DULUTH, Minn. — Invasive earthworms from Europe that came over with early settlers and have been moving across North America ever since are causing sugar maple trees to decline in northern Minnesota  forests.

That was the conclusion of a research project published in the latest issue of the journal Biological Invasions — the second major project in as many years pointing to earthworms as the culprit in northern Minnesota forest problems.

Scientists in the latest study, led by Michigan Technological University biologist Tara Bal, looked at plots of maples in Michigan’s Upper Peninsula, Northeastern Minnesota’s Superior National Forest and northern Wisconsin, and found maple trees experiencing “dieback” due to disturbances on the forest floor.

That disturbance was worms eating the leaves.

Dieback is when top branches that should be full of leaves instead had bare spots. That’s a sign of tree decline, where trees stop growing or die. In some areas, timber companies said maples were dying and becoming worthless even before they could be harvested.

From 2009 to 2012, Bal made annual visits to the same sites to measure what was happening.

“We went into this not knowing what was causing it, thinking it could be (forest) management or insects or something else,” Bal told the News Tribune. “But going out to over 120 sites over several years, you could see it was the worms and the lack of duff. We didn’t see any older maple leaves because they’re the leaves worms like to eat first.”

That invasive earthworms are a problem isn’t new. Scientists in northern Minnesota have been reporting issues for years. But the maple situation appears to be worse than others.

“It’s not necessarily a death knell for maples. You can have trees and worms,” Bal said. “But when you have worms and then you add on a drought or high temperatures or heavy deer browsing, they are combining to impact maples pretty severely. It’s changing the forest out there.”