How did earthworms invade Minnesota?

A University of Minnesota Ph.D. student holds a juvenile Lumbricus Rubellus, one of the most damaging worm species to the forest floor. (The Minnesota Star Tribune) The first to arrive were European earthworms. Settlers brought them along in the dirt of potted plants – both live worms and eggs that hatched later, said Frelich. St. Paul and Minneapolis were likely the epicenter of that early invasion.

Some ships also used bags of soil as ballast. They then would ditch the dirt – worms and all – in the port where they landed, he said. That included Great Lakes ports such as Duluth.

European earthworms made similar incursions even earlier in New England. But their spread is much more advanced in Minnesota (and nearby states like Wisconsin and Michigan) because of the wide distribution of so many lakes and streams, Frelich said.

That geography compounded the situation when anglers in Minnesota began using worms as bait back in the 1930s.

"People took fishing bait into every remote corner of the state, didn't use it all and ended up dumping it on the shore, because everybody thought earthworms were good for the environment," Frelich said.

Introduced near thousands of lakes, the creatures spread out from the shores, covering the state's entire landscape in just decades.