I recently had the most remarkable conversation about ash trees in New Hampshire. It was remarkable because, for the first time since Emerald Ash Borer arrived here in 2014, it wasn’t all doom-and-gloom.
“It’s just in the last two years we’ve turned the corner and been a lot more positive about the future of ash in New Hampshire,” said Kyle Lombard, administrator of the state’s Forest Health Bureau, part of the Division of Forest and Lands. If he is getting more positive, then so am I!
As I’m sure you know, EAB is an Asian beetle that burrows into ash trees and lays eggs that create larvae that eat so much that they basically starve the tree to death. Tree mortality is close to 100% within just a few years.
Ever since the beetle showed up on this side of the Pacific Ocean, many people, including me, have said that American ash trees are going to disappear from the landscape, a depressing echo of the American chestnut, a majestic arboreal species that was killed off by invasive bugs decades ago.
That gloomy scenario is no longer inevitable. What has changed? Biocontrols, specifically three species of EAB-eating wasp imported from China that are thriving in the Granite State, are building up populations.
“No one really knew how well they would acclimate here. It’s all about our winters,” Lombard said. “It’s been surprising how well we’ve been able to establish all three species — how well they’re working, how well they’re doing on their own. In southern New Hampshire, almost any ash stand you go into, you can find at least two of the three species we’ve released over the last 10 years.”
It now seems likely this trio (and perhaps a fourth which hasn’t yet moved this far north) will thrive to the point they can start knocking back the population of EAB.
Getting back to balance
This doesn’t mean that soon we’ll return to the days when ash made up a sizeable chunk of New Hampshire forests — because the idea isn’t to use the wasps to eradicate EAB. “That’s not how biocontrol works,” Lombard said.
The idea is to reduce the beetle’s population so that when it gets combined with other population controls, such as bug-eating birds, weather and competition from other insects, ash tree populations can regenerate even though emerald ash borers are still around. Over decades, the hope is ash trees will slowly return to our woods.
“We want to keep the population low enough, balanced enough, that there will never be an outbreak phase that destroys a whole generation of trees,” he said.
As an example of what the state is aiming for, Lombard pointed to the bronze birch borer, a native pest that operates like EAB and “kills about 5% of our birch trees every year.” It has wreaked this destruction forever, but the birch trees are still here, an example of species give-and-take that is part of any ecosystem.
Foresters are sufficiently optimistic that as part of the New Hampshire Forest Health Program they are growing ash seedlings from seed collected across the state in 2022. The seedlings were grown at the State Forest Nursery in Boscawen, and some 1,300 have been replanted at 13 sites throughout New Hampshire. The hope is that by the time they become big enough to attract EAB, biocontrol wasps will have knocked down the invaders’ population to manageable levels and the ash population can regrow.
Unfortunately, this biocontrol doesn’t work for mature ash trees because the wasps can’t get through the thicker bark. The immediate apocalypse will still happen. Trees we know and love like the two in front of my barn are still doomed unless you regularly inject them with insecticide to kill the EAB larvae.
Obviously regular tree injections are not practical on a forest-wide basis, but the state is doing it on 100 sites of mature ash trees so that future generations can see what an ash forest is like as they wait for today’s seedlings to mature.
“Imagine if we had saved 100 sites of chestnut. We would be able to walk through mature chestnut forests,” said Lombard, sounding slightly wistful. “Wouldn’t that be great?”
Fears of biocontrol
Mention of biocontrol brings up legitimate concerns because history is riddled with cases of it getting out of hand: Creature A is a pest so we import Creature B to eat it, but Creature B decides it likes to eat other things, as well, or it mutates into a locally disastrous form, and now we have more of a pest than Creature A.
Lombard said these concerns are well known and that the wasps in question have been tested by the U.S. Department of Agriculture for perhaps a decade. Nothing is absolutely certain in biology, of course, but the risk-to-reward ratio seems acceptable here.
If nothing else, it gives me a chance to write a column about environmental change that doesn’t end with holding my head in my hands and moaning with despair. Which is good, because that always annoys my coworkers.
