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Call it a can of worms.

Night crawlers and angleworms have been favorite angling baits for generations of Minnesotans, and when the fishing season opens this weekend, many anglers will pin their hopes of a fish dinner on a worm impaled on a hook.

But it turns out the iconic wiggly critters have a little-known dark side: They are an invasive species wreaking havoc on the state’s forests. And anglers have helped spread the little critters around the state and nation.

“We don’t have any native earthworms in Minnesota,” said Lee Frelich, a researcher and director of the University of Minnesota’s Center for Forest Ecology.

For the past 11,000 years since the glaciers receded, forests in Minnesota and other Great Lakes states developed without earthworms.

Instead, the angleworms and night crawlers we find in our yards and gardens and use as fishing bait are imports, originally brought here with soil and plants by European settlers in the 1800s and 1900s. But unlike modern-day invaders such as zebra mussels, spiny waterfleas or Asian carp, these invaders have gotten little attention.

Until recently.

Frelich began studying their impact about 15 years ago, and he says they are having a major effect on the state’s forests, altering the ecosystem.

“The worms eat leaf litter on the forest floor, so you end up with bare ground,” he said. “And they compact the soil and cause nutrients, especially phosphorus and nitrates, to leech out of the soil into the water.”

The results? “The growth of the forest is actually stunted,” Frelich said. “We’ve seen about a 30 percent reduction in sugar maple growth rates.”

Without the leaf litter on the forest floor, trees are more sensitive to drought conditions.

A main reason Frelich began studying earthworm impacts was a phone call he got from Bruce Dayton, Gov. Mark Dayton’s father, around 2000.

“He called me one day and asked why his trillium disappeared in his woods in Orono,” Frelich said. “I said I didn’t know but would look into it.” Trillium are white, low-growing native wildflowers.

Frelich examined the woods, which Dayton later donated to the state, and found they were heavily infested with earthworms.

“The soil was heavily compacted and the nutrients were poorer than it should be for an old-growth forest that had never been logged,” Frelich said. And he believed earthworms were responsible for the disappearance of the trillium.

“Earthworms hit trillium, violets and orchids really hard,” he said.

The habitat changes also can affect wildlife.

“Ovenbirds, which make their nests on the ground, are less abundant in areas with earthworms,” Frelich said. He’s not sure how other wildlife are being impacted.

All of this has come as a revelation to many people, anglers included.

“The more work that’s been done, the clearer it is that earthworms are having a huge impact here,” said Laura Van Riper, DNR terrestrial invasive species coordinator. “It often surprises people.”

Earthworms have spread through all of southern Minnesota and good portions of the north. Anglers have inadvertently helped spread night crawlers and angleworms by dumping unused bait on the ground.

“About 50 percent of the boundary waters is invaded, and outside the boundary waters, there still are some earthworm-free areas scattered about the landscape,” Frelich said. “… There wouldn’t be invasions in the boundary waters if it wasn’t for the use of live bait.”

Voyageurs National Park has banned live bait on interior lakes, hoping to prevent the introduction of invasive species. Quetico Provincial Park, just across the border from the Boundary Waters Canoe Area Wilderness, has done the same.

Though the DNR has spent lots of money publicizing and fighting the spread of aquatic invasive species, earthworms have gotten much less attention. Van Riper said the agency has discussed earthworms at State Fair exhibits.

But the DNR’s fishing regulations booklet contains only one reference: “You cannot dispose of unwanted minnows and leeches in Minnesota waters. Unused bait, including night crawlers, should be put in the garbage.”

Besides Voyageurs, there’s been no talk of restricting or prohibiting use of worms elsewhere in the state.

“No one is saying people should stop using earthworms as bait,” Van Riper said. “If you have extra worms, don’t put them on the ground or in the water, put them in a trash can.”

Frelich said one thing appears certain: Earthworms will continue to spread, and there’s likely no turning back the clock.

“We haven’t found any way of getting rid of earthworms,” he said. Besides being spread by anglers, they are being transported north with sod or nursery plants.

More wiggly problems are on the horizon: Asian earthworms, which can reach higher densities and cause more problems. They’re already in Minnesota.

“They are the dominant worms on the St. Paul campus [of the University of Minnesota],” Frelich said. And he believes they will spread.

“It’s just a matter of time,” he said.

Doug Smith • dsmith@startribune.com