A growing threat has inched into Minnesota's forests, traveling across the state one bag of mulch at a time.
Jumping worms that have upended local gardens and lawns for several years have now infested 100-plus-year-old forests and state and regional parks scattered throughout the Twin Cities metro area.
The worms, which turn healthy soil into a sickly, lifeless mess that resembles cat litter or coffee grounds, have been found in about 250 locations surrounding the metro area and Rochester, said Lee Frelich, director of the University of Minnesota's Center for Forest Ecology.
"If they spread 100 feet in every direction, they're going to have total coverage" of the Twin Cities, Frelich said. "The question is, can we keep it out of remote forested areas?"
Frelich found the first jumping worms in Minnesota in 2006, in Minneapolis' Loring Park. Since then, they've mostly been garden pests, destroying soil and harming lawns in urban areas. Foresters have long feared that the worms would find their way into forests, where they can do far more damage. Once the worms establish a foothold in a wooded, remote place, it's unlikely they will ever be removed.
The pests have been found in two old-growth forest areas of the U's Minnesota Landscape Arboretum in Chanhassen as well as under the towering basswood and sugar maple trees of Lake Rebecca Park Reserve in Rockford, Minn. In both cases, they likely got there through mulch, applied to the arboretum's own gardens or at a neighboring home, Frelich said.
"As far as we can tell, any place where there's a house — which is just about everywhere because even remote areas have cabins with gardens — they can spread into the forests from there," he said.
Jumping worms are a type of earthworm originally from Asia. They look very similar to other worms, but they move around much more, appearing to wiggle or jump.
There is no good way to get rid of them. The U is researching ways to chemically control them in gardens, but those chemicals would never be a practical way to kill worms in a forest.
They are particularly dangerous to Minnesota because no earthworms are native to the Upper Midwest. The glaciers of the last ice age would have killed off any that may have once lived here, and so the forests that grew up over the last 11,000 years evolved without them.
Jumping worms eat the duff — the forest floor — and all the fallen leaves they come across. They live within the first few inches of the top soil and ruin it, breaking apart the roots and connections that hold the soil together.
"The detached soil washes away in the rain, so you see trees with their roots exposed," Frelich said. "You can imagine the seedlings of the native plants trying to get rooted in something like cat litter."
As the soil washes away, nutrients go with it. The forest becomes less bountiful, producing smaller, more sickly trees and plants. Species such as trilliums either die off or greatly reduce in number. When plants go, the insects and animals that rely on them follow.
Other types of earthworms and nightcrawlers, as common as they have become, also caused tremendous damage to the state's woods upon arrival. They likely came with European settlers in the 1800s, crawling in the soil attached to an imported plant. Several studies have shown that earthworms made forests less diverse.
Over the last two centuries, many forests have been lost or fragmented into smaller areas. Deer populations and densities have increased, all of which adds more pressure to the plants trying to survive in a forest, Frelich said.
"We keep adding barriers to the success of our native plants," he said.
Jumping worms, like all worms, spread very slowly naturally — it would take them about 100 years to travel half a mile. But they travel extremely fast when carried across town in mulch and yard waste and at leaf collection or compost sites, said Angela Gupta, a forester at the University of Minnesota Extension.
"Their eggs and cocoons are about the size of poppy seeds, so they're really hard to spot," she said. "And they're all within those top inches of the soil, so as we're raking our leaves in the fall, if a yard is infested, there's a high probability we're moving worm eggs."
The best prevention methods are the simplest ones, said Laura Van Riper, terrestrial invasive species program coordinator for the Minnesota Department of Natural Resources.
"Throw your unused bait worms in the trash," she said. "Clean the mud from boots and equipment so we're not accidentally moving cocoons from one place to another. If you live in the Twin Cities but have a cabin Up North, please don't dig up any plants and move them up there."
Some types of mulch have higher risk of carrying eggs or worms, Van Riper said.
Most compost sites, for example, have to be heat treated, which will kill them off. And wood chips are generally free of worms and their eggs, since the pests don't live in trees. But both wood chips and compost soil can become infested after the fact if they are stored on a site that already has jumping worms, she said.
Yard waste sites, however, are not required to be as rigorously treated and can more easily spread the worms.
"The best thing is to just talk to companies, and ask them if they're aware and find out the steps they are taking to keep them out," Van Riper said.