This is a second-wave invasion

America didn’t always have worms. At least not of the familiar earthworm variety.

European earthworms were once an invader to North America, too. When they arrived from across the Atlantic in the 1600s, much of the continent had been free of a meaningful earthworm population since at least the last ice age. When they got here, they brought their share of changes to the landscape, including altering native forests. But in the intervening centuries, we have learned to live with — and sometimes even love — them.

Amynthas worms, by contrast, are slightly newer, second-wave invaders. Although the first documented observations of them in the US reach back to the 1930s, their arrival in many regions has been within just the past decades or even years. When such a vigorous organism moves in, the early results can be shocking, especially with jumping worms. “There are so many of them, and they’re so active, people get really disturbed by them,” Williams says.

The Amynthas species we have in the US (most commonly Amynthas agrestis and Amynthas tokioensis) are primarily from Japan and the Korean peninsula. In their home habitats, they evolved along with the local ecosystems — and the ecosystems along with them. But here, “just like any other invasive species that are displaced into a brand new habitat that might not have controls, they’re able to take advantage of that and go gangbusters,” says Brad Herrick, an ecologist at the University of Wisconsin-Madison Arboretum.

But buried in this issue is a big and more concerning mystery: Researchers don’t know why, over the past decade and a half, these worms seem to be spreading so much farther and faster.

The worm invasion may be getting worse

It’s believed Amynthas worms are primarily spread through moved mulch and compost; soil transported with plants or on vehicles; streams by natural distribution and use as fishing bait; and, of course, by snaking their way across the landscape. (Part of Amynthas’s success lies in the hardiness of their tiny cocoons, which are just 1 to 3 millimeters in diameter, can survive temperatures ranging from about -15 to 103 degrees Fahrenheit, and some of which are thought to hide cryptically in the soil for more than a year before hatching.)

Why are we now seeing so many more of them, and in so many more places? Part of it might be due to increased awareness, but Herrick and others also think there is more to it than that. Climate change could be one possibility, he says, opening up more northern latitudes to their liking. Another is that they have reached a population tipping point that makes mass spreading more likely, Herrick notes.

Although much remains unknown about these worms, we do have some good reason to worry about them — and to do our best to limit their spread.

Take the way they move through the soil, for example. European earthworms, on the one hand, are wide-ranging feeders. They make their way through surface, mid-, and lower levels of the soil. In this ambling habit, they circulate nutrients (ingesting some debris here, leaving their castings there) and break up the soil among strata, providing air and water to the layers below.

Amynthas worms, on the other hand, stick to the surface. So not only do they not perform the helpful mixing, but they also leave all of their castings — which Herrick likens to “coffee grounds or taco meat” — on the surface, where they are easily washed away by rain and irrigation. “They can transform the soil in one growing season,” Herrick says. This can cause problems for cultivated landscapes, such as gardens and urban areas, as they lose nutrients in runoff and have less stable upper soil layers for plants to root into. (Their potential impact on US agriculture has not yet been well studied, although heavily tilled and treated cropland is a less welcoming habitat for them.)

They also seem to be altering forests. In North American forests, which evolved over more than 10,000 years without earthworm populations, earthworms of any kind can undermine the soil’s density and change its composition. Amynthas worms also pose a threat to the many organisms — plants, bugs, microorganisms — that make up the established understory ecosystem. “Once this layer disappears, this whole biodiversity disappears, and impacts forest ecology as a whole,” explains Katalin Szlavecz, a soil ecologist at Johns Hopkins University. This disturbance can also make it easier for other invasive species to move in, Herrick adds.

And then there’s jumping worms’ uncanny ability to push out established European earthworm populations. They clearly seem poised to outcompete their more methodical relatives. After an invasion, “It’s almost like War of the Worlds: what happened?” says Williams.

The reason for the decimation remains unclear. “Is it a virus? Is it an associated nematode? Do they have a chemical release? There’s a huge mystery here,” she says.

“The can of worms is open, and you can’t put them back in”

In light of these unhelpful doings, some states have tried to slow the spread by listing Amynthas worms as prohibited species. And to try to beat back existing infestations, researchers have investigated using everything from controlled burns to sulfur treatments, with moderate localized success. But, says Szlavecz, “I don’t think, on a large scale, any of these are efficient.”

Some commercial processes might help stop them. For example, Herrick has found that heating the cocoons to 104 degrees for three days kills them. And others are investigating different types of soil applications, including worm-killing fertilizers and fungi.

Gardeners, meanwhile, have been fighting their own battles against Amynthas. Some are still trying to prevent them from entering by erecting a shallow barrier of metal flashing to serve as a subterranean wall. Williams recommends also not picking up roadside compost, mulch, or plants, and asking nursery staff about the potential for jumping worms in products. There may be some that get in anyway: “you can’t stop birds from flying, you can’t stop worms that like to wriggle across the soil,” Williams says.

Still, others dealing with current infestations can try solarizing soil with plastic in the spring or forcing worms to the surface with a “mustard pour” — mixing powdered mustard with water and pouring it over the soil surface — and then handpicking them out.

While most land management experts encourage all of the reasonable steps we can take to control these voracious worms, there is little hope of eradicating them from North America. “The can of worms is open, and you can’t put them back in,” Williams says.

In other words, we now have our own adapting to do.

Herrick and his colleagues are currently enlisting local gardeners and others to help learn what native and ornamental plants might survive well or even thrive in jumping worm-modified soil.

“There are more question marks here,” Szlavecz adds. Which is why, she argues, continued research — as well as individuals’ observation — of these worms needs to continue. She argues for a rebranding as well. Not only do they not jump, “they’re not ‘crazy’ — it’s a big enough problem that they are invasive. Calling them ‘crazy’ just adds to the panic.”

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