Emerald Ash Borer

Biology and importance

The emerald ash borer is an invasive beetle that severely damages ash trees across North America. First found in Michigan in 2002, it has spread to other areas very rapidly (including Colorado), killing millions of ash trees in forests, cities, and neighborhoods.

What does the beetle look like?

Larvae are white with characteristic bell-shaped body segments (see photo, below left); they burrow into the inner tree, then feed and create S- or serpentine-shaped patterns. Larvae will emerge as adults in spring, leaving D-shaped exit holes in trees. Adults are small, bright metallic green beetles with a copper/red underside and are approximately ½ inch long (see photo, below middle). They are active throughout the summer.

What are symptoms of infestation in my ash trees?

• Dieback of ash trees, or tree death of branches and twigs that starts near the tips and moves inward. This occurs in multiple branches and twigs, not just one. (See photo below at right — this is likely one of the first symptoms noticed)
• Excessive branching on tree trunks
• S- or serpentine-shaped markings under tree bark
• D-shaped exit holes on the outer areas of trees
• Increased woodpecker activity (birds will attempt to eat larvae under the bark)

Why is emerald ash borer a problem?

Ash trees are very common in the Western United States due to their drought tolerance and were extensively planted throughout Colorado’s Front Range. Larvae feed under tree bark, disrupting water and nutrient flow, leading to severe tree damage and even death within 3 to 5 years if left untreated. Tree removal, replacement, and treatment can lead to huge economic impact for homeowners and cities. Loss of ash trees can disrupt local ecosystems and reduce shade cover.

What should I do if I find emerald ash borer in my yard or a public ash tree?

First, confirm the infestation by looking for signs of emerald ash borer (upper tree canopy dieback, D-shaped exit holes, serpentine galleries under tree bark — see photos below). Report the infestation by contacting your local Colorado State University extension office, your city or county division of forestry, or the Colorado State Forest Service. Make sure to share specific details about the location and signs of infestation and try to get a sample of the wood and the insect itself when you detect it.

If the tree is in your yard, contact a professional arborist in your area to discuss a plan of action. If the infestation is in the earlier stages, insecticide treatments may be recommended to protect the tree. If the tree is heavily infested, removal may be necessary to prevent risk of injury. Note that canopy dieback of 30% or more means that a tree is no longer safe for an arborist to climb for removal. If a tree is removed from your yard, proper wood disposal is necessary. Shredding the tree into wood chips is an option, and research indicates that wood chips one inch or smaller cannot harbor EAB. Boulder County has a helpful decision guide to aid you in determining an effective plan of action.

Recommendations for homeowners

As emerald ash borer is present in our area, it is important to be aware of any infestations. Consider these steps.

Conduct regular monitoring of ash trees on your property:

Regularly inspect your ash trees for signs of emerald ash borer, such as the characteristic dieback at tops of trees, excessive branching around tree trunks, D-shaped exit holes, serpentine galleries under tree bark, and increased woodpecker activity. Keep a regular watch over your own ash trees, being aware of their location on your property and overall health status.

Take preventative measures:

When using firewood for camping or other activities, avoid moving firewood or other ash tree materials from infested areas to prevent spreading emerald ash borer. Make sure to purchase firewood when you reach your final destination. Consider planting diverse species of trees and shrubs to improve resilience of your landscape.

Work with professionals to treat or remove trees:

Contact a professional arborist to help assess the health of your ash trees and come up with a best plan of action. If treatment is recommended, rely on professionals to apply systemic insecticides. These treatments are most effective when applied before a heavy infestation and must be applied at regular intervals thereafter (approximately every two years). If a tree is heavily infested and is past the point of being saved, it will likely need to be removed to ensure safety. Note that canopy dieback of 30% or more means that a tree is no longer safe for an arborist to climb for removal. Work with a professional arborist or tree removal service to safely remove and properly dispose of infested trees. If you have a young ash and don’t want to deal with long-term maintenance, consider cutting the tree while young. You can find more information here about how to seek help from a professional.

Stay informed of information on emerald ash borer:

Keep up to date with local emerald ash borer management guidelines and resources. You can find guidance from the City of Fort Collins here and information from Denver extension here.

Management information

Contact a professional in your area

If you suspect a tree on your property is affected, it is crucial to take immediate action. Emerald ash borer is highly destructive and can cause significant damage to ash trees. However, dealing with this pest issue requires specialized knowledge and equipment. Please do not attempt to handle this problem on your own. Instead, contact a professional arborist in your area. These experts are trained to identify, treat, and manage emerald ash borer infestations safely and effectively. Attempting to address the issue without professional help can lead to further damage to your trees and potentially pose safety risks.

Why is it so important to manage an emerald ash borer infestation?

• Tree health and safety: emerald ash borer feeding disrupts the overall health of ash trees and negatively impacts the tree’s ability to transport water and nutrients. This can lead to tree death, which leads to an increased risk of falling branches or entire trees, leading to property damage or personal injury.
• Environmental impact: Ash trees are extensively planted in Fort Collins and throughout Colorado’s Front Range, providing shade and important habitat for wildlife. The loss of ash trees can be disruptive to local ecosystems.
• Economic considerations: The cost of removing dead or dying ash trees is significant.
• Preventing spread of emerald ash borer: Managing an infestation helps prevent the spread of emerald ash borer to other areas. Management is vital for protecting other ash trees on neighboring properties and public spaces.

Insecticide information

Before taking any steps to apply insecticides to your ash trees, you should first consult with a professional arborist to assess the health of your ash trees and determine the best treatment plan. A professional will be able to help you identify the most appropriate treatment plan. You want to ensure that, if you decide to use insecticides to treat your trees, your plan is safe and effective.

Dinotefuran and imidacloprid were formerly recommended treatments, but the latest research indicates they are no longer effective against emerald ash borer. Research is ongoing in Colorado to provide more data regarding insecticide efficacy against emerald ash borer. For now, we can glean information from other universities.

A study conducted by Michigan State University evaluated the effectiveness of two insecticides, emamectin benzoate (applied as trunk injections) and imidacloprid (applied as soil drench), in controlling emerald ash borer on green ash trees over three years. Trunk injections of emamectin benzoate were more effective in reducing emerald ash borer larvae and exit holes compared to soil applications of imidacloprid. Another study conducted by Michigan State University examines the effectiveness of different insecticides in controlling emerald ash borer over six years. Researchers treated green ash trees with imidacloprid, dinotefuran, or emamectin benzoate at various intervals. Emamectin benzoate trunk injections were most effective, significantly reducing EAB larvae even three years post-treatment. Dinotefuran trunk sprays also helped, while imidacloprid was less effective.

Where to report findings

Report the infestation by contacting your local Colorado State University extension office, your city or county division of forestry, or the Colorado State Forest Service.

Resources for WIPM outreach and implementation grant collaborators

If you’re part of our EAB outreach and implementation grant team, you can access our shared folder with resources here.

References and helpful links