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Emerald Ash Borer

Emerald Ash Borer, Agrillus planipennis

Emerald ash borer is an invasive pest of ash trees. Larvae of this pest feed beneath the bark, eventually killing the tree. Emerald ash borer was found in Boulder, CO in 2013 and is actively spreading.  

EAB larvae.

EAB larvae.

Adult emerald ash borers and the characteristic D-shaped exit hole

Adult emerald ash borers and the characteristic D-shaped exit hole.

EAB tunneling on an ash with bark removed

EAB tunneling on an ash with bark removed.

EAB damage on ash tree

EAB damage on an ash tree.

Description and Life Cycle

The emerald ash borer (EAB) is a flatheaded beetle in the order Coleoptera, family Buprestidae. The adult is a dark metallic green beetle, about 0.37-0.5 inches long. The larvae are cream-colored with bell-shaped body segments and can reach 1.5 inches when fully grown. Larvae are the destructive stage of this insect. After hatching from eggs deposited in the bark of ash trees, larvae bore into the cambial layer just beneath the bark. Larvae wind back and forth as they feed, creating distinct S-shaped tunnels, or “galleries”. The larvae will feed beneath the bark for a year before pupating and emerging as adults in late May through July. Adult beetles create a D-shaped exit hole in the bark as they chew their way out of the tree. EAB adults typically live three to six weeks and fly up to a half-mile from where they emerge to find a new host tree. After mating, female beetles will lay 60-90 eggs, one at a time, in the crevices of ash tree bark. Distribution of this pest over long distances is possible due to the transportation of infested firewood, logs, nursery stock or other wood.  



EAB attacks all true North American ash trees (Fraxinus species), which are commonly planted in urban and community landscapes. Injury is caused by the tunneling of larvae in the cambium, just beneath the bark. Tunneling by the insect reduces the ability of the tree to transport water and nutrients producing a progressive weakening of the plant. As the amount of wounding increases, the tree will become weaker and show signs of distress. These include thinning of the leaf canopy, as smaller leaves are produced, branch die-back, bark splitting along the branches, and new sprouts on the trunk. There may also be increased woodpecker activity on the tree, as they feed on EAB larvae. Once EAB has infested a tree, the injury it causes will increase over time so that the tree will ultimately be killed, unless treated with insecticides. It may take two to four years for an infested tree to die, and the speed of decline depends on many other factors. These include the original health of the tree, weather, and the number of emerald ash borers developing in nearby trees.   


Detection: The first step in protecting a landscape from EAB is determining whether there are ash trees present and scouting for the presence of emerald ash borer. This webpage from CSUFS can be used to identify ash trees. Presence of emerald ash borer can be determined by symptoms of injury on trees (like a thinning canopy), D-shaped exit-holes in the bark, or S-shaped galleries when the bark is peeled away.  

EAB poses a serious threat to Colorado’s urban and community forests, where ash trees make up a significant portion of tree populations. The life cycle of this pest protects it from predation and makes it difficult to treat. There are no established biological or mechanical methods of controlling EAB and no varieties of ash resistant to this pest. Complete control (eradication) of EAB is not possible and suppression will require considerable investments. Protection of existing ash trees is achieved through insecticide treatments, which are applied as soil drenches or trunk injections. These insecticides can be used preventively, applied when the insect is first detected near the neighborhood, or when populations of the insect are still low, and tree injury is minimal. Insecticides used to manage EAB are systemic types which are absorbed by the plant and move into cambium, where it kills feeding larvae. When these are applied, it is necessary to consider possible non-target effects to pollinators or beneficial insects. Additionally, treatments are only effective if less than 30% of the canopy is affected by EAB, and they must be repeated for the life of the tree. To minimize the impact of invasive pests in the long-term, urban landscapes should contain a more diverse array of tree species. Ash trees are no longer recommended to plant in Colorado and alternative species should be planted instead. The movement of EAB to other areas can be slowed by not moving wood (including firewood) to other parts of the country.   

Quick Facts:

  • Emerald ash borer is a green flat-headed beetle that develops in ash trees (Fraxinus spp) 
  • It is an invasive insect that is native to Asia. It was first detected in Detroit in 2002 and was found in Boulder, CO, in September 2013 
  • Emerald ash borer wounds ash trees by tunneling under the bark.  When wounds become extensive, trees show symptoms of decline and, ultimately, are usually killed. 
  • Systemic insecticides are typically applied as either soil drenches, trunk sprays or direct injections into the trunk, and can suppress early emerald ash borer infestations. 

View CSU Extension Fact Sheet

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