Invasive Pests Or Beneficial Friends?

Most of us who live in North America were raised to think of earthworms as beneficial garden allies. We were taught that these wiggly weirdos were invaluable for healthy soil, and as food resources for hungry birds.

The general idea was that if you dug into the soil and found a handful of these worms, then said soil was super healthy. Furthermore, you could also use these wrigglers as fishing bait!

Imagine how surprising it is to find out that common earthworms are invasive in North America. Furthermore, they aren’t just invasive: they’re actually quite harmful to our local ecosystems. This is because the same traits that make them great for our gardens wreak havoc on forest systems.

How Do Invasive Earthworms Harm Local Ecosystems?

Gardeners, botanists, and biologists are very familiar with earthworms’ role in soil health. These worms break up the soil as they wriggle through it, thus aerating it and releasing nutrients from it.

Water then moves through the soil more easily, carrying nutrients through the earth, and up into the plants growing there.

Now, the problem is that native North American plant species evolved over millennia to adapt specifically to their local environments. Forest plants in particular learned how to get nutrients from the soil as efficiently as possible. Invasive earthworms mess with all that.

“Mor” vs “Mull” Soils

If you’ve ever taken a walk in a forest, you’ve noticed that the forest floor is covered in detritus. This organic matter consists of fallen leaves, slightly broken-down leaf mold, fallen conifer needles, dead insects, and fungi. The soil in a forest is known as “Mor” soil, and consists of this detritus laying atop lower, mineral-rich soil, rather than being mixed with it.

Now, when earthworms are introduced to these soils, they change them drastically. Within just a few years of being introduced, the worms churn these layers together. As such, the “Mor” soil becomes “Mull” instead.

The layers are mixed together more homogenously, which wreaks havoc on indigenous plants’ health. Basically, the plants that have evolved to thrive in the various layers now don’t know what to do with themselves.

All Effects Ripple Outwards

Invasive plant species root more shallowly than native species, which needed to adapt to draw minerals from lower soil layers. As such, these invasive species end up thriving whilst indigenous species miss out on nutrients. [1]

It’s not just plants that miss out on these nutrients, either. Remember that nothing in nature exists in a vacuum, and this nutrient redistribution affects soil microbes and insect health as well. In fact, the altered soil biology can also damage mycorrhizal health, which in turn will affect all the other life nearby.

In fact, every change spirals outwards and affects other species. For example, the native plants that aren’t thriving as well won’t be able to nourish the pollinating species that evolved to feed on them.

In turn, those insect species won’t be as plentiful for local or migratory birds, nor small mammals. As you can imagine, the predators that feed on these birds and small mammals (e.g. hawks, owls, foxes, and such) see their food sources diminish as well.

Over time, what had previously been a thriving forest full of interdependent species becomes unbalanced. Invasive species thrive while native ones die off, and may never recover in that area again.

Where Did These Earthworms Come From?

Three of the most common invasive species, Lumbricus terrestris, Octolasion tyrtaeum Savigny and Dendrobaena octaedra Savigny, were introduced to North America in the 17th century. [2]

Scientists assume that these worms hitched rides to this continent in produce root bulbs. They (and their eggs) likely stowed away inside cabbages, onions, and other food plants that European immigrants brought to the new world.

Then they went on their merry way once introduced into the soil, dropping offspring as they wriggled onwards. These worms don’t require mates in order to reproduce, so there was nothing hindering their procreation.

Considering how many earthworms exist on this continent now, you can see how prolific they’ve been!

Can We Take any Steps to Reduce Their Populations?

These worms are widespread and prolific, as mentioned. As such, it would be very difficult for us to significantly reduce their numbers. That said, there are some actions we can take to reduce their populations on our own properties.

Oust Them with a Mustard Vermifuge

Choose a day when the weather is quite lovely, and the soil is already moist. For example, on a warm summer morning or evening after a rainstorm. Then dissolve 1/3 cup of powdered dry mustard in a gallon of water, and drench an area of soil with it.

There’s an organosulfur compound in mustard called allyl isothiocyanate, which is what gives mustard its slight burning effect. This component is also present in radishes, wasabi, and horseradish.

It irritates the worms’ skin as it seeps into the soil, and they’ll try to crawl away from the burning sensation. This drives them up and out of the earth, at which point you can grab them and toss them in a bucket.

From there, it’s up to you whether you want to relocate them, kill them off, or use them for another purpose. For example:

Feed Them to Wild Animals

You’ve likely noticed that you bring a lot of worms to the surface when you till your soil. Every time you turn that soil over, hundreds of worms—and their eggs—get exposed. Local animals have learned to recognize this smorgasbord and congregate every time we till. Then flocks of all species descend to devour the worms before they can disappear again.

You can team up with your local wildlife to help reduce earthworm populations. Collect exposed worms in a bucket as you turn your soil over. Then spread them out on a tray near your bird feeders. We also give them (and various grubs and slugs) to our raccoon friends to munch on.

If you’ve used the mustard pour (aka “mustard drench”) as mentioned above, you’ll need to wash off the earthworms before feeding them to any animals. While the mustard won’t hurt birds or mammals, it’s not an appealing flavor to them.

In fact, most of them will avoid it. As such, rinse the worms off thoroughly with a hose to get the lingering mustard powder off. Then give your wild friends the worm buffet they’ve always dreamed of.

Let Your Poultry or Fowl Eat Them

If you have chickens, Guinea fowl, ducks, quails, or other domestic/game birds, you’re in luck. Earthworms make tasty, nutritious treats for all of these species. As such, let your birds run wild in the garden after you’ve tilled so they can gorge themselves.

Alternatively, you can also offer them a worm and grub buffet similar to the wild bird snack board mentioned above. Just wash off the mustard as above if you’ve done a drench to draw them out.

Use Them as Fishing Bait

Do you live close to a river, lake, or ocean? Then dig up some of these earthworms and use them as fishing bait. Ocean fish such as cod, halibut, mackerel, and flounder love these worms. So do freshwater species like bass, catfish, trout, yellow perch, and bluegills.

Although using worms as fish bait won’t drastically reduce populations of earthworms, every little bit helps.

Reduce the Spread

Avoid moving soil from one area of your property to another, especially if you live close to a forest. If you do live near the woods, use those mustard drenches around the edges of your property. This will discourage the worms from moving in that direction.

Additionally, if and when you buy compost or mulch from a garden center, ensure that it’s been heat-treated. This treatment sterilizes the contents so you won’t spread soil-borne pathogens. As an added bonus, the heat also kills off earthworms and their eggs so they can’t establish themselves and spread onward.

It’s important to recognize the impact that human activity has on ecosystem health and well-being. By being aware of how every action ripples outward, we can be more diligent about invasive species in the future.

We may not be able to eliminate invasive earthworms from North America, but hopefully, we can refrain from doing additional damage while we’re here.

References:

  1. Qiu, J., Turner, M.G. Effects of non-native Asian earthworm invasion on temperate forest and prairie soils in the Midwestern US. Biol Invasions 19, 73–88 (2017). https://doi.org/10.1007/s10530-016-1264-5
  2. Nico Eisenhauer, Stephan Partsch, Dennis Parkinson and Stefan Scheu. 2007. Invasion of a deciduous forest by earthworms: changes in soil chemistry, microflora, microarthropds, and vegetation. Soil Biology and Biochemistry 39: 1099-110.doi:10.1016/j.soilbio.2006

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