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Emerald ash borers are killing NH's ash trees. Meet the wasps that could save them.

One of the wasp species are tied to a tree during their release so they can emerge in their own time.
Bill Davidson
One of the three wasp species the state's Division of Forests and Lands uses are tied to a tree during their release so they can emerge in their own time.

Earlier this summer, during a scheduled tree removal, Portsmouth city arborist Max Wiater and his crew experienced something like never before.

“The tree was so inundated and so infested with emerald ash borer, all the adults started to fly out of the tree when we began cutting it down,” said Wiater.

He said the flying green bugs started to land on the workers, covering their bodies. Wiater said the whole ordeal was “alarming.” He said his team had only started to notice symptoms of infestation in the city’s ash trees this spring.

The pest has been spreading across the state since 2013, when it was first detected in Concord.

But the emerald ash borers might have met their match in the form of stingless wasps.

Tiny, but fierce 

The wasps are tiny, ranging from one millimeter to three millimeters in length. They also don’t have a stinger like you might imagine, said Bill Davidson, forest health specialist with the New Hampshire Division of Forest and Lands.

“Like what a hornet might use to sting somebody, these wasps actually use it to lay their eggs in their host insect,” he said.

Through a U.S. Department of Agriculture-approved, biological control program, New Hampshire was eligible to introduce three different wasp species to prey on the emerald ash borer. Davidson said they first started to release the wasps around Concord and Canterbury in 2014.

“The goal is to get these things out across as broad a region as possible so that they can start establishing in our environment and build up their populations and begin to spread so they can move with the emerald ash borer,” said Davidson, “And hopefully at some point, offer some level of protection to our trees.”

Currently, Davidson said they’ve released the wasps in about 28 sites across the state.

The wasps are sent over from a rearing facility in Michigan.

“Once a week throughout the summer, I'll receive a shipment, just a cardboard box that has the wasps inside,” said Davidson.

Since the wasps are fragile, either Davidson or other staff try to do the releases the day they are received, he said.

Each of the three wasp varieties arrive packaged differently. One of them comes in clear plastic cups with mesh lids. Another comes as larvae attached to their hosts in a piece of ash wood. The other comes in the form of parasitized ash borer eggs on coffee filters placed inside small medicine bottles.

The releases are carried out differently also. The wasps can either fly directly out of a cup, chew their way out of the ash wood, or are tied to a tree to emerge in their own time.

Finding the right predator 

The process of even finding these “natural enemies” to emerald ash borer took a lot of troubleshooting, let alone how to complete the releases, said Juli Gould, an emerald ash borer biocontrol program researcher with the USDA.

The USDA and the U.S. Forest Service traveled to China and Russia in order to find these wasps. Researchers then brought them back to the U.S., where they held the wasps in a quarantine facility.

“We tested all the wasp species, and we felt they were safe, that the parasitoids were not going to attack the non-target insects that were native to this country,” said Gould. “We went through a lot of paperwork, a lot of studies and a lot of scrutiny. And finally in 2007, we got permission to release them.”

They first completed the releases in Michigan, where emerald ash borer (or EAB) was first discovered in the United States. The biological control program is now in its 20th year, and 30 of the 35 states that came into contact with the emerald ash borer now complete the wasp releases.

“Between these parasitoid wasps and native woodpeckers, which also really like to eat EAB, we have unbelievable control of emerald ash borer,” said Gould.

In a study Gould published in 2022, she found that 54% to 81% of young ash trees (or saplings) had no living emerald ash borer in them in the areas she surveyed in New York following the introduction of the wasps ten years earlier. This means the pest population is dropping, and regeneration of ash trees is ongoing.

“We want to maintain ash as a viable part of the North American forests, but it's going to take time,” said Gould.

Ash trees come in three different varieties: white, black and green. At least before the introduction of the emerald ash borer, they made up around 6% of trees in New Hampshire. While that number might not seem notable, the role that these trees play in forest ecosystems is.

For example, black ash trees, not only significant to indigenous communities in the Northeast, play an important role in regulating the water table, said Davidson.

“When those black ash trees die, we don't really have another tree that's comparable to black ash that can live in those areas and suck up the amount of water they do,” he said.

Although the wasps may not be able to save many of the state’s mature ash trees, Davidson said there’s still hope for their place in our forests.

“We're optimistic that we'll be able to keep some trees around and that at some point generations into the future, those trees that survive will be able to grow into healthy, mature, reproducing trees and repopulate future generations of ash.”

Back in Portsmouth, city arborist Wiater and his staff are still in the process of removing infested ash trees. They’ve already taken out nearly 30 since the start of this year, and over 20 more that are either scheduled to be removed or soon-to-be proposed to be removed. Wiater said the city’s normal range of tree removals is around 50 to 70 a year.

As far as Wiater knows, the wasps haven’t reached the Portsmouth area, so he said their ash trees are “defenseless.” But that could change in the future with a connection Wiater is hoping to make.

“Bill Davidson. I’m actually writing this name down,” said Wiater.

Adriana (she/they) was a news intern in the summer of 2023, reporting on environment, energy and climate news as part of By Degrees. They graduated from Northwestern's Medill School of Journalism in June 2023.
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