Earthworms are not native to much of North America. If there were earthworms in the past, glaciation probably killed them off. In southern regions, native earthworms are greatly outnumbered by invasive species brought here by European settlers. Now, earthworm species from Asia are a new threat. It’s a problem not just in North America, but around the world.
Earthworms are commonly used as bait for fishing, and this is one way they spread — leftover worms dumped on the soil will survive and thrive. Gardeners who use earthworms for composting also spread them; using species that can’t survive in the wild is a safer option.