Assuming it is able to adapt to the environment, the tendency to populate and consume in excess is what usually results in harm to the ecosystem. This short video from National Geographic outlines the impact of various types of invasive species and how they harm the environment of non-native territories.
When provided with optimum conditions, they can quickly reach population capacity within a worm bin and begin suffering from limited food resources. However, compost worms have not demonstrated cold tolerance that scientists believe to be essential to thriving in an invasive manner.
A study published in the American Midland Naturalist found evidence that red wiggler eggs did not survive well in winter conditions, limiting the likelihood of the species becoming invasive (source). I have read studies where it was suspected that their tendency to populate and consume in excess could result in limited resources for other species.
Invasive issues aside, composting worms provide an excellent method of dealing with discarded food scraps. While we often think of this as a backyard hobby, worm farming can be put to use on a massive scale to offset landfill waste of kitchen scraps.
An airport in North Carolina, for example, composts leftover food from travelers using almost 2 million red wiggler worms (source). Compost worms pose the potential to be invasive due to their tendency to reproduce and consume in excess.
Given the abundance of natural prey, however, along with an inability to thrive in colder conditions, it is unlikely that they will pose a threat to an ecosystem where they are transplanted. Responsible management of red wigglers is recommended to lessen the likelihood of invasive behaviors in local areas.
So if you are worried that your compost worm farm is going to wreak havoc on the ecosystem, you can likely take a deep breath and relax. Even if they did, the glaciers killed any native North American earthworms in our region.
There are over 100 species of native North American earthworms in glaciated areas such as the southeastern U.S. and the Pacific Northwest. However, native species have either been too slow to move northwards on their own or they are not able to survive Minnesota's harsh climate.
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. In areas heavily infested by earthworms, soil erosion and leaching of nutrients may reduce the productivity of forests and ultimately degrade fish habitat.
They also help incorporate organic matter into the mineral soil to make more nutrients available to plants. For instance, their castings (worm excrement) can increase erosion along irrigation ditches.
Most of our native hardwood forest tree seedlings, wildflowers, and ferns grow best in these conditions. Less infiltration combined with the removal of the duff and fallen tree leaves results in increased surface runoff and erosion.
If we stop introducing them we can retain earthworm free areas for a long time. Jumping earthworms (also known as Months worms) are a new species starting to spread in the Great Lakes states.
However, if they or other species are able to survive winter and escape from compost piles they could further harm native forests. See the brochure A B C's of Composting with Earthworms Safely by Great Lakes Worm Watch for more info.
Red worms are excellent at breaking down decaying organic materials, creating a nutrient-rich substance for your garden. Placed directly into a garden with nothing for them to eat, red worms will likely die or move to other areas where they can find food and an environment more suitable for their needs.
They thrive in shallow environments such as plastic bins, fitted with worm bedding, a little soil and the same types of matter you would use in a standard compost bin: yard clippings and plant-based kitchen scraps. If you see worms atop the lawn or garden surface at night, it's these earthworms or nightcrawlers feeding upon decaying matter.
Create a trench near the garden plants 6 to 8 inches deep -- it can be any length or width. Shred a bunch of corrugated cardboard or newspaper to create a moisture-absorbing layer for the trench, then add some partially decomposed plant matter from the yard.
Add some kitchen vegetable scraps, red worms and a layer of cardboard or yard matter atop that. The worms will enjoy their new environment and recycle the decaying matter into nutrients for your garden plants.
A simpler version involves placing decaying plant scraps around the garden, then adding worms, but this may be unattractive. Add crumbled strips of newspaper and corrugated cardboard as worm bedding, a small bit of soil to help the worms' digestion and then scraps from yard plants and plant-based kitchen scraps.
On that note, I’d like to take the opportunity to say thanks to my good friends Mark (“from Kansas”) and Allison Jack for helping to get this topic back on my radar screen. Both of them have been in contact with researchers in this field, and they were able to get me pointed in the right direction, so I could learn a lot more myself.
As luck would have it, I happen to live about 15 minutes from a world-leading earthworm researcher and worm ID specialist, Dr. John Reynolds. As soon as the invasive issue ended up on my radar screen I decided to get back in touch with Dr. Reynolds to chat about potentially sending in some worms for identification (more on that in a minute).
What’s intriguing is the fact that I’ve come across multiple mentions of key “surveys” examining the presence (or lack thereof) of L. rubella in samples obtained from worm farms in Australia, Europe and North America. The picture with L. rubella is somewhat confused, the report in the Permaculture Technology book on the survey on worm farms dates back to the 1980s and is no longer valid.
For instance, an earthworm farm in Illinois run by Bill Crater. My own sneaking suspicion is that L. rubella is not employed in typical vermicomposting situations quite as much as some people might think.
According to Dr. Reynolds, separating L. rubella from E. fetid is “not difficult” based on “a couple of characters which can be seen with a magnifying glass, even if they are not mature.” (including common features like coloration, and morphology of tail region). As such, I think there needs to be a lot more focus on educating people about the differences between these species.
African Nightcrawlers (Surplus Eugenia) blue Worms (Period excavates) I’ve had Blue Worms in outdoor systems previously, and while they certainly did well during the summer, once cooler temps of fall arrived their activity declined rapidly, and they started to disappear.
Policy and management responses to earthworm invasions in North America. Evidence for human-mediated dispersal of exotic earthworms: support for exploring strategies to limit further spread.
Exotic earthworm effects on hardwood forest floor, nutrient availability and native plants: a masochism study. Earthworms (Oligochaeta: Lumbricidae, Sparganophilidae) of the Atlantic Maritime Eco zone.
The earthworms (Oligochaeta: Acanthodrilidae, Lumbricidae, Megascolecidae and Sparganophilidae) of northeastern United States, revisited. The earthworms (Oligochaeta: Acanthodrilidae, Eudrilidae, Glossoscolecidae, Lumbricidae, Lutodrilidae, Ocnerodrilidae, Octochaetidae, Megascolecidae and Sparganophilidae) of southeastern United States.