Have you noticed an increasing number of large dead trees along roadways, and in wooded areas near you? Many of these are green or white ash trees, and they are being killed by an invasive pest first spotted and identified in the Detroit, Michigan/Windsor, Ontario, Canada, area in 2002, and now rapidly spreading throughout the range of the host species. The emerald ash borer (EAB), scientific name of Agrilus planipennis Fairmaire, an invasive insect native to Asia, has killed millions of ash trees (our Fraxinus species) in urban, rural and forested areas across much of the U.S.
As of December 2018, emerald ash borer infestations were known to be present in 35 U.S. states and five Canadian provinces. Up to this time, all of our native North American ash species have shown little natural resistance to this insect borer. Within a few short years following EAB introduction, arborists began protective treatments initially across the Great Lake states to protect individual trees.
There are several insecticide options for controlling the EAB. They include soil drenches, stem injections, stem sprays and tree cover sprays. By far the most effective and environmentally safe is stem injections. Additionally, multiple economic analyses over time have concluded that treating trees with effective systemic insecticides is much less costly than removing trees. This would be true wherever individual ash landscape trees provide real benefits to the home or landowner.
Recent research has shown that ash trees of all sizes can be effectively and consistently protected over multiple years, even up to three years, and in areas with high densities of the beetle and at the lowest label rates.1 The current two EPA-labeled stem injection products used by arborists that are certified pesticide applicators within their respective states are Tree-age or ArborMectin. Both of these products contain Emamectin benzoate as the active ingredient.
EAB larvae feed under the bark in “tunnels” they make called galleries. These galleries sever wood vessels, in the phloem and xylem, which affects the transport of nutrients and water within the tree. The beetle galleries under the bark are serpentine, or snakelike, in form and are vertically oriented. This is why it takes a few seasons for the beetle to kill the individual tree. If these inner galleries were orientated horizontally, like other species of insect borers, then the ash trees would die in just one season
It is also these interior wood vessels under the bark in the sapwood that carry the systemic insecticide throughout the upper live crown of the tree, preventing the beetle larvae from becoming established.
The signs of infestation are crown dieback and the branch and stem woodpecker holes. The larvae under the bark are a good food source for our native woodpeckers and the woodpeckers have no trouble finding all of them. As the woodpeckers attack the infested parts of the tree, they knock the dark outer bark off and reveal the lighter bark underneath. This is called “blonding,” and is another sign of advanced borer infestation.
Emerald ash borer adults emerge in the spring from infested trees feeding on the foliage of live ash trees, then mate and lay eggs for re-infestation of the same tree or new ash trees. Adult emergence in the spring coincides with the white flowering of locust trees in bloom.
The infestation in my region, Virginia, is well advanced. At this stage, the background populations of the EAB have collapsed. This happens at about 50 percent mortality of the infested trees within a particular region. Now a lower background population of the EAB will survive and reproduce on young ash trees in sizes down to one-half inch diameter. These surviving EAB populations will gradually rebuild in individual older unaffected trees (“escapes”) as well as treated trees that have reduced protection over time. Despite the very significant losses of the ash tree in North America, there is always hope; and similar to American chestnut and American elm to name a few, genetic resistance has been observed within the native ash trees. In a long-term study (15 to 20 years) on EAB infestations in Ohio forests, U.S. Forest Service researchers have discovered a 1% survival rate. So there is some genetic resistance in the ash trees and the potential for developing future ash trees resistant to the EAB is out there.
Recent treatments in Virginia
While there may be other examples of public cost-share programs out there, recently in Virginia, the Virginia Department of Forestry decided it will pay a portion of the cost, 50%, in a cost-share program for the treatment of ash trees threatened by the ash borer. Trees must be at least 12 inches in diameter and retain at least 70 percent live crowns. A full description of what the department will cover can be found at
In practice and going by the labels, EAB treatments are intended to be preventative. They are meant to protect individual ash trees from infestation before they begin. The labels state that effective injection treatments are favored by a full canopy with leaves and a healthy vascular system, and this is generally true. It’s been rare nationally that arborists would attempt to treat infested ash trees with advanced level of crown dieback.
In Rockbridge County, Va., from May to July 2018, we treated 11 infested white ash trees that met the minimum of 70% live crowns, ranging from 10% to 30% crown dieback. This year all of these infested trees not only survived but retained the percent live crowns present at the time of treatment in 2018. These treated white ash are now rebuilding their crowns, and thus demonstrate that an opportunity to save partially infested trees exists. These kinds of results would not be isolated to Rockbridge County or other parts of Virginia or other states but would be the same where arborists are effectively treating similar ash trees by these methods.
In the shorter term or over the next few years, property damage and public safety will be an issue as dead ash trees will shed large dead branches and parts of whole stems. Dead ash trees get brittle and will fail quickly. Given the level of ash mortality obvious now in my region, within the next two to four years the percent of dead ash tree failures will reach 80 to 100%. Felling dead trees can be dangerous. There is no reliable internal hinge wood in these dead trees, thus gravity takes over and your tree can land anywhere, so be careful.
Trees have value and provide real benefits. An interesting website is www.treebenefits.com. To use this site, you will need to know the name of your tree species and its size, or its diameter in inches at about 4.5 feet above ground. At this site, you can see the value of the benefits your tree provides. Trees are like people in that they will benefit from long-term health care to extend their useful lives.
1 Herms DA, McCullough DG, Clifford CS, Smitley DR, Miller FD, Cranshaw W. 2019. Insecticide options for protecting ash trees from emerald ash borer. North Central IPM Center Bulletin. 3rd Edition. 16 pp.
Ed Hayes is a forest pathologist, entomologist and Certified Arborist. He lives in Rockbridge Baths, Virginia. This story was excerpted in part from an article written by Hayes for The News-Gazette of Lexington, Virginia.