From the Chicago Tribune: China's ash trees may hold salvation for U.S. cousins

By Gerry Smith and Tara Malone | Tribune reporters


Glenn Weiler sounded like he was delivering a eulogy, speaking in the past tense used by arborists who now deem the ash tree's demise inevitable, its enemy unstoppable.

"Ash was so nice because it handled the tough soils," said Weiler, who stopped growing the species at his Zion tree nursery two years ago. "It's becoming a thing of the past, I think."

But some researchers still see a future for the ash tree, and they believe it lies somewhere in the Chinese countryside. Six years after the emerald ash borer was first spotted in the Midwest, researchers are turning their attention across the globe, to the region where the tiny, metallic-green beetle originated.

One ongoing effort—reminiscent of past efforts to rescue the American elm—is to identify Asian ash trees that have developed natural defenses against the pest, then develop a hybrid species for the American landscape that will withstand the devastating beetle.

"It's reflective of the idea that the emerald ash borer is here to stay and it's going to take a long-term solution," said Dan Herms, an associate professor of entomology at Ohio State University who is involved in the project.

Although still years from development, the pursuit of borer-resistant ash trees has become part of the evolving strategy for combating the ash borer, which has killed about 25 million trees across the Midwest and Canada.

Hope in Far East
Officials in Michigan found some hope in the Far East last fall, when they began unleashing tiny, stingless Chinese wasps that kill the pest's eggs and larvae.

And this spring, Illinois officials are rigging traps designed to snag the flying beetles after sweeping inspections and a state quarantine have helped contain their spread.

Also, researchers have been encouraged by the early success of a new pesticide that may become widely available this year. Researchers have found the pesticide, which contains the chemical emamectin benzoate, kills more than 99 percent of ash borer larvae in treated trees and virtually all adult ash borers that feed on them.

Meanwhile, evidence of the pest continues to emerge in ash trees across the Chicago area since first appearing two years ago in a Kane County subdivision. Last month, work crews with the Kane County Forest Preserve District cut down and burned nearly 100 trees infested with the ash borer.

Already, the telltale white larvae of the beetle have been spotted in ash trees everywhere from south suburban Hazel Crest to the tree-lined North Shore.

With few proven strategies for eradicating the ash borer, researchers across the Midwest are trying to find out how Asian ash trees manage to live with their enemy.

Later this year, Kris Bachtell plans to collect female ash tree seeds in China's Gansu province and join other researchers who are trying to identify, then replicate, the DNA in Asian ash trees with natural defenses against the ash borer.

"The best chance to find resistance is to go where the pests and plants co-evolved," said Bachtell, director of collections and grounds at the Morton Arboretum in Lisle.

The approach, which could take several years to yield results, was once used to revive another species after a similar scourge.

Learning from elms
During the 1950s, the American elm was nearly done in by Dutch elm disease, a plague that felled hundreds of trees in Chicago's Grant Park and nearly every elm on the campus of the University of Illinois at Urbana-Champaign.

But then researchers, led by the Morton Arboretum's George Ware, began experimenting with cross-breeding Asian elms in the late 1970s. Since then, the arboretum has introduced thousands of disease-resistant elms across the country.

Ware, a dendrologist at the arboretum, remains hopeful a similar effort could lead to a resurgence of the ash tree in the American landscape.

"It'd be a glorious thing if someone said, 'This is a species of ash that the ash borer simply does not like,' " he said. "The search is intense now, so I think there will be some kind of breakthrough."

Above all, officials said they've learned one important lesson from the outbreaks of both Dutch elm disease and the emerald ash borer: diversification.

Elms once represented about 40 percent of the tree population in metropolitan Chicago, but they were replaced after the Dutch elm outbreak by the ash tree, which makes up about one-fifth of Chicago's trees.

Now, as communities plant new trees where ash once stood, they're drawing from different species to guard against another pest destroying entire blocks of trees. Many suburbs cap the use of any single species at 5 percent of their total tree stock.

"When elm trees were removed and they replaced them with ash trees, there was a sense of urgency. They just wanted to replace them," said Jennifer Statz, a forester in Wilmette overseeing the removal and replacement of 2,855 ash trees on public lands during the next four years. "Now we've learned that's really not the wisest management choice."

Two years ago, when the emerald ash borer was discovered less than a mile from his Kane County tree nursery, Matt Zerby started bulldozing hundreds of ash trees. This month, Zerby plans to replace some of them with hybrid elm trees, hoping someday he'll be able to plant ash trees again.

"Who knows, maybe 40 years from now we'll have an emerald ash-borer resistant ash tree," Zerby said.