Earthworms: Villains in Disguise

(originally posted May 6, 2011)


If you asked me to tell you about earthworms five years ago, I would

Apparently this kindergartner
wants a snack.

have told you about their importance to the soil; how they aerate the soil with their tunnels and even create it though their excrement (hooray for worm poo!). Everybody knows that worms are an important part to the soil—even a kindergartner will tell you that. Right?

The truth is the beloved earthworm and all of its cousins are exotic invasive species to the northern United States and Canada.

How is this possible? Way back when glaciers covered these areas (about 10,000 years ago) any worms that may have inhabited these areas were wiped out, leaving forests to evolve in the absence of worms. Plants became accustomed to and dependent on thick duff layers of rotting leaves, twigs and other vegetative debris for insulation and nutrition. Then when European settlers arrived in their ships, they dumped the weighted ballasts of their ships, which besides containing soil and rock, would also contain earthworms and their eggs. And thus the invasion of the earthworm began.

Earthworms consumed the duff layer of northern forests, leaving the forest floor barren of nutrients and insulation for young plants. And thanks to the transportation of plants and worms used for composting and fishing bait, these seemingly harmless invertebrates were quickly and continuously given a free ride across North America. This was a pretty good deal for a creature that can only expand its range by a half mile over a period of 100 years! In southern areas where worms were not eliminated by glaciers, these exotic worms now compete with native worms and overwhelm forests.

A sugar maple forest before earthworm invasion

A sugar maple forest after earthworm invasion
(images courtesy of the Great Lakes Worm Watch)

If worms are so bad, why are we taught about their benefits? In cities and agricultural areas, where our soil is compacted and low in nutrients, earthworms do provide the benefits we learned about in grade school. And it wasn’t until recently that studies have been done revealing this new information on the earthworm invasion.

So what can we do about this villain in disguise? Is all hope lost? Fortunately, there are steps we can take to keep the last few worm-free areas safe from invaders and to curb existing populations. If you use worms as bait, do not return any leftovers to the soil. Give them to the fish—they’ll appreciate the free meal! If you use “red wigglers” for composting, freeze your compost before distributing it over your garden to kill any worms and their eggs. And finally, you can learn more by visiting the Great Lakes Worm Watch website. Perhaps with more research, future kindergarteners will learn a different story about the not-so-common earthworm.