Oobius Agrili
As emerald ash borers chew their way through America's woodlands, foresters are hanging their hopes on two tiny wasps. Under a millimeter in length, Oobius agrili lays its eggs in the ash borer eggs, as shown. Photo by Houping Liu
Tetrastichus Planipennisi
The elegant Tetrastichus planipennisi parasitizes the borer's larvae. Photo by David Cappaert
Andrew Storer
Andrew Storer coaxes a swarm of Tetrastichus planipennisi onto an ash tree infested with emerald ash borers. If all goes well, they will throttle the beetle's population explosion and, ultimately, help save America's ash trees. Photo by Mariusz Nowak
“The analysis indicated that taking no action to suppress the emerald ash borer could potentially put the nation's ash tree resources and their associated habitats at risk.”

A Natural Solution

by Jennifer Donovan

Beetle-eating wasps


They call it fighting fire with fire. Or in this case,fighting bugs with bugs.

The emerald ash borer, an invasive, iridescent green beetle with a voracious appetite for ash trees, was found in the eastern end of the Upper Peninsula of Michigan in 2007. In 2008 a population over 200 miles to the northwest was identified in an abandoned cemetery in Laurium, about ten miles north of Michigan Tech on the Keweenaw Peninsula.

Adult emerald ash borers lay their eggs on ash trees, and the larvae tunnel beneath the bark. There, they eat the living part of the tree, known as the phloem, through which nutrients flow from the roots to the leaves. Ultimately, they kill the tree.

Now Michigan Tech's Andrew Storer, a forest insect ecologist, is heading Houghton County's battle with the bugs. Part of a $2.2 million, four-county project funded by the American Recovery and Reinvestment Act (ARRA), the emerald ash borer management program is called SLAM (SLow Ash Mortality). Michigan Tech received $651,000 from the federal grant.

Two of the more unusual weapons SLAM is using are wasps—Oobius agrili and Tetrastichus planipennisi specifically. The wasps, which originate in China, are bred in this country by the US Department of Agriculture Animal and Plant Health Inspection Service (APHIS) and a Forest Service lab in southeastern Michigan.

They are tiny, stingless insects whose favorite meal is emerald ash borers. In fact, their taste is even more refined than that. Oobius agrili favor emerald ash borer eggs, while Tetrastichus planipennisi prefer the ash borer larvae.

One muggy August day, Storer and the SLAM team are tromping through a wooded area near Calumet. They stop at a tree where a purple trap has caught emerald ash borers infesting the tree.

Stripped bark reveals a heavy infestation of ash borers, the eggs already hatched into larvae. Storer lifts a jar of Tetrastichus planipennisi—the larvae-eating wasps—from a battered cooler.

"This is the technical part," he quips. "We take the lid off."

What flies out is nothing like a swarm of wasps. In fact, the tiny parasites are almost too small to see. Storer taps the open container against the bark. "The idea is to get them to go on the tree," he explains.

The female wasps can sense the ash borer larvae through the bark, so they lay eggs where they know their offspring will find food.

The team moves on to a tree where ash borer eggs that have not yet hatched have been identified.

There, they open a flask of Oobius agrili and urge the miniscule wasps toward the tree.

There are those who raise questions about releasing one exotic species to control another. Isn't there a risk that the wasps in turn will become the next problem?

"It's true that once we release the wasps, the genie is out of the bottle," says Storer. "But there has been extensive testing of these parasites against closely related and unrelated species, and the results show that the wasps are specific to the emerald ash borer. That's essentially all they eat, so they don't pose a threat to native insects."

Emerald ash borer–munching wasps have been released successfully in lower Michigan and Minnesota. They have become established at a number of sites, and the scientists hope that they will help to control the beetle populations there with no side effects.

APHIS conducted an environmental assessment comparing two options: taking no action against the emerald ash borer and releasing the parasitic wasps. "The analysis indicated that taking no action to suppress the emerald ash borer could potentially put the nation's ash tree resources and their associated habitats at risk," said Leah Bauer, a research entomologist with APHIS in East Lansing. "Host-specificity testing in China and in laboratory studies in the US indicated that the wasps are not expected to attack other insect species besides the emerald ash borer."

Storer adds, "You have to weigh the risks against the benefits. The benefit is that reducing the density of emerald ash borer populations could slow the rate at which ash trees die. That could save our ash tree population, which is a valuable resource."

"Given the devastating impact of emerald ash borers and the apparent high specificity of the wasps," Storer says, "this is considered to be an acceptable risk."

Storer calls the wasps "one tool in a pest management toolbox, not a silver bullet. I believe that a successful management strategy will involve multiple tools."

The SLAM project is also using other approaches, including girdling groups of trees to act as "sinks" that draw the ash borers away from other trees in the vicinity; injecting trees with a powerful insecticide; and removing severely infested trees that are not suitable for other kinds of treatment. To track the pests, Storer's team has hung purple prism traps in ash trees and in girdled trap trees in a grid around known infested areas.

Girdling involves cutting a shallow ring of bark off the tree, stressing it, which seems to attract the opportunistic ash borers. The insecticide, called TREE-age, is costly and impractical on large scales such as in natural forests. Removing infested trees works, but one of the goals is to save the ash trees, not chop them down.

Also at work in Delta, Schoolcraft, and Mackinac Counties, the SLAM project involves collaboration with Michigan State University, the Michigan Department of Agriculture, the US Forest Service, and the state Department of Natural Resources and Environment. The overall project has created thirty-eight jobs, including ten at Michigan Tech.

"This work is labor-intensive," says Storer. "We need people out in the field, setting and checking traps, treating infested trees, collecting data, and assessing results. We couldn't have done it without the ARRA funding."

Once the SLAM project is completed, the data it generates will be used to develop models for effective management of the emerald ash borer. So far, the beetle has been identifed in thirteen states and Canada.