Scientists Hope the Emerald Ash Borer Succombs to Tiny Chinese Wasps

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Dr. Houping Liu checks the eggs of Chinese wasps in an incubator. Entomologists at Michigan State University hope the wasps bring the tree-killing Emerald Ash Borer under control.
Dr. Houping Liu checks the eggs of Chinese wasps in an incubator. Entomologists at Michigan State University hope the wasps bring the tree-killing Emerald Ash Borer under control.

Tree lovers are waiting for the ‘silver bullet’ that will bring the Emerald Ash Borer under control. Investigators believe the tree-killing beetle entered the U.S. near Detroit from China, probably as a stow away in wooden shipping material. In the eight or ten years since, EAB has killed an estimated 20 million trees in Michigan – and has likely infested millions more in Ohio and other states. Now scientists are wondering if another Chinese import might finally bring EAB under control.

In a cramped laboratory at Michigan State University in East Lansing, entomologist Debbie Miller is putting fresh greenhouse grown ash tree leaves into a container housing a pair of ash borers.

“It’s just a regular drinking cup that has had a window put into it for ventilation,” Miller says. “And they’re quite happy in there.”

So happy, in fact, they’re mating.

“Twice a week we replace the foliage so that they’ll have fresh food and favorable living conditions,” Miller says.

The U.S. Forest Service is hatching thousands of ash borers in another lab down the hall. They were brought to East Lansing on the bark of logs cut from areas in Michigan infested with EAB.

“Every week we set up a certain number of beetles in order to collect the eggs for the parasitoid,” says Miller.

A parasitoid is a parasite, which in this case has a deadly attraction to the Emerald Ash Borer. EAB has almost no enemies in the U.S., though a small number are eaten by woodpeckers. But there aren’t enough woodpeckers to to eat the rapidly expanding EAB population.

But in the past few years scientists have found several types of tiny, stingerless Chinese wasps that feed on EAB larvae.

“These little wasps are not a benign parasite,” says Leah Bauer, a Forest Service entomologist who’s directing the research in East Lansing. “These actually in a way are like predators.”

“These tiny little egg parasites will lay an egg in an EAB egg and the larva of the wasp grows inside that egg and it kills it,” Bauer says. “Instead of an ash borer hatching out, a little wasp will hatch out and fly away and look for more eggs. And they’re so tiny they can crawl all over the tree bark and in between the layers of bark and find the stages of Emerald Ash Borer they need to attack,” she says.

“Bauer says the wasp finds an ash trees the same way the borer does, searching for the tree’s aroma. She says the wasp can even hear borers eating beneath the bark. Because it’s so tiny, the wasp can move in and out of the crevices where the borer has laid its larvae.

This type of biological control has been used for decades and it’s not without risk. But Bauer says the project has passed federal scrutiny.

“The idea behind that is to bring in these natural enemies that have co-evolved with the pest in the country of origin and then you bring them over if they are safe to bring over in terms of risk to non-target organisms. In fact the two that we are working on here in our lab we found that they kill about 75 percent of them. And that’s nice. They become established in the population of the pest. We’ll never get rid of Emerald Ash Borer. All we can do is hope to manage it at a density low enough that the trees can actually survive,” says Bauer

The federal government’s stamp of approval came earlier this summer. Now the tiny Chinese wasps are being released in certain parts of Michigan. The problem is the complexity of rearing more, a task in the hands of research associate Houping Liu. He says an indispensable element is wood from an ash tree.

“This is how we do it,” Liu says. “We use different size of ash sticks; uninfested ash sticks. So first of all we need to dig out EAB from the field or cut uninfested trees from the field. And then insert the EAB larvae in this kind of ash stick, then put the bark back, wrap it up, then put in some water and honey as food.”

The entomologists in East Lansing also have to go to great lengths to get the borers to lay their own eggs. Debbie Miller says wrapping ash twigs with ribbon gets the job done.

“This is a stick wrapped with ribbon,” Miller says. “Since the ribbon creates a situation where the insect has a place to lay the egg underneath a surface. It does not like to lay eggs exposed. It doesn’t seem like much but it’s numbers and that’s where the damage occurs; it’s the numbers that cause the problem. An ash tree would easily survive if there were few Emerald Ash Borer present on it. But it’s destroyed because it’s too many.”

Even if the Great Lakes region’s ashes are saved, Leah Bauer does not believe they’ll ever return to their grandeur. Their wood may be so scared by borer attacks that baseball bat manufacturers may be forced to select another hardwood. But if the parasitoid approach is successful, millions of ash trees might have a fighting chance.

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