Earthworms seem pretty harmless. But they’re causing problems for Michigan’s multi-million dollar sugar maple industry.
That’s the finding of a study by Tara Bal, a research assistant professor of forest resources and environmental science at Michigan Technological University.
Bal and her team received a number of calls from foresters noticing that their maple trees were starting to look unhealthy at the crowns - a condition called dieback.
“And it basically looks like just an empty branch there hanging out at the top of the tree where it used to have leaves, and now the leaves have died and it’s started pulling resources back into the trees,” Bal explains.
The researchers concentrated on sugar maple forests in the Upper Peninsula, northern Wisconsin and Minnesota to find out what was happening.
“The amount of duff and litter layer and leaf litter that’s on the forest floor was the biggest thing that we were finding in all these sites that had a lot of dieback,” she says.
That leaf layer on the forest floor was disappearing - and that’s a problem.
“So you think of the forest floor, it’s sort of like the mulch layer that you might have in your flower bed or your garden at home, or the compost layer that you want on top of the soil to kind of protect it, keep some of the moisture underneath,” says Bal.
She says if the forest floor is compromised, trees can lose protection from frost and drought. This can cause damage to the roots of the trees.
And the source of that forest floor damage? Some very hungry earthworms.
“They are essentially little ecosystem engineers and they are directly eating on and feeding on that forest floor litter layer,” says Bal.
The earthworms we have in Michigan aren't native to this region.
“In the south, you have different soil types, and you have areas where earthworms have been there longer. And even further south where you have unglaciated soil, so places where the glaciers never came before, you have native earthworms further south in the U.S.,” says Bal.
“So here, in this part of the country, and in Wisconsin, where glaciers came down 10,000 years ago, it wiped out earthworms from here. So trees here over the last 10,000 years have not had any earthworms. Most of the worms that we have in Michigan are all Asian and European species that have been brought over.”
Although earthworms aren’t native to the region, at this point they’re almost everywhere. And there isn’t much to be done about that.
“The kind of disappointing thing people hear about is that there’s no wormicide, there’s no pesticide or something you can spray to get rid of earthworms," she says. "So really monitoring your sites more frequently, if you know that you have a really drought-y summer and that stand has earthworms in it, go back and check in the fall and see if you have a lot of dieback because maybe you’re going to want to return sooner so you don’t lose the economic value of those trees.”