Earthworms: Invasive, Destructive, Plentiful.
I saw my first robin of the spring today. And whenever I see robins, I think of only one thing: earthworms. And lots of them.
Every spring rain—and summer rain, and autumn rain—our sidewalk from front door to driveway is overwhelmed not only with water but with hundreds of slimy earthworms escaping the flooding of their tiny underground homes. We can’t make it to our cars without squishing an earthworm beneath our shoes every step of the way. It’s a distasteful experience, really.
Of the fifteen earthworm species living in Minnesota, zero are native to our state. (There are two native North American earthworm species, but they prefer warmer climes, and Minnesota has a native water worm species.) Why doesn’t Minnesota have its own worms? According to Smithsonian Magazine, they were wiped out by a glacier some thousands of years ago. Guess they couldn’t quite escape that deep freeze. The earthworms you see today were brought in from Europe and Asia by settlers via potted plants and other loamy medium as early as the 1600s.
The big (HUGE) problem with these invaders isn’t merely their tendency to overwhelm sidewalks when the rains come. Fact is earthworms are downright dangerous to our local wild ecosystem, even if they do provide a tasty meal for robins or an excellent bait for fishermen. In 2000, scientists from the University of Minnesota announced that invasive earthworms are posing a real and continuous threat to the health of our state’s hardwood forests, eating plant matter on topsoil and destroying the ability for native plants and trees to take root and thrive. Up to “ninety-nine percent of the populations of native plant species normally found in hardwood forests…are destroyed in affected areas [and] In many areas, the remaining bare soil is simply eroding away." This affects the health of trees, plant life, and so many other living things, from microbes and insects and on up the food chain.
99% of native plant species in MN's hardwood forests are destroyed by earthworms.
Earthworms destroy the spongy loam called “duff” that provides a base for native plants to grow, including berries and fruits that bears and birds thrive on. So, part of the bear population “paradox”—where there are more bear sightings even though there are actually fewer bears—is that there is a lack of natural food to eat in the forests themselves. As a result, bears are venturing to farmer’s fields to graze. Before they were happily stuffing themselves in the cover of forests; now they have to show themselves to find food, making them easier targets for hunters and putting them in greater physical danger from harvesting machinery.
Do you like bears and birds, forests and trees, foraging and flowers? Then help get rid of earthworms! So, how do you do that?
Like many folks who might “dump the leftover worms near the lake” in a well-meaning but misinformed effort to help the environment thrive, I used to try to avoid stepping on my sidewalk worms. Now, I’d rather they die. Scientists also suggest just killing them, encouraging fishermen to toss leftover fishing worms into garbage bins. You can kill sidewalk worms with a sprinkling of table salt (or just leave them and let the sunshine dry them out and do the dirty work for you). Follow the Minnesota DNR’s advice to refrain from purchasing worms for fishing or composting purposes and to inspect mulch, plants, soil and bait for worms (and disposing of said hitch-hikers) before relocating or transferring potted plants to the ground.
There are also ways you can help forests recover from this worm invasion. Planting native grasses, flowers, shrubs, and trees is a great start (even if you’re not planting in the middle of the forest, the seeds and pollen of those you do plant will spread). If you’re more into a quick-fix control measure for your own personal lawn, pesticides containing carbaryl as an active ingredient should do the trick, according to GardenGuides.com. It’s no secret I try to avoid using chemicals for weed and pest control, and the same applies here. The downside to using chemical control methods for earthworms is at least double-fold: one, you’ll have to reapply the pesticide a few times throughout the year and, two, MN DNR warns “There are no known methods for controlling invasive earthworms on a large scale in natural settings. Chemical treatments that would kill earthworms would kill beneficial soil organisms as well.” Bummer.
Both the beauty and tragedy of the ecosystem that our Creator put in place is that everything is a cog in a vast natural machine.
Should we sacrifice a few for the sake of saving the many? That’s the question at hand, at least for me. Both the beauty and tragedy of the ecosystem that our Creator put in place is that everything is a cog in a vast natural machine. None are a separate entity, compartmentalized in its own cocoon, living and dying and reproducing without effect on the rest. Even something as seemingly harmless as the lowly earthworm is wreaking havoc on bigger members of the local wild: trees, bear, birds, deer, turkey, and, in many ways, even us humans. Ironically, this ripple effect will or has already reached the fish, whether fishermen who love using worms as bait want to admit it or not.
There is currently no large effort to stop the sale or importation of earthworms to Minnesota for fishing or gardening purposes, and putting something like that into effect would be a logistical headache. The best the individual can do is to choose to abstain from purchasing earthworms. Instead, if you must have earthworms for your tasks and activities, consider digging them up yourself and using ones already here.
Also, cheer on the robins! They can eat up to 14 feet of earthworms a day, and are certainly doing their part to keep the invasive worm population at bay. I also kept a Peking duck who had an affinity for gobbling down earthworms. She didn’t mind getting wet when it rained and of course that’s when her favorite snacks surfaced, too! What a win-win!
But do wild ducks—more specifically, mallards—eat earthworms? Stick around for our next Local Wild to find out. Until then: Stay dry and cheer on the robins!
Just For Fun: An interactive frost depth map is provided by the National Weather Service and can be viewed and used at www.weather.gov/ncrfc/LMI_FrostDepthMap.
 McCormick, Tori J. “Earthworms are bait. They’re also a nightmare for healthy Minnesota forests.” Star Tribune, 21June2018. www.startribune.com/earthworms-are-bait-they-re-also-a-nightmare-for-healthy-forests/486178521/.  “Jumping Worm (Amynthas species)”, Minnesota Department of Natural Resources, © 2020.  Nodjimbadem, Kate. “Is the Earthworm Native to the United States and More Questions From Readers”, Smithsonian Magazine, March 2016. www.smithsonianmag.com/smithsonian-institution/earthworm-native-united-states-more-questions-from-readers-180958094/.  University Of Minnesota. "Earthworms Threaten Minnesota Forests." ScienceDaily. ScienceDaily, 7 September 2000. www.sciencedaily.com/releases/2000/09/000904130558.htm.  Kennedy, Tony. “Minnesota’s overall bear shortage a paradox amid soaring hunter success”. Star Tribune, 04September2017. http://www.startribune.com/minnesota-s-overall-bear-shortage-a-paradox-amid-soaring-hunter-success/442555133/.  “American Robin”. Wild Birds Unlimited, Inc. © 2020. www.wbu.com/birds/robins.