Emerald Ash Borer

What is an emerald ash borer?

The emerald ash borer is an insect native to Asia. It was introduced into North America sometime during the 1990s. It is an extremely destructive insect to all true native North American ash trees because our ash trees have very little resistance to this new pest. EAB attacks and kills all true native North American ash trees, including green, white, black and blue ash, as well as “autumn purple ash,” a popular white ash variety in Colorado.

This pest kills stressed and healthy trees and is so aggressive that ash trees may die within two years after they become infested. It is possible for EAB to infest an ash tree for up to four years before visible signs of decline in the tree occur.

The half-inch long, dark green, adult beetles are active from late May through July, as they feed on ash trees and lay eggs on the bark. After hatching, the resulting EAB larvae tunnel into the bark to feed in the phloem and outer sapwood layers of the tree, producing galleries that girdle and ultimately kill the tree within two to four years. These expanding S-shaped tunnels, or galleries, can be viewed when the bark is removed. EAB adults typically fly up to a half-mile from where they emerge to infest new trees. Distribution of the pest over much longer distances is possible due to people transporting ash firewood, logs, nursery stock or other wood.

Where are EAB in Colorado?

EAB was first discovered in 2013 in Boulder County. Since then, it has been identified in Broomfield, Larimer and Adams counties in 2019 and more recently in Erie and Thornton in 2021. In June 2023, the first confirmed case of EAB was discovered in Littleton. The Colorado State Forest Service maintains data on impacted areas within the state.

Should Castle Rock residents be concerned?

Although EAB has not yet been discovered in Douglas County, it has been identified and confirmed in a small area of Arapahoe County, within the City of Littleton near the border with Englewood, Greenwood Village and Centennial. Castle Rock residents should learn more, prepare and consider their options to protect their ash trees now before the EAB pests are found in this county.

What should I do if I have ash trees?

You have a decision to make as to whether you remove or treat your ash tree. Many factors play into that decision, from tree health to the size of your tree. If you are uncertain whether you have an ash tree, versus an elm or boxelder tree, learn how to identify ash trees.