I grew up gardening on the farm. My mother taught me that seeing earthworms was a sure sign of good, workable soil. I took her at her word, and never considered that the earthworms I was seeing might be an invasive species.
Unfortunately, the earthworms we commonly see in mid-Michigan are mostly invasive types. The problems they cause are especially serious in wooded areas, where their deep burrowing and feeding significantly change the soil, and over time, cause a reduction in the understory plants. This, in turn, affects birds and small mammals, insects and other woodland life.
Earthworms have been in Michigan for hundreds of years, coming in soils of plants brought with settlers, as well as in soils used as ballast in sailing ships. We continue to spread them today, primarily through moving plants, soils on treaded tires of ATVs and tractors moved without first washing down the vehicles, and through discarding night crawlers and other bait worms out in nature.
It is important to note that the Red Wigglers (Eisenia fetida) used in vermicomposting are also non-native. They do not over-winter here due to our cold soil temperatures; however they may be contaminated with eggs or adults of species that will overwinter and spread. Keep your vermicomposting bins inside and above freezing, and do not release into the external environment. A newer and highly-concerning import, the Amynthas species or Jumping Worms, are very prolific and active, and present an even greater threat to our woodlands.
A great source of further information can be found at www.greatlakeswormwatch.com . A well-written booklet with excellent photos, for use by older children or adults, is Earthworms of The Great Lakes by Cindy Hale.