Denise Schreiber is a columnist, lecturer and the author of Eat Your Roses. She has more than 25 years of experience in professional horticulture, ranging from interior plantscaping to greenhouse production to public horticulture.

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Emerald Ash Borer
by Denise Schreiber       #Advice   #Feature   #Pests

The adult emerald ash borer is about ½ inch long and is a bright metallic green color.1

There are wanted posters out everywhere! Emerald ash borer (Agrilus planipennis Fairmaire), is an invasive insect native to Asia that has killed tens of millions of ash trees in urban, rural and forested settings. It is known to be a hidden hitchhiker on that firewood you bought the other day to take to your camp because it was a good price. With the U.S. Interstate system, it has easily found its way and invaded many counties and states.

This beetle was first discovered in 2002 in Southeast Michigan and Windsor, Ontario. EAB infestations have now been detected in at least 20 states: Colorado, Connecticut, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Maryland, Massachusetts, Michigan, Minnesota, Missouri, New Hampshire, New York, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia, and Wisconsin. In Michigan alone, it has caused almost $12 million dollars’ worth of damage and the toll continues to rise as EAB moves through the states.

A mature ash tree showing signs of infestation. Dieback begins in the top one-third of the canopy and progresses until the tree is bare.2

The emerald ash borer favors all 16 different species of ash trees found in the United States. Ash trees are as important ecologically in the forests of the northeastern U.S. as they are economically important. Ash trees fill gaps in the forest and are highly desirable for urban tree planting, providing shade and habitat. Ash wood is valued for flooring, furniture, sports equipment (such as baseball bats, hockey sticks, oars) and tool handles. Ash trees and wood are also significant to Native American cultures for traditional crafts and ceremonies.

Think of EAB as the bubonic plague of ash trees. While there are some natural controls, they are not effective enough to slow the relentless march of the borer through the forests. According to Dr. Houping Liu, Ph.D., forest entomologist for the Pennsylvania Department of Conservation and Natural Resources, “The effectiveness of EAB parasitoids is still being evaluated; field release of those parasitoids will continue as long as the Feds maintain financial support to the rearing lab in Michigan. We are not expecting large enough numbers of parasitoid offspring in the field just yet, as biological control is a long-term strategy that will take time.” He goes on to say, “Climate has an effect on the parasitoids the same way it has on EAB.” He also makes the not-so-encouraging prediction that it is possible that emerald ash borer will eventually affect every ash tree in the U.S.

The life cycle of EAB starts from mid-May to mid-August when the ash borer lays its eggs on ash bark. From May to August the eggs hatch and tunnel into the tree. From August to October they feed under the bark. From May to June the adults hatch and leave D-shaped exit holes. Adults are roughly 3/8 to 5/8 inches long with metallic-green wings and a coppery red or purple abdomen. They will feed on foliage before laying eggs on the bark of the tree.

The larvae of the EAB are creamy white and legless with flattened, bell-shaped body segments. The terminal segment bears a pair of small appendages. Larval feeding disrupts the tree’s circulation of water and nutrients.4

The larvae feed just under the bark and create S-shaped galleries. Galleries weave back and forth across the wood grain and are packed with frass.3

The first inkling that something is wrong with your ash tree is the crown of the tree gets a little thinner. You may notice a few dead branches. You will also notice little D-shaped holes in the trunk of the tree. These holes can occur anywhere on the tree trunk. If you peel back some bark, you will see galleries made by the larvae of the borer. They look like S-shaped tunnels throughout. As the larvae feed, they can clog up the xylem and phloem, which are the transport systems in a tree, carrying water and nutrients. This is the time to have your tree evaluated by a certified arborist (not the local “landscraper” looking to trim your trees for you). If at least two-thirds of the tree canopy alive and there are no epicormic shoots (side shoots from the base of the tree), it can be treated. Untreated, a large ash tree can die within two to three years.

At this time, there is no organic method to subdue EAB. According to information at, insecticides that can effectively control EAB fall into four categories: first, systemic insecticides that are applied as soil injections or drenches; second, systemic insecticides applied as trunk injections; third, systemic insecticides applied as lower trunk sprays; and last, protective cover sprays that are applied to the trunk, main branches and (depending on the label) foliage.

There are two main treatments currently being used, although other chemicals also are used. One treatment is imidacloprid and the other is a product containing emamectin benzoate. Imidacloprid products are either applied as a soil drench, foliar spray or as a trunk injection. Emamectin benzoate is a trunk injection only treatment. (There are a few more products that have limited success against the borer.) Both of these products are applied by professional pesticide applicators with specialized equipment and protective gear. All treatments have a particular time frame in which they should be applied (which will be on the label). The emamectin benzoate treatment has been most successful so far in treating ash trees and will last at least two years (unlike other treatments). It is suggested that if you have a high-value tree – one that is large and well rounded, historical or has some other value to the community – that you should spend the money on treatment.

If you have missed the initial diagnosis and your ash tree is beyond redemption, you should look to have the tree removed as soon as possible. Affected ashes can topple without warning within a few months of tree death.

So the best thing you can do is be proactive about your ash trees and remember not to move any firewood except into the fireplace.

Woodpeckers Tell the Tale

Another telltale sign of EAB infestation is the hammering of woodpeckers. Although the usual cacophony of the hammering is for mating purposes, the EAB larvae have become a delicacy for the ruby-topped birds that thrive on the insects. As the woodpeckers go after the larvae, which are just under the bark of the tree, they will loosen the bark to get at them and actually remove large portions of bark by chipping at it with their beaks.  5  


1. Photo by Marianne Prue, Ohio Department of Natural Resources - Division of Forestry
2. Photo by Steven Katovich, USDA Forest Service
3. Photo by Christopher Asaro, Virginia Department of Forestry
4. Photo by Kenneth R. Law, USDA APHIS PPQ,
5. Photo by Jeff Rugg

From State-by-State Gardening January/February 2014.


Posted: 04/30/14   RSS | Print


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