Meet the parasitoid wasps scientists hope will save (some) of Vermont's ash trees
On a warm afternoon in early fall, Vermont Department of Forests, Parks and Recreation protection forester Chloe Sardonis taps a tiny parasitoid wasp out of a compostable coffee cup and onto an ash tree near Plainfield’s Spruce Mountain.
The wasp is small and black, and at first glance, it doesn’t look like much more than a gnat. Except that it’s going to hunt for emerald ash borer larvae through this tree’s bark.
The invasive beetle emerald ash borer was first spotted in Vermont in 2018. Since that time, it’s spread to 13 of the state’s 14 counties. The insect is on track to kill hundreds of thousands of mature ash trees in Vermont in the coming decades. But some scientists hope these tiny wasps might give the next generation of ash trees a fighting chance at survival.
Sardonis explains the process as she works.
“We walk around and find a tree with symptoms… walk over to that, take some info down for the government about tree size, etc., open it up, take the lid off and tap ‘em out,” she says, adding to the wasp, “Be free! Do your thing!”
This wasp’s “thing” is to paralyze the emerald ash borer larva and inject it with its own eggs, which then hatch and eat the grub.
State scientists are releasing three of these special wasp species at three sites across Vermont — here in L.R. Jones State Forest, as well as at a campground on South Hero and at another site in Bennington.
The wasps don’t sting people. They have a very specific taste for beetle larvae, but only at certain phases of their life cycle. One species being released targets the emerald ash borer’s eggs.
Today’s batch of nearly 1,000 wasps traveled to Vermont from Michigan by way of UPS, in cups with a little honey.
“It’s a very delicate process of encouraging them to leave their honey-filled home,” Sardonis says as she taps a few out.
The wasps are part of a nationwide effort to save future forests from this super invasive shiny green beetle.
Emerald ash borer was first found in Vermont four years ago. By the time you can tell an ash tree is infested, it’s already too late. The beetles eat away at the cambium, the part of a tree that moves water and sugars up and down the trunk. They can kill a mature ash tree in just a few years.
Josh Halman is a forest health specialist with the Department of Forests, Parks and Recreation. He helps keep track of invasive species and their impacts on Vermont’s forests, and he says the beetles basically girdle trees.
“They’re pretty much strangling the tree and making it impossible for those nutrients to get up to those tissues in the upper crown, where they’re needed,” Halman says.
Emerald ash borer arrived in the United States in 2002. It’s likely the beetles made their way here in wood packing materials. Vermont watched for the insects for years before finding them here in 2018.
Now that they’re in Vermont, scientists expect emerald ash borer will kill most of the mature ash trees in the state, as they did in many parts of the Midwest.
But some scientists say these wasps offer a glimmer of hope for the next generation of trees.
“The eradication of emerald ash borer is pretty much impossible here and elsewhere,” Halman says. “But what these insects can do, is they can reduce the population to the point where the next cohort of ash trees that are growing up will have a fighting chance to defend themselves against emerald ash borer as they grow.”
The wasps are native to eastern Russia and China, where they hunt emerald ash borer larvae and eggs in the wild. The U.S. Department of Agriculture spent about a decade running tests in a quarantine facility in Michigan before they released them, to try to make sure they wouldn’t harm other species in the U.S.
Jian Duan is an entomologist and the lead scientist working on this at the USDA.
“To get regulatory approval, we need to conduct safety testing… just to make sure these natural enemies do not attack our native insects,” he says.
So far, things look promising.
Duan says the wasps are very specific about how they hunt, following the vibrations the larvae make under an ash tree’s bark when they feed.
That specificity made them a good bet for biocontrol — when scientists or farmers use a pest’s natural enemy to control its population.
In fact, Duan says one species of the wasps is so particular that: “If you peeled the ash bark and took the emerald ash borer larvae out of the tree, and put them into a petri dish, these wasps cannot attack the larvae… because they have to go through this process.”
Vermont is the 31st state to release these wasps. Duan says in parts of southern New England, they’re killing as much as 50% of all emerald ash borer larvae.
But this approach has its limitations. It’s expensive to raise the wasps.
“We always struggled with the number of parasitoids the lab can produce,” Duan says. “It is very labor-intensive to rear this biocontrol agent.”
Additionally, the wasps won’t save trees that are already facing infestation.
In fact, 20 years after emerald ash borer first came to the United States, the federal government has largely given up on efforts to quell its spread. In 2021, the USDA lifted regulations on moving firewood across state lines.
Instead, the feds say they’re putting those resources towards rearing more wasps.
Elise Schadler would like to see more federal money set aside for something less glamorous: removing dead ash trees.
“Tree removals — I mean, excuse my French — but it’s just not sexy,” Schadler said.
Schadler runs Vermont’s Urban and Community Forestry Program. They help towns manage their urban trees, everywhere from town forests to village greens to city streets.
She says it’s not the first time Vermont has faced something like this. A hundred years ago, the American elm dominated New England city streets. They were favored for their shade and their shape.
But when Dutch elm disease — a fungus — came through from 1940 ro 1960, it killed some 11,000 elms in Burlington alone.
Many of those elms were replaced with fast-growing, salt-tolerant green ash trees.
In fact, Schadler’s organization has mapped more than 90,000 ash trees in public right-of-ways around the state. And she says there are more, squeezed between old houses in Burlington’s north end or growing wild along town roads.
She says if all these trees die at once, it will be an expensive proposition for towns to remove them.
“You know, you’d think we would have learned our lesson after Dutch elm [disease] came through, but we certainly did not,” Schadler said.
As for Vermont’s ash trees, Schadler says towns can save money by planning ahead and removing trees now, or working with a licensed arborist to treat them annually with a state-approved non-neonicotinoid insecticide.
That option has some downsides, though. While it’s not terribly expensive to treat trees, the pesticides can seep into the soil or nearby waterways, and they also kill native insects.
Some towns are already doing this work and have been for a long time. But Schadler says the vast majority of Vermont towns don’t have an arborist on-staff. This work gets done mostly by volunteers, and she says this problem is too big for volunteers to take on alone.
Schadler’s organization can help towns with the logistics and with securing funding. She says replanting with more diverse trees is key in the face of climate change, which could bring more forest pests.
“We don’t know how … changes in extremes and temperature increases are going to impact the urban forest like everywhere else. So diversity, diversity, diversity — that’s not just species diversity, but age diversity,” Schadler said.
Forester Chloe Sardonis with Forests, Parks and Recreation says that’s important for all of Vermont’s forests.
“There’s a suite of issues that comes with climate change or introduced pests,” Sardonis said. “They tend to compound on each other … It’s like a one-two punch for our native species.”
It’s too soon to know if the wasps will be enough to give younger ash trees in Vermont a fighting chance.
Vermont started releasing them during the pandemic, and data from the USDA show it will take about three to five years for them to make a dent in the emerald ash borer population here.