I have read about the problem with earthworms invading forests and overeating the duff, forest floor, disturbing the local biosystem, etc.
What about in our Puget Sound area? Is this a threat, and should we be concerned about worms escaping from worm bins and vermicomposting projects?
If you have a worm bin, be sure that the worms inhabiting it are red wiggler worms, Eisenia fetida (the kind which are ideal for vermicomposting), and never dispose of the contents of a worm bin in a wooded area.
According to former UW Botanic Gardens director Sarah Reichard’s book, The Conscientious Gardener (University of California Press, 2011), “where native worms already exist, new introductions […] may overconsume local food sources. But perhaps the most serious problems occur where there are no native worms, such as the northern temperate forests of North America […]. There, the worms modify the soil structure, affecting the flora and fauna.” Reichard recommends that gardeners not use soil with worms in it as fill dirt, and not dispose of nursery material with worms in it, especially in a wooded area. To be cautious, you can freeze your worm bin compost for one to four weeks to kill the worms. Be sure the supplier of your composting worms is knowledgeable about the species of worm they are selling. Reichard says: “…better yet, do not use worms at all: let naturally occurring organisms break down the waste.”
The invasive worm problem is a major issue in places like Minnesota. Here is information about that state’s attempt to prevent earthworm invasions of forest land. Note that they say Eisenia fetida (red wiggler) is safe because it will not survive winter temperatures. Our winters are not as cold, but I did not find any documentation to suggest that red wiggler worms are a problem in the Pacific Northwest.
Lady Bird Wildflower Center (in Texas) replied to a question similar to yours:
Excerpts:
“The bottom line is that in the far northern deciduous forests there is great concern over the presence of exotic species of earthworms because of their effect on the soils; while in the unglaciated southern regions, although there is some concern, the impact of the exotic species on soil processes is of minimal concern.
“The habitat and available food will define where exotic species can live. Eisenia fetida lives in surface soil and its food is leaf litter, microbes, and soil with high organic content. Hendrix and Bohlen (Hendrix, P. F. and P. J. Bohlen. 2002. Exotic Earthworm Invasions in North America: Ecological and Policy Implications. BioScience V. 52, no. 9, pp. 801-811) say: ‘Earthworms species from northern latitudes (e.g., European lumbricids and some Asian megascolecids) are poor colonizers in tropical or subtropical climates (except in localized temperate situations), and vice versa. For example, despite continued and widespread introduction throughout the United States, Eisenia fetida, the lumbricid “manure worm” commonly used in vermicomposting, is not often found in natural habitats in the southern United States.”
As far as I know, the only endangered native earthworm species in our state is the Giant Palouse Earthworm. Nevertheless, it’s prudent not to release your vermicomposting worms into the wild anywhere.