Fall 2008

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James E. Appleby/University of Illinois

1.     It’s here! Native to Asia, the emerald ash borer—a small, green beetle that bores its way underneath the bark of ash trees—was first discovered in the United States in 2002, near Detroit. Since that time, the invasive pest has spread to 10 states, killing some 25 million ash trees throughout the upper Midwest. The first positive identification of emerald ash borer in Wisconsin came this summer, when the insects were found in ash trees in Ozaukee and Washington counties, just northwest of Milwaukee.

2.     EAB has likely been here for a while. While this summer’s finding generated a lot of media attention, the number of beetles found strongly suggests that they didn’t just show up in Wisconsin. Trees can be infested for two or three years before they show any outward signs of stress, which means the beetle has probably gone undetected in Wisconsin for at least a couple of years. That’s not all bad news—it shows that the insect is fairly slow in establishing itself and spreading to nearby trees.

3.     Wisconsin has more than 765 million ash trees. And that number means we should take emerald ash borer seriously. Because this beetle is not native to Wisconsin, our trees have no known natural defense to ward off the beetles’ attacks. If emerald ash borer showed up in Madison, where almost 30 percent of the terrace trees are ash, the results could be devastating.

4.     We can fight EAB—but there are no silver bullets. Research shows that several types of pesticides—including both over-the-counter and professionally applied insecticide treatments—can be effective in controlling emerald ash borer. The best results come from pesticides that contain the compounds imidacloprid or emamectin benzoate, two neurotoxins that have been shown to kill emerald ash borer. But remember that we have only known about this bug since 2002, and we don’t yet have comprehensive data on any treatment. Nothing has been proven to work all the time, and there’s no evidence that higher-cost, more invasive techniques are any more effective than do-it-yourself ones.

5.     We humans are likely the biggest reason for the spread of EAB. On its own, an emerald ash borer typically flies less than one-half mile per year. It spreads far faster by getting a ride in infested firewood or other ash products. That means the best strategy for controlling the insects’ spread is what Wisconsin has already done—establish a quarantine zone around known infestations and block shipment of any wood from those areas. The highest area of risk lies within 10 to 12 miles of an EAB infestation. Outside of this area, I don’t think there is much reason for landowners to worry, at least for now. However, if it brings you peace of mind, you can certainly treat your high-value or specimen ash trees with insecticide.

Chris Williams is an associate professor of entomology and an insect-control specialist for UW-Extension. He is researching the effectiveness of several insecticide and application technologies in preventing the spread of emerald ash borer, and he also maintains a web site on the beetles at

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