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College of Food, Agricultural, and Environmental Sciences


Exotic Tree Pest Generating Concern in Forestry/Nursery Industries

July 18, 2002

WOOSTER, Ohio - A new tree pest, discovered last month in Michigan, could pose a threat to Ohio's nursery industry and native forests if it ever crossed the state border.

The beetle, known as the emerald ash borer, has been identified as Agrilus planipennis, and is generating concern among natural resource professionals, entomologists and forest service officials because of the insect's destructive impact on white and green ash trees. Most wood boring insects only target weakened trees, but observations indicate that this insect targets healthy trees, as well.

"This is an alarming find because it suggests that rather than just killing stressed trees in landscapes, the emerald ash borer might be able to attack and kill ash trees in native forests," said Dan Herms, an Ohio State University entomologist with the Ohio Agricultural Research and Development Center. "That could have quite a devastating ecological impact. Ash trees are common both in landscapes and natural forests throughout Ohio and the eastern United States, for that matter." The insect has yet to be found in Ohio.

The emerald ash borer, discovered in southeast Michigan this year following reports over the past two to three years of declining and dying ash trees, is a native of east Asia, including China, Japan, Korea and eastern Russia. U.S. Department of Agriculture researchers speculate the insect was brought to the United States via infested wooden shipping crates and has been established in Detroit and surrounding suburbs for at least five years.

Little is known regarding the insect's biology and to what extent it attacks and damages ash trees, but Herms speculates the insect behaves similarly to other insects in the genus Agrilus, such as the bronze birch borer, a key pest of white-barked birch trees.

Research on the bronze birch borer initiated at OARDC over 20 years ago by Dave Nielsen found that native North American birch trees are highly resistant to the insect, which is also a North American native. European birch trees, on the other hand, are extremely susceptible.

"We analyzed 200 individuals of various species of birch trees and every last one of the European and Asian species was killed. While over 75 percent of the native trees survived despite the fact that they endured huge bronze birch borer outbreaks without the benefit of protective pesticides, irrigation or fertilization," said Herms. Oftentimes, trees that lack an evolutionary history with a particular insect or disease are much more vulnerable than are trees that have a long association with the pest, said Herms.

"With the ash borer, we've got an exotic insect attacking native trees and it may be that the ash trees don't have a natural defense to protect themselves," he said.

The emerald ash borer, like the bronze birch borer, often infests a tree for several years, causing the tree to gradually decline and eventually die. "A heavy infestation can kill a tree in one season, but a gradual decline over several years is more common," said Herms. Infestations prove fatal because the larvae feed under the bark, disrupting the tissue that carries nutrients from the roots to the canopy.

USDA researchers, in conjunction with entomologists at Michigan State University, have found that the emerald ash borer appears to exhibit a one-year life cycle that is similar to many native Agrilus species. The insect overwinters under the bark as a worm-like larva. In late spring, larvae pupate into metallic green adults that emerge from the tree at the onset of summer, leaving behind a characteristic small D-shaped emergence hole in the trunk.

Adult emerald ash borers also appear to fly to other areas when a host is not readily available, making their spread to new trees that much easier. USDA officials are currently trying to delimit the range of the pest through nursery inspections in southeastern Michigan and analyzing potential sources for spreading the insect. There are native species of ash borers, which are moths rather than beetles, but they only attack severely stressed trees. Although they can be a problem in urban forests and landscapes, they pose no threat to natural forests.

The emerald ash borer is just the latest in a growing list of exotic pests and diseases that threaten forested ecosystems in the United States, including Dutch elm disease, chestnut blight, beech bark disease, the gypsy moth, hemlock wooly adelgid and the Asian longhorn beetle.

Candace Pollock
Dan Herms