Emerald ash borer continues to decimate trees 10 years after it was discovered in Wisconsin

TOWN OF GRAFTON – All Jim Tonelli has to do to see the ravages of emerald ash borer is walk out his back door. 

Since last winter, he has been forced to cut 70 ash trees on his 1.25-acre Ozaukee County property. More are dying and will have to be sawed down. 

“It’s sad to see them go,” Tonelli said.

But as the march of the invasive pest has moved across the state and sickly and dead trees began enveloping his subdivision, he and his wife, Kathy, both master gardeners, were prepared for it.

“It’s nature,” Jim Tonelli said, bullfrogs croaking in a pond on his property. “We've got to roll with nature and invasive species.” 

Ten years ago this summer, emerald ash borer first turned up in Wisconsin near the Milwaukee River on the border of Ozaukee and Washington counties. Since then, the Department of Natural Resources estimates that dying and dead ash trees now cover several thousand square miles of the state.

Experts believe the Asian beetle probably arrived in the state about 2004, and went undetected. It got here no doubt from a load of firewood or other wood products from another infected area.

It was first discovered in North America near Detroit in 2002. Illinois officials reported a find in 2006. As of May, it had been found in 33 states.

'Day of reckoning'

“You can drive down Highway 41 from Oshkosh to the Illinois line and never lose sight of an infected tree,” said Bill McNee, a forest health specialist with the DNR.

In 2008, “emerald ash borer was a theoretical thing,” said August Hoppe, a third-generation arborist and president of Hoppe Tree Service.

“Today, it’s a day of reckoning. Everywhere you look, there are dead and dying ash trees,” he said.

Hoppe estimates about 30% of the tree removal component of his business involves ash trees. Five years ago, it is was 5% or 10%.

The destruction has been most acute in southeastern Wisconsin.

Each year, the insect’s impact spreads a little more, occasionally leapfrogging long distances with the help of humans moving firewood and logs. Emerald ash borer is now found in counties bordering the Mississippi River and is as far north as Douglas County, in northwestern Wisconsin.   

Forty-eight of Wisconsin’s 72 counties have infested trees, and on March 21, state agriculture officials placed the entire state under quarantine.  

Despite its impact, emerald ash borer can almost seem invisible where other species are abundant; the infestation has not yet taken hold; or where ash have been treated with a potent larvae-killing cocktail.

Since the discovery, a big plus is that scientists have found effective insecticides, particularly in the early stages of infestation.

In other research, they are also experimenting with using parasitic wasps that feast on the larvae and eggs of ash borer, but present no threat to humans. Since 2011, tens of thousands of the wasps have been released, including in many areas of metropolitan Milwaukee.   

But in the short term, the best line of attack has been the use of chemicals that destroy the larvae of the beetle. At that stage, larvae inhibit the flow of nutrients beneath the bark, killing the tree.

A checkerboard of stumps

Tonelli’s yard slopes down to an old farm pond that he shares with several neighbors. Ash stumps sprout like mushrooms everywhere.

He paid an arborist to treat eight of the trees by injecting a pesticide between 2011 and 2016. It didn’t work. Only one of those trees is still standing.

“I can’t tell you why it has made it,” he said. “But it probably doesn’t have long.”

In its most recent guidance, University of Wisconsin Extension entomologists say that while it can be difficult to assess the extent of infestations at early stages, insecticides can help and are most effective when infested trees have less than a 50% thinning of the canopy.

Insecticides are either injected into a tree by a professional, or property owners can pour a solution over the base of trees.

Early intervention

The City of Milwaukee has been injecting ash trees on streets every other year since 2009 with an insecticide, emamectin benzoate, at a cost of $1 million a year. 

Milwaukee was fortunate because it got a head start. Treatments started three years before emerald ash borer was discovered in the city. 

Officials say that the cost of removing 28,000 ash and planting new trees, as many communities are doing, would be greater. Also factored in are aesthetic benefits, less air pollution and storm water runoff (trees consume a lot of water) and energy savings provided by shade. 

“I think it’s made a world of difference,” said David Sivyer, forestry services manager for the city, which hosted a group of scientists from the United Kingdom on June 11 to see the results firsthand.

Rebecca Lane, city forester in Oak Creek, agreed. 

“For them, treatment has been a great option,” said Lane, who has removed 1,000 street trees since emerald ash borer was discovered in Oak Creek in 2009.

Oak Creek continues to get frequent calls from property owners wanting to know what to do. “At this point if you have not been treating, and live in Oak Creek, it’s very likely too late,” she said. 

On Leroy St., near Wilson Park on Milwaukee's south side, a tall green canopy arches over the road. Ash trees are flourishing on the boulevard because of the insecticide injections. 

The few ash that reside in yards on the street have lost most of their leaves, or are nothing more than clusters of dead branches. 

“It’s going to have to come down,” a homeowner said tersely about a dead ash in his backyard, declining to say anything more. 

The man’s neighbor, Paul Nourse, lives next to St. Roman Catholic Church, where at least a half-dozen ash trees are dying. They, too, will have to be removed, according to the city. 

“All the city trees are good.” Nourse said. “But the ash trees that have not been treated? Eh. 

“I like the trees. They provide a lot of shade. It’s nice in the summer.”

Anna Groves of the Journal Sentinel staff contributed to this report.