“Why would anyone want to destroy earthworms?” we asked in our July article “Advice from 1683: Killing Earthworm Pests with Tobacco.” In the 1600s it was to keep unsightly worm castings from blemishing gravel paths, but today there’s a different reason.

“In northern parts of the country, earthworms are not native,” our good customer Robin Schachat of Shaker Heights, Ohio, explains. “It’s now thought that the greatest loss of the duff layer in our native forests is due to earthworm infestations, not to deer browsing.”

Although worms can be beneficial in the disturbed and compacted soils of our backyards, since the last glaciers retreated some 14,000 years ago there have been no native worms in the area once covered by them —north of NYC and the Ohio and Missouri rivers, and in the Rockies and parts of Washington — so in those areas forest ecosystems evolved without worms.

Recent research at the University of Minnesota has shown that “without worms, fallen leaves decompose slowly, creating a spongy layer of organic ‘duff.’ This duff layer is the natural growing environment for native woodland wildflowers. . . . Invading earthworms eat the leaves that create the duff layer and are capable of eliminating it completely. Big trees survive, but many young seedlings perish, along with many ferns and wildflowers.”

Although there are native earthworms in unglaciated parts of the country, if you live further north we encourage you to learn more at the Invasive Earthworms page of the Minnesota DNR and at the Great Lakes Earthworm Watch.