Emerald Ash Borer

Need to Know

  • Emerald ash borer (EAB) is a highly destructive invasive beetle that attacks all species of ash native to North America.
  • The larval stage feeds beneath the bark and disrupts water and nutrient flow within the tree, which leads to tree death, typically in 2 to 4 years unless treated.
  • Signs and symptoms of EAB include, canopy die-back, the formation of epicormic shoots, S-shaped feeding galleries under the bark, D-shaped exit holes, and increased woodpecker activity and damage. 
  • Preventative insecticide treatments are available to protect the ash tree but they must be started before damage is observed to be most effective.  Regular treatments will be required for the rest of the tree's life.
  • Adult females typically lay eggs on ash within a few miles of where they emerged. However, this pest is easily moved long distances by moving infested firewood, logs, and nursery stock to un-infested areas. 

The emerald ash borer (EAB), Agrilus planipennis, is an invasive beetle from Asia discovered in southeastern Michigan in 2002. Since then, the beetle has spread widely across the U.S. and Canada. EAB was first identified in Iowa in 2010 and the pest is now found in most Iowa counties. 

EAB Infestation Status in Iowa By County (IowaTreePests.com)

Photo of Adult Emerald Ash Borer
Emerald ash borer adults are small elongated oval beetles that are metallic green in color.

EAB larvae feed on the inner bark of ash trees and cut off the living water and nutrient conducting vessels, causing tree death. EAB has killed hundreds of millions of ash trees in North America and cost municipalities, property owners, nursery operators, and forest products industries hundreds of millions of dollars. 

Description of Emerald Ash Borer

Adult emerald ash borers are bright metallic green in color with very short antennae. They are ½ inch long and one-eighth inch wide. Emerald ash borer larvae are creamy-white in color and have flattened segmented bodies. Older larvae grow up to an inch long.   
More Information: Native Borers and EAB Look-alikes  (pdf) (E-2939)

Life Cycle of Emerald Ash Borer

Adults are active from May to September, with the peak of their activity happening in June or July.  During this time EAB adults consume ash foliage (causing insignificant damage) and mate.  Females lay their eggs (average 60-90 per female) individually or in small clusters between layers of bark and in bark crevices primarily in the trunk and major branches. Tiny white larvae hatch in about one week and burrow directly from the egg through the outer bark to the cambial regions where they feed from mid-summer through the next spring.  As the larvae feed they create serpentine or s-shaped galleries filled with frass.  Larvae overwinter and pupate in the spring.  The pupal stage lasts for about 1 to 2 weeks, then the new generation of adults emerges in early summer through a D-shaped exit hole.  

The life cycle usually takes one year, although two-year life cycles do occur, especially in newly infested trees.  

Damaged Caused by Emerald Ash Borer

photo of tree with crown dieback and epicormic shoots due to EAB
Trees infested with EAB develop thin crowns, dead branches, and epicormic shoots on the lower part of the tree. This tree has too much canopy thinning to be a candidate for successful treatment with systemic insecticides.

While EAB adults feed on the foliage, the damage is insignificant.  The primary damage results from larval feeding of the cambium layer just under the bark.  This damage disrupts the flow of water and nutrients in the tree and causes progressive canopy thinning and dying branches, usually from the outer or upper parts of the canopy down.  Decline typically happens over 2 to 4 years but can progress in as little as 1 or as many as 5 years.  Epicormic sprouts (suckers) form low on the trunk and major branches and usually there is noticeable flecking of bark by woodpeckers foraging to feed on the larvae. Occasionally vertical fissures on the bark are observed with the galleries or tunnels visible under the split. 
More Information: Signs and Symptoms of EAB (pdf) (E-2938)

Management of Emerald Ash Borer

There are two options to manage trees infected or likely infected with EAB.  You can remove a failing/declining ash tree and replace it with another species, or you can use preventive insecticide treatments to preserve and protect your ash tree.

Without preventative systemic insecticides, infected ash trees will die. These preventative treatments are most effective on those trees that exhibit no EAB damage and are otherwise healthy with a full canopy. Trees must have less than 50% canopy thinning or less than 30% of branches dead to be a candidate for treatment.  Once the entire crown has visibly small leaves (thinning) and there are only large leaves in the middle portion of the tree (on the suckers) there has already been too much damage for insecticides to 'save' the tree.  Ash trees that are in decline from other causes should be removed and not treated as preventing EAB damage will not reverse the decline in trees impacted by other diseases or insects.

Several insecticides are available and can be applied by trunk injection, soil drench, soil injection, or as a basal trunk spray, depending on the product.  Insecticides are best applied from mid-April to mid-May.  A few products are also effective when applied in early August to mid-September, but control is more effective with spring applications. While some products, primarily soil drenches, are available for homeowner use, most can only be applied by licensed pesticide applicators.  Applications of insecticides will be needed every 1 to 2 years depending on the product.  If insecticides are not reapplied within the appropriate timeframe, trees are open to becoming infected.

Detailed information on emerald ash borer management options is contained in this publication available from Iowa State University Extension & Outreach.

Removal of Infested Ash

If you have determined your ash is too damaged to be treated you should remove it as soon as possible.  Ash branches become brittle once dead and will fall which poses a hazard to people and property.  We recommend utilizing an arborist to remove ash trees on personal property.  Ash trees do not need to be treated or chipped, but if you do plan to keep the wood for campfires or fireplaces use it on your property and do not transport the wood to new locations.  Any firewood brought indoors (especially the first year after being cut) can have EAB beetles emerging in the house.  The EAB that emerge indoors will not infest any items in the home and will not live long.  They will likely end up near windows where they can be vacuumed and removed.  Do not treat firewood with insecticides. 

Frequently Asked Questions on Emerald Ash Borer

Photo of woodpecker damage from feeding on EAB larvae
Bark flecking and holes created by woodpeckers are common on EAB infested ash trees.
Photo of D-shaped exit hole from EAB adult
The small D-shaped exit hole located in the center-right of the photo is created by the emerging adult.

Additional Information on Emerald Ash Borer

Recognizing Emerald Ash Borer and Its Damage

Management of Emerald Ash Borer

Ash Identification and Replacement Suggestions

Distribution Maps of Where EAB Has Been Confirmed

Images and Videos

Other Websites

Información del barrenador verde esmeralda en español

photo of s-shaped galleries under bark
S-shaped galleries under the bark created by feeding EAB larvae.
Photo of tree with epicormic shoots due to EAB
Epicormic shoots (sprouts) form low on the tree when infested  with EAB.

Do you live in Iowa and have an insect you would like identified?

The Iowa State University Plant & Insect Diagnostic Clinic will identify your insect, provide information on what it eats, life cycle, and if it is a pest the best ways to manage them.  Please see our website for current forms, fees, and instructions on preserving and mailing insects.   

Contact information for each state's diagnostic laboratory for U.S. residents.  If you live outside of Iowa please do not submit a sample without contacting the Plant & Insect Diagnostic Clinic.

Last reviewed:
July 2022

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on September 12, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.