What is the Emerald Ash Borer and what
does it do?
Emerald Ash Borer (EAB) is a highly destructive, non-native insect that infests and kills all North American true ash species (Fraxinus spp.) including green, white, black and blue ash, and their cultivars (including “autumn purple ash,” a popular white ash varietal in Colorado). 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 two to four years.
Emerald Ash Borer in Colorado:
Emerald Ash Borer (EAB) was found in Boulder, CO, in 2013 and now has been found in Denver in 2025. As a non-native insect, EAB lacks predators to keep it in check. EAB only attacks ash trees in the genus Fraxinus (so mountain ash are not susceptible).
Approximately 15% of the trees that make up Colorado’s urban forest are ash. There are an estimated 98,000 in the city of Boulder alone. The Denver Metro area has an estimated 1.45 million ash trees. EAB is responsible for the death of millions of ash trees in the United States.