Beating the Beetle
Andrew Storer is hot on the trail of one of the baddest bugs in town. The emerald ash borer is responsible for the deaths of millions of ash trees, primarily in urban areas and parks in southeastern Michigan, and it is munching its way north, south, west, and even east, into Canada.
"The point of entry was somewhere near Detroit, and it was already pretty widespread before it was identified," says Storer, an entomologist and associate professor in the School of Forest Resources and Environmental Science. By the time people noticed the ash trees dying, in 2002, the emerald ash borer had already been in the US five years or more. It eats trees in the Fraxinus genus, including green, black, and white ash. (The familiar mountain ash isn't a true ash tree and is not attacked by the emerald ash borer.
The delay in spotting the problem is no surprise. "So often, new exotic species have already colonized a large geographic area before we find them," Storer notes. Now, however, his team has been finding plenty of them in trap trees they've set throughout Michigan over the past three years as part of his Emerald Ash Borer Detection Project. Selected ash trees are girdled-a ring of bark is removed from the trunk-and a sticky band is wrapped around the tree to catch visiting bugs. It seems stressed trees are more likely to attract the opportunistic pests. "We need to know where the emerald ash borer is, if we are going to try to eliminate localized populations or prioritize forest management that reduces its food source," Storer said.
The adult beetles lay their eggs on ash trees, and the larvae tunnel underneath the bark. There, they eat up the phloem, the living portion of the tree through which nutrients flow between the leaves and the roots. After a number of years of infestation, the tree dies.
One method of controlling emerald ash borers is to cut down all ash trees within half a mile from an infested tree. That might work quite well but for one unfortunate circumstance: emerald ash borers have been hitchhiking through Michigan, courtesy of vacationers who inadvertently give them a lift in bundles of firewood. Once that mechanism was identified, the Michigan Department of Agriculture has worked to prohibit long-distance transport of firewood, and the Michigan Department of Natural Resources has banned ash firewood from state parks and campgrounds.
"Our message is, don't move firewood," Storer says. The rules can slow the spread of the ash borer, but they probably won't stop it in its tracks. When seven thousand campfire woodpiles were examined in state, federal, and other campgrounds throughout Michigan, about 4 percent contained ash, enough to pose a threat.
Given that reality, Storer thinks that the ash borer will leapfrog slowly throughout midwestern forests. The only infested tree the Emerald Ash Borer Detection Project has found so far in the Upper Peninsula was in Brimley State Park, many miles from the next-nearest infested tree. The bugs were almost certainly carried in by an unsuspecting camper from Lower Michigan; it's virtually impossible that they would have crossed the Straits of Mackinac on their own.
The response from forest managers was swift and typical for a newly discovered isolated population: they cut down all the ash trees within half a mile of the sick tree. Given that American ash species are so vulnerable to the emerald ash borer, some experts are investigating even more drastic measures, namely biological controls.
Scientists are studying natural enemies of the emerald ash borer, such as parasitic wasps found in its native Asia. The ultimate goal would be to introduce them into the US, where they would prey on the emerald ash borers and bring their numbers under control.
Storer has not leapt upon the biological-control bandwagon, which he sees as potentially laden with unintended consequences
"Rarely does everything work out as planned," he notes. "There are inherent risks. What if the control goes after native species that are related to the emerald ash borer?
Numerous relatives of the emerald ash borer are found in Michigan's forests, including some about which very little is known. "What if an introduced parasite were to go after a common native relative such as the bronze birch borer," asks Storer, "and woodpeckers lose them as a food source? We have to be very careful how we respond to this problem."
In Asia, the emerald ash borer is not the pest it is in North America, and its enemies have been hard to identify. And while Asian ash tree species do host the beetle, they are not usually killed by it.
Storer thinks that Asian ash species have developed a natural resistance to the emerald ash borer. North American ash species may be able to develop a similar level of tolerance, if emerald ash borer populations can be controlled. He and fellow Michigan Tech researchers Associate Professor Linda Nagel and graduate student Tara Eberhart have developed a model for forest managers that could cut populations of the beetle by reducing their food source, namely, ash trees.
The Ash Phloem Reduction Model is founded on the premise that if ash trees are few and far between, then emerald ash borer populations will not get so large and overwhelm the trees. Based on the amount of ash tree surface area in a stand of timber, the model can recommend how many trees to cut and of what size.
"Successful forest management of insect pests often uses some type of silvicultural tool like this," says Storer. Foresters wanting to harvest ash would manage for a few large trees. "On the other hand, if they want genetic diversity, they can keep the small trees." That tactic could help native ash trees express resistance to the emerald ash borer.
Over time, he says, the emerald ash borer and American ash trees could reach an accommodation. "There is a possibility that the insect would behave like an endemic species and come into balance with our natural system," Storer says. Native birds are already feeding on them: "Woodpeckers are better at finding emerald ash borers than we are," Storer notes.
"We need to minimize the impact of the emerald ash borer and minimize any negative effects of control efforts," he says. The risk of introducing another new species to the ecosystem is huge, so resource managers should fight the temptation to stampede toward biological controls-not always easy in a political realm where decision makers are being pressured to "do something."
"There are a lot of people who say ash will disappear," Storer says. "But I'm not convinced that we'll lose an entire genus of trees. We need to be very, very careful or we could lose much more than that."