FORT COLLINS, Colo.  – Earlier this month experts from Colorado State University Extension confirmed the presence of emerald ash borer (EAB) in the City of Thornton. According to Thornton’s tree inventory, there are approximately 1,600 ash trees on public property, and there are significantly more ash trees on private property in Thornton.

“If all the ash trees are lost to the EAB, it will have a significant impact on Thornton’s tree canopy and general landscape,” said Thornton Parks, Golf and Forestry Superintendent Paul Burkholder.

With ash trees estimated to comprise 15 percent or more of all urban trees in Colorado, this non-native, invasive pest poses a serious threat to urban forests. EAB attacks and kills both stressed and healthy ash trees and is so aggressive that trees typically die within two to four years after becoming infested.

EAB was first confirmed in Colorado in 2013 in the City of Boulder. Since then, EAB has spread to other cities and towns on the Front Range. Thornton is the first Colorado city east of I-25 to confirm EAB. Thornton Parks & Forestry made the initial find in an ash tree in Park Village Park, located near the center of the city.

“It’s probable that the EAB has already infested other ash trees in Thornton,” Burkholder said. “The larger ash trees in Park Village were treated in 2021, so this recent find tells us EAB has been there for several years prior to treatment.”

EAB Tips for Front Range Residents

  • Determine now if you have any ash trees. Identifying features of ash trees include compound leaves with 5 to 9 leaflets; leaflets, buds and branches growing directly opposite from one another; and diamond-shaped bark ridges on mature trees. More information about ash tree identification, including a helpful app, is at
  • If you have an ash tree, start planning. Decide if the overall health of the tree and the benefits it provides merit current or future treatment, or if it would be best to remove and replace it with a different species. If you are not sure, contact your local CSU Extension horticulture agent or an ISA Certified Arborist. If you do plan to treat the tree, the CSFS offers recommendations for selecting a tree care company.
  • Plant trees. Replace ash trees in poor health with diverse species. The Colorado Department of Agriculture offers a database of registered nurseries and landscape contractors.
  • Recognize signs of EAB infestation. Property owners with ash trees should be on the lookout for thinning of leaves in the upper tree canopy, 1/8-inch D-shaped holes on the bark and vertical bark splitting with winding S-shaped tunnels underneath. Report suspect trees by calling the Colorado Department of Agriculture at 1-888-248-5535 or filling out their EAB Report Form at
  • Help prevent further spread of EAB. Do not transport ash or any hardwood firewood, or any other untreated ash wood products, to other locations outside the Front Range. Dispose of ash wood safely by chipping, composting, milling into lumber or taking to a landfill.

Learn more about ash tree identification, the symptoms of EAB, treatment options and how to use ash wood.

Emerald ash borer (EAB) is a non-native, wood-boring beetle that is responsible for the death or decline of tens of millions of ash trees in the United States and Canada. This insect was first discovered in Michigan in 2002, and since then it has spread to at least 35 states, including Colorado. As a non-native insect, EAB lacks predators to keep it in check. EAB only attacks ash trees in the genus Fraxinus, but has also been documented infesting white fringe tree. Mountain ash and other tree species are not susceptible.