Emerald Ash Borer on Acreages

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Mark H. Shour, Ph.D.
Program Specialist, Entomology
Iowa State University Extension & Outreach


A very small (½ inch long x ⅛ inch wide), metallic green beetle is moving/being moved across Iowa and is destroying ash trees in its wake. More than twenty counties are now considered infested by the Iowa Department of Agriculture and Land Stewardship, and this count is expected to increase in the coming year.

emerald ash borer

The emerald ash borer (EAB), Agrilus planipennis, hitchhiked from Asia in wood packaging materials into the United States sometime in the 1990s. It established itself in ash trees in the greater Detroit, Michigan area, nearly undetected until 2002 when an adult specimen was submitted for identification.  The news of this exotic, recent introduction into the USA set off queries about previous literature, only to find two papers about its physical description and known Asian hosts. The great majority of information about EAB has been observed in the past 13 years, and much more research is needed to answer the many questions arising from this devastating insect pest.

Although the adult stage feeds on its host’s leaves, it is the larval (immature) stage that inflicts the primary damage.  Larvae (pictured below) feed beneath the bark in the cambium area, cutting serpentine tunnels (reminds the author of mountain switch-back roads) thus severing water and mineral transport to the top part of the tree, and also preventing sugar transport from the leaves to the roots.  In essence, the tree dries out and is starved at the same time. Multiple larval feeding ‘galleries’ along the trunk and branches will kill the tree in 3 to 5 years.

emerald asah borer

The life cycle can be completed in one year. Adults emerge in June and July, feed, mate, and lay eggs on the tree bark. Larvae hatch from the eggs and chew through the bark to the cambium area, feed, and overwinter. The insect transforms into the pupal stage in the early spring, then to the adult stage.  Adults emerge by chewing a “D-shaped” exit hole through the bark to complete the cycle.  It has been reported that EAB took 2 years to develop in healthy trees, with most of the life cycle spent in the larval stage.

Host trees

EAB has an insatiable appetite for ash trees – Fraxinus species. In Iowa, green ash (F. pennsylvanica) and white ash (F. americana) are fairly common in landscapes and forests, while pockets of black ash (F. nigra) and blue ash (F. quadrangulata) also occur naturally. Approximately 6% of trees in Iowa’s forests are ash species. The ash component in urban landscapes is much higher (60%+), as this genus was used as a replacement for American elms once Dutch elm disease moved through Iowa’s towns and cities.

Unfortunately, EAB’s attention is not focused on declining or weakened ash trees, but healthy trees as well. There are some ash trees still standing in Michigan despite EAB’s feeding activities, and researchers are trying to determine if this is the result of just being missed or if there is tolerance or resistance involved.

A new host, white fringetree, Chionanthus virginicus, was found in Ohio in 2014.  This olive family relative of ash appears to allow the complete development of EAB, so add another genus to its narrow host list.  Fringetrees, traditionally a more southern species, are fairly recent introductions in Iowa’s urban areas by landscapers.

How do you know if you have an ash tree?  Below is a picture of an ash leaflet and the bark texture of green or white ash.  

ash tree leaves

ash tree bark

(Photo credit for the two photos above - Paul Wray, Iowa State University Extension, Bugwood.org.)

You could also take a leaf into the county Extension office (remember it is a compound leaf) or visit the following sites for the do-it-yourselfers:


Symptoms of EAB activity

Telltale symptoms of EAB activity in an ash tree include:

  • Top of the tree dieback– EAB starts feeding at the top of ash trees.

ash tree die back

(Photo credit - Mike Kinter, Iowa Department of Agriculture and Land Stewardship.)

  • Epicormic sprouting (‘water sprouts’) along main branches or upper tree trunk– since larvae cut off sugars from the tree top, the tree pushes out new suckers to try to regain sugar production (As shown in photo above.)
  • Woodpecker feeding (“flecking”) – large pieces of outer bark are flecked off by woodpeckers looking for EAB under the bark.

ash tree damage

(Photo credit - Mike Kinter, Iowa Department of Agriculture and Land Stewardship.)

  • D-shaped exit holes chewed by adults emerging from under the bark. Unfortunately, these are first seen at the top of the tree where EAB begins its feeding; by the time you see the exit holes low on a tree, it is usually heavily infested.

emerald ash borer hole

(Photo credit - MH Shour, Iowa State University Extension and Outreach.)

  • Serpentine tunnels chewed by larvae under the bark. In many instances, the bark above this tunneling cracks and is easy to pop off.

emerald ash borer damage

(Photo credit - Mike Kinter, Iowa Department of Agriculture and Land Stewardship.)

Combined together with the positive identification of an EAB life stage (larva, pupa, and/or adult), these are used to declare a ‘find’ infested by state and federal regulators.

Native borer activity

Iowa has its set of native insects that feed on dying/declining ash trees.  Their job is to assist in the natural wood recycling effort. Ash-lilac borer, redheaded ash borer, flatheaded appletree borer, banded ash borer, and eastern ash bark beetle are five native borers that make tunnels under the bark of ash trees, and can be confused with EAB at first glance. For more information on this topic, visit:


How to manage EAB infestations

After you confirm you have an ash tree, the next step is to determine if the ash tree is in vigorous health. Trees must be healthy and growing on a good site for treatments to be effective. Compromised trees that have mechanical injuries, loose bark or thin canopy or are struggling to grow in poor sites with limited rooting area, compacted soil or other stresses are not worth treating. If the tree is apparently healthy and is valuable in your landscape, then treatment options may be considered. Please visit https://store.extension.iastate.edu/Product/Emerald-Ash-Borer-Management-Options to read the ISU Extension & Outreach publication Emerald Ash Borer Management Options.

Specimen trees in the landscape can be protected from EAB. This process is done either every year or every other year (depending on the active ingredient used), and is a long-term commitment (at least 10-15 years into the future).  If the tree is 20” diameter at breast height (measured 4.5’ high on tree’s trunk) (corresponds to 60” circumference), the landowner could use a homeowner treatment – see page 3 of EAB Management Options. If the tree is larger, a certified pesticide applicator can determine which of the four treatments (soil drench, soil injection, basal trunk spray, or trunk injection) is best for the site and tree.

Treating woodlot trees is not practical or cost effective, especially where timber production is the primary goal.  Healthy windbreak ash trees might be treatment candidates if the landowner is willing to invest in the necessary equipment for trunk injection ($2500 - 3000) and become a noncommercial certified applicator (if using emamectin benzoate sold as Tree-Age). A description of this applicator category can be found at: https://www.extension.iastate.edu/psep/. Two tests would need to be passed, the Iowa Core Test and the Category 3OT test.  Study materials (IC-445 and CS-15) for these tests can be obtained at your county Extension office or at https://store.extension.iastate.edu/ProductList?Keyword=pesticide%20applicator.  Testing information is available at https://iowaagriculture.gov/pesticide-bureau.

Insecticide control measures against EAB should not be used unless you live within 15 miles of the confirmed EAB infestations. All ash trees within this zone are at risk of EAB attack.  Trees outside the 15-mile risk zone should be monitored for signs of infestation.

For a full list of EAB detection and education activities, please visit our website at https://www.extension.iastate.edu/psep/emerald-ash-borer-eab.

Date of Publication: 
June, 2015