What Is EAB?

An aggressive, destructive pest

Emerald ash borer (EAB) is an insect native to Asia. It was unintentionally introduced into North America during the 1990s. The specific timeframe is unknown due to the years it takes for trees to show symptoms from an EAB infestation.

EAB attacks and kills all true ash (Fraxinus spp.) native to North America, including green, white, black and blue ash, and their cultivars, including Autumn Purple® ash, a popular white ash variety in Colorado. EAB also infests olive trees (Olea europaea) and white fringetrees (Chionanthus virginicus) but not mountain ash (Sorbus spp.) since it is not a true ash species.

This pest kills both stressed and healthy trees and is so aggressive that ash trees may die within two years after they become infested. However, depending on the tree’s health, it is possible for EAB to infest an ash tree for three to four years before visible signs of decline in the tree occur.
 

Life history of EAB

EAB adults are approximately a half-inch long with a metallic, emerald-green head and back and a coppery reddish-purple abdomen. The adult beetles are active from late May through September. They consume ash foliage but cause little damage to affected trees, which allows them to remain unnoticed by homeowners until the tree begins to show signs of decline.

Females lay eggs in bark crevices in early summer. After one to two weeks, the eggs hatch and the worm-like larvae chew their way under the bark. The larvae are cream-colored, consist of bell-shaped segments and can average 1 inch in length when fully grown. EAB larvae feed on the inner bark of ash trees, creating S-shaped galleries that harm the tree by disrupting the movement of water and nutrients. The tunneling and feeding under the bark is what eventually kills infested trees.

The larvae mature into adults in the spring of the following year when they emerge through the bark, leaving small, D-shaped exit holes 1/8-inch wide. EAB normally completes its lifecycle in one year. The beetles may fly up to several miles to infest new trees or lay eggs in a new portion of the same tree from which they emerged.

Distribution of the pest over much longer distances is possible due to people transporting infested ash firewood, logs, nursery stock or other wood products.

A threat to Colorado’s urban and community trees

EAB poses a serious threat to Colorado’s urban and community forests, where ash trees comprise an estimated 15% or more of all trees. A 2025 study from the Colorado State Forest Service estimated that 18% of the trees in the Metro Denver area are ash trees, amounting to tens of thousands of trees in the Denver area alone. The future costs of EAB in Colorado, in terms of ash tree treatments, removals and replacements, could exceed several billion dollars.

Many of the state’s ash trees are located on private property and in parks and other community areas. As a non-native insect, EAB has few native predators to keep populations in check. Without ongoing treatment, any infested ash trees will die.

Everyone in Colorado can help fight this pest by not transporting firewood, because human-assisted spread is the only way EAB can move over long distances.