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 Photo: David cappaert, michigan state university, bugwood.orgWhen emerald ash borer was discovered in the United States in southeast Michigan in 2002, it had likely been there, eating away at the trees, for a decade. One of the defining characteristics of the invasive insect is that by the time it’s identified it might be too late to save the tree or the spread of infestation to surrounding trees. As of November 2010, EAB had been located in 15 states and two Canadian provinces.
Lawn & Landscape spoke to Deborah G. McCullough, a professor in the departments of entomology and forestry at Michigan State University, about the Asian native killing North American ash trees.
EAB’s lifecycle
The adult beetles are active for much of the summer nibbling on ash tree leaves.
“After they have fed for about three weeks, the females will start laying eggs,” McCullough says. “They can deposit one egg or they might have two or three eggs in a row that they lay on the tree. The eggs hatch in a couple of weeks; the larvae go right to the bark and start feeding on the phloem and cambium under the bark. It’s the larval feeding that causes the damage. Usually we see the very first larvae starting to feed about the middle of July.”
The larvae chew chambers into the sap wall or even the outer bark. The larvae stay through winter and then pupate the next spring, emerging in May or early June.
Identifying EAB
The insect is attracted to certain types of ash trees in North America.
“Green ash appears to be one of their most favorite hosts,” McCullough says. “White ash is the other common landscape species and it’s less preferred, but they’ll certainly move to white ash, especially as the green ash trees start to die.”
Even in the trees more susceptible to housing EAB, it’s hard to identify a new infestation because the tree won’t show symptoms for years. By the time an infestation is found in a new area, EAB might have lived in that tree for four to eight years.
“If you don’t peel all the bark off the tree, you wouldn’t know that anything was going on,” McCullough says of an early invasion. “It’s only when the population of ash borers on the tree builds up that you start to see the canopy become thin, branches will start to die, you may start to see bark cracks. Woodpeckers are a really good early warning sign. The woodpeckers will feed on the large larvae usually in the winter time and the early part of the spring.”
The problem is if one tree is infested, many around it could be, too. The beetle can fly, for up to half a mile.Ash trees within an already quarantined country are probably at risk.
Controlling EAB
Today, there are a dozen or so products for treating an EAB infested tree, but studies show the best outcomes are produced when insecticides are used on still relatively healthy trees. If a tree has already lost more than 50 percent of its canopy, it’s probably too late to save.
“There are actually insecticide products that you can inject into the tree or you can apply them to the soil and let the tree take them up,” McCullough says. “There is one product you can apply by spraying the lower three or four feet of the trunk.”
Most products need to be applied once a year. But an insecticide containing the active ingredient emamectin benzoate is controlling EAB 99 percent of the time for two years.
“Other products are getting better,” McCullough says. “They’re improving in terms of figuring out how much has to be applied per year. We’re learning that spring is the best time to apply insecticides. Usually bigger trees can be harder to protect than smaller trees. It’s not really simple, but there are certainly effective products now.”
The author is associate editor of Lawn & Landscape. She can be reached at clawell@gie.net.
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