensing and Listings

Milling your ash wood

Ash is a hardwood that can be transformed into art and useable products. 

List of Sawers and Sawmills with EAB compliance agreements pdf

Are You Treating a Public Ash Tree?

Please note: The use of Neonicotinoid pesticides, such as Imidacloprid and Dinotefuran, are prohibited on public property in the City of Boulder. Common EAB products that contain Imidacloprid include “Bayer Advanced”, “Merit”, and “Safari”. These products should not be used to protect public ash trees.

Emerald Ash Borer Photos

Crown Dieback in large ash treeEAB Adult in barkExcessive sprouting in lower branchesWoodpecker DamageWoodpecker Damage 3D shaped exit holeD shaped exit hole and bark splittingBark SplittingD shaped exit holes 2EAB Larvae and S shaped gallery under bark

See all related photos in the Photo Gallery