The Dirt on Worms

November 13, 2009 09:38 AM

The earthworm has long been considered the farmer's friend. Their burrowing has always been thought to aerate the soil while their d

Most people don't realize that many of the earthworms in our soils are imports from Europe and Asia. Unearthing the truth behind them is shedding new light on what we've perceived as old friends.

roppings provide fertilizer. No-till farmers count on these tireless workers to improve soil tilth. And on their worst day, earthworms sacrifice everything to land us a lunch from our favorite fishing hole.

So it may come as a surprise to learn that the beloved earthworm is now getting a more slimy reputation. It turns out most earthworms beneath our feet are exotic intruders from Europe or Asia. These sub-terranean immigrants have moved farther into northern states and Canada, where earthworms haven't existed, and in some cases have displaced the more beneficial native species in Southern states.

"Soils with a lot of earthworms crawling around are generally considered good in agricultural systems,” says Diane Stott, a research soil scientist with USDA–Agricultural Research Service located at Purdue University's National Soil Erosion Research Laboratory in Lafayette, Ind. "But all earthworms aren't alike, and the exotic species can have harmful effects even in agricultural settings. Earthworms create macropores in the soil. A burrow that reaches 6' deep might be good for drainage if you've got a lot of water but bad if you don't.

"Also, the problem with earthworm channels is that recently applied nutrients can be directly transported below the root zone during the next rainfall,” she adds.

Nuked forests. It is damage in hardwood forests that developed since the last ice age that has gained the most attention. Without earthworms, fallen leaves decompose slowly, creating a spongy layer of organic "duff.” This duff layer is the natural growing envi-ronment for native woodland wildflowers and tree seedlings, provides habitat for ground-dwelling animals and prevents soil erosion.

Invading earthworms eat the leaves that create the duff layer and are capable of eliminating it entirely. Big trees survive, but young seedlings and much of the understory plants perish.

Cindy Hale, a University of Minnesota Duluth ecologist, spearheads research and raises public awareness about the issue through the Great Lakes Worm Watch. She says that unlike agricultural soils, the soil in earthworm-free hardwood forests is naturally loose because of the thick duff layer. When earthworms invade, they actually increase the compaction of the soil, which can result in decreased water infiltration and more surface runoff and erosion.

Welcome to America. "We have no evidence that earthworms ever inhabited Minnesota before European settlement,” Hale notes. "Even if they did, the glaciers killed any native North American earthworms in our region.” Scientists figure these European species of earthworms were introduced on purpose or by accident by colonial settlers—coming along for the ride in potted plants or in soil used for ballast in ships.

More recently, the widespread use of earthworms as fishing bait has spread them to more remote regions of the country. Anglers—thinking they are doing the world a favor—are famous for freeing their unused bait in the forest or into the water.

All common bait worms are non-native, including those known as night crawlers, leafworms or angleworms. So are red wiggler earthworms, which are sold and shipped all over the country for home compost piles and vermi-composting (worm composting) operations. Hale says red wigglers used in composting don't like Northern winters and have been shown to be more invasive in Southern regions if they escape the composting operation.


Night crawlers might be good for fishing, but they are robbing some forests of nutrients. Scientists urge you not to introduce new earthworms into the environment.

Most recently, Asian varieties called Alabama jumpers, or jumping worms, have been showing up as uninvited residents. They're called that because these worms have a very muscular and hyperactive behavior that causes them to flip and jump.

"Fisherman like them because of this activity,” Hale says. "The scary thing is that they are able to live at high den-sities. We get a lot of ‘nothing will grow here' calls from these invasions. We are concerned that these earthworms have the potential to have negative consequences in ecosystems where you traditionally think of earthworms being good, such as gardens and landscape,” she adds.

Not all foreign earthworms are destructive, however. Of the thousands of species around the globe, only approximately 15 of the European and Asian varieties are considered to be bad guys. Hale says that despite distinct differences in these worms' anatomy, color and behavior, most people never really notice and think "a worm is a worm.” But research is showing that different species can have very different environmental impacts.

Perhaps the biggest challenge, Hale explains, is just getting people to believe that earthworms are an issue. "There's this mythology that earthworms are always benevolent creatures that increase the fertility and mix the soil for us,” she says.

"It's one of the ecological concepts people seem to really get. For many people, earthworms are commonly known to be good—end of discussion. That may be true in many agricultural systems, but not necessarily so in native forests.”


Contain Those Crawlers

There's not much to be done about earthworms that have already invaded the landscape, says Cindy Hale, author of a book on earthworms and an ecologist with the Natural Resources Research Institute, University of Minnesota Duluth.

"Controlling crawler introductions is the best way to contain the problem,” Hale says. Scientists calculate that earthworms spread only a short distance on their own. Here's how you can help:

  • Don't dump extra worms in the woods or in or around ponds.
  • Dispose of unwanted bait in the trash.
  • If you use earthworms for composting (vermicomposting), freeze them solid for at least one week (one month is better) to kill the earthworms and their egg cases.
  • Don't transport leaves, mulch, compost or soil from one place to another unless you are confident that there are no earthworms or cocoons present in the mixture.
  • Avoid planting non-native plants and shrubs. If you do, examine the root balls and destroy any worms you find. Do not plant non-native plants near woodlands or water.

You can e-mail Pam Smith at

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