Emerald Ash Borer

Emerald ash borer

The Emerald Ash Borer (EAB) is an invasive insect that infests and kills ash trees. The larval stage of EAB feeds under the bark of trees, cutting off the flow of water and nutrients. Infested trees gradually die over a period of approximately three to five years.  

Detecting Emerald Ash Borer is very difficult. Visual survey is not as useful as branch peeling; branch peeling is more accurate. If an ash tree is experiencing dieback or looking unhealthy, have the tree examined by a professional tree company. You can also send pictures of your ash tree to Master Gardener Help Desk at CSU Extension in Arapahoe County for assistance with your tree.

Infestation signs include:

  • Sparse leaves or branches in the upper part of the tree
  • D-shaped exit holes about 1/8 inch wide
  • New sprouts on the lower trunk or lower branches
  • Vertical splits in the bark
  • Winding S-shaped tunnels under the bark
  • Increased woodpecker activity

Identifying Emerald Ash Borer

Learn more about ash tree identification and the signs and symptoms of EAB

Not sure about your tree’s id? Send photos of your tree with close ups of leaves, twigs, and bark to the Colorado Extension office (the more photos you can provide the better).

Treatment Options

We encourage you to treat any of your healthy ash trees! The decision of WHEN to start treatment can vary based on how close you live to the current infestation. As of June 2024, EAB has been confirmed in a small area on the west side of Centennial. If you live on the west part of Centennial, treating your ash trees sooner than later is advisable. Learn more in Arapahoe County’s EAB FAQ and Planning Guide.

Here is a great resource for learning about emerald ash borer treatment options. Four insecticide treatments are available. The frequency of the application will depend on the product but will range from one or two years. The options available are all systemic insecticides meaning the roots will uptake the product. It will take time for the product to thoroughly move through the tree, and it is important to keep the tree well-watered during this time. 

When Hiring a Tree Care Professional

  • Ask for credentials. Arborists should have a current State of Colorado Pesticide Applicator License, should be insured and ideally be certified through the International Society of Arboriculture.
  • Don’t be afraid to ask for a specific contract of work and customer references.
  • Do your research and get quotes from multiple companies.

Search for a Certified Arborist at Trees Are Good.

Learn More

Learn more about treatment options, how to dispose of infected wood, tree care, when to begin treatment and EAB look-alikes. Download our EAB FAQ and Planning Guide.

For general questions on EAB, treatment options, or how to make decisions about your trees and possible diagnosis, contact CSU Extension-Arapahoe County at MasterGardener@arapahoegov.com or 303-730-1920.

On May 2, 2024, CSU Extension hosted a webinar EAB: An Update for Homeowners. See videos below.

English version


Spanish version


For more information

Helpful Resources