Threat to American’s Ash Trees
By John Weems, WVU Core Arboretum Specialist
Reprinted from WV Nursery and Landscape Association Bulletin
There were some interesting presentations at the recent Mid-Atlantic State Cooperators Forest Health Meeting in Morgantown. (The meeting is an annual affair that rotates among the six states of the region.) Session topics included hemlock wooly adelgid, Asian longhorned beetle, sudden oak death, beech bark disease, gypsy moth, and other threats to forest health. There was much discussion of drought.
However, the topic that may have scared me the most was the emerald ash borer, an exotic beetle that could devastate America’s ash trees. The emerald ash borer, Agrilus planipennis, is apparently a minor forest pest in its native Asia. It was identified in this country for the first time very recently, in July 2002, in southeastern Michigan. It has many close relatives in Asia, but few in this country, so it has a niche all to itself here. It seems to reproduce and multiply with abandon.
Scientists identified the beetle when they examined dead and dying trees, mostly along suburban streets and in lawns. Michigan already had a rapid response contingency plan (set up in anticipation of the Asian longhorned beetle) in place, and officials have done a remarkable job of getting on top of the situation early, but there is little prospect of preventing the rapid spread of the emerald ash borer. Both male and female adults can fly.
Borers in general are difficult to control because they spend most of their lives, and do most or all of their feeding, inside trees. There are no existing control protocols for emerald ash borers. Woodpeckers eat the larvae, but birds can’t be expected to keep up with the sudden expansion of a new food source. Officials in Michigan have identified the “new” pest in 5 counties, and established a quarantine of those and surrounding counties. Infested trees have been cut down. Transportation of ash firewood, logs, lumber, bark, and brush is illegal within the quarantine area, or from that area to other regions. At best, though, this quarantine is unlikely to do more than reduce human-aided spread of the beetles, which seem well equipped to spread on their own.
Each emerald-colored female lays 65 to 90 eggs, individually, in bark crevices. (The adults are shorter than the diameter of a dime.) Hatching takes place in mid-summer. Larvae eat through the bark, then meander through the cambial region of the phloem (inner bark) and sapwood, eating as they go. Infestations result in the girdling of the trees. Stems usually die within 2 or 3 years. Heavy sprouting at the base of the trunk often follows, and helps distinguish emerald ash borer infestations from other ash problems. Larvae overwinter inside the bark, then pupate there in the spring. Adults emerge head first through distinctive, D-shaped exit holes. Adults may fly into the canopy to mate, eat foliage, or lay eggs, but most of their flying is within two meters of the ground. White ash and green ash, America’s major species, are known hosts. The borer will probably infest our black and blue ashes as well. In Asia, it is also found on some species of elm and walnut.
The arrival of this beetle is bad news. Forest Service scientists and others are scrambling to study and deal with it, but early work has revealed nothing likely to control emerald ash borers. Most tree mortality so far has been in suburban areas, but there is no reason to believe forest ashes have any resistance. The borer has killed stressed trees and vigorous, irrigated and fertilized trees. Climate can limit the range of an insect, but in Asia the emerald ash borer lives from Mongolia and Russian Asia, with brutally cold and dry winters, to Taiwan, with sweltering summers. The conference’s presenter of information about the emerald ash borer suggested that the beetle could do for ashes what chestnut blight did for American chestnut--reduce them from important forest trees to minor forest shrubs.
I love ash trees. I hope I never see an emerald ash borer at the Arboretum. I’ll be looking, though, for the signs: bright green beetles crawling on ashes, trees with declining tops but heavy sprouting from the bases, and D-shaped holes through the bark. If I suspect the presence of emerald ash borers, I’ll collect insects for prompt identification by entomologists.
Before assuming the end is near for ash trees, let’s remind ourselves how little is known about the emerald ash borer. Bacteria, viruses, or other as-yet-undetected pathogens may help control it. Insects can spread fast, but spores and microorganisms can spread even faster. Dense populations of beetles and larvae amount to a golden opportunity for potential pathogens. Drs. David L. Roberts and David Smitley of the Michigan State University Extension Service give the following control recommendations for the emerald ash borer.
Maintenance of good health and vigor of the ash trees is paramount. Borers are attracted to stressed trees and trees can become stressed from either lack or excess levels of mulch, water and nutrients. Irrigation and fertilization to keep the trees at optimum levels of hydration and nutrients will help increase their resistance to attack by borers. Choose only the best sites for planting young ash trees, in terms of soil conditions, especially drainage.
Pesticides that have been effective...[for homeowners include] Bayer Advanced Lawn and Garden Multi-Insect Killer (cyfluthrin) to trees in June and July. Again, thorough coverage of foliage, branches and the trunks of the ashes is essential. Homeowners can also use Bayer Advance Tree and Shrub Control (1.47% imidacloprid) as a drench evenly over the entire root area at the rate of 1 oz. of product per inch of tree circumference. Read the label!
Note: As always, use extreme caution when handling pesticides. We recommend consulting your County Extension Agricultural Agent if emerald ash borer presence is suspected.