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Scientists hope wasps can contain emerald ash borer

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Stingless wasp
A closeup of the stingless wasps the Minnesota Department of Agriculture is releasing in southeastern Minnesota in an effort to control the invasive emerald ash borer.
MPR Photo/Nate Howard

Several thousand new insects burrowed into trees in southeastern Minnesota on Wednesday as scientists released stingless wasps along the Mississippi River.

Their target: the emerald ash borer, an invasive pest that feeds on ash trees and kills them. Borers have been found from the Twin Cities metropolitan area to Houston County in the far southeastern tip of Minnesota. 

State and federal officials delivered the wasps to a forested island just east of Houston County, a control site for the first release. The wasps are a natural predator of the emerald ash borer.

Monika Chandler, a project coordinator for the Minnesota Department of Agriculture, stepped ashore holding a large, plastic cooler in her hands. 

"This is the most fun part of it all -- the actual release," she said.

Stingless wasps
Minnesota Department of Agriculture officials release stingless wasps as a biological control for emerald ash borer in rural Houston County. The release site, Blackhawk Park, is located 30 miles south of La Crosse, Wis.
MPR Photo/Nate Howard

Chandler walked toward a towering ash tree marked with bright orange tape signaling it's infested. After placing the cooler on the ground, she peeled off the tape that sealed it shut.

Inside, were dozens of sealed plastic cups containing several hundred tiny wasps -- each about the size of a freckle.

At the tree, Chandler gently tapped a cup until all the tiny wasps disappeared into the bark.

"Our hope is that if we release biological control agents now, we can reduce the damage of an outbreak level of emerald ash borer," she said.

By inserting wasps into the emerald ash borer population, she said, officials hope to control their spread, not eradicate them.

The reason emerald ash borer is so worrisome, is the sheer number of ash trees around -- in Minnesota, about nine million statewide. The U.S. Department of Agriculture developed the three species of wasps at a facility in Michigan. Researchers believe they prey on the larvae and eggs of the emerald ash borer.

Release wasps
The Minnesota Department of Agriculture (MDA) invites media to view the state's first release of stingless wasps as a biological control for emerald ash borer. MDA will release two species of stingless wasps that were tested and approved by USDA for biocontrol to limit the ash borer population.
MPR Photo/Nate Howard

Minnesota officials have been surveying areas for infested ash trees so they can remove or treat those affected by the insect. By releasing the wasps, they aim to introduce a natural control that over time will move with emerald ash borer as it spreads.

"The wasps reproduce faster than emerald ash borer," Chandler said. "They're three to four generations per year of the wasps. And so we're hoping that with this numbers game, that we can catch up to the emerald ash borer."

Researchers believe biological control is the best option for cost-effective, long-term emerald ash borer population reduction. 

Clyde Male, an assistant refuge manager for the U.S. Fish and Wildlife Service, said although a variety of insecticides are available to treat individual trees, using them is not an effective way to control the pest on a large scale.

"We're not spraying insecticide on them," Male said. "We're not using a mechanical means. We're using an insect to control an insect."

Male said researchers have thoroughly tested the wasps and determined they do not harm humans, native plants or other animal species.

"It's very host specific," he said. "The battle will take place between those two insects pretty much undetected until we come back in several years."

On those visits, researchers will inspect the sites to see that battle's effect on the emerald ash borer. They also hope to learn if the wasps can survive the cold Minnesota winters.

Male said they will monitor the site for five years to track which of the wasp species succeed in controlling the ash borer. Two of the species kill the pests by inserting their eggs into emerald ash borer larvae. The developing wasps feed on and eventually kill the larvae.

The metallic green emerald ash borer have killed tens of millions of trees in other Midwestern states.

The release was part of an effort by the U.S. and Minnesota departments of agriculture and the states of Minnesota, Iowa and Wisconsin.