The Trouble with Worms
GET TO KNOW
Common European Earthworms & the New Asian Jumpers
American gardeners tend to believe that earthworms are a wonderful way to improve soil, but this is not always the case. Often called “soil architects,” earthworms are essentially soil processing machines. However, each species of earthworm has evolved to generate soil consistent with the needs of the plant community in its native range. Therefore, worms are not general purpose – “one worm fits all” – soil magicians.
No worms are native to Michigan. The glaciers that covered this area until about ten thousand years ago wiped them out and the system that evolved as the glaciers receded did not include them. The nightcrawlers, red worms and any other worms you find are all European or Asian. These worms do not support the native ecosystems of the Great Lakes regions, and have been found to be harmful to our northern forests.
WARNING FACT: Plant and animal nicknames are often very misleading and do not provide reliable information regarding the origin of a species. For example, Asian jumping worms are being sold on the internet under various names including: “Alabama Jumpers,” “Georgia jumpers,” “crazy worms,” and “snake worms.”
Because Asian jumping worms are known for surviving an extended period of time underwater, they have become a favorite bait for fishermen. Please spread the word and ask the fishermen you know to avoid these worms.
Northern forests across the US and Canada are dependent on the presence of a stable and substantial layer of decaying leaf litter. This surface layer of plant litter, also known as duff, provides habitat for the many species of microorganisms, small creatures, and forest floor plants that contribute to the overall balance of the ecosystem. When earthworms are artificially introduced to such communities, they speed up the decay of leaf litter. Eventually there is a breakdown of the duff, and many researchers believe that ensuing die-offs of native species are the direct result of earthworm introduction. Unfortunately, once the native community dies-off, a pathway opens for colonization by invasive plant species.
Research has shown that once earthworms are introduced to Great Lakes region forests, they “come into an area with a thick organic mat, and two to five years later that layer is gone.” (per Peter Groffman, from the Cary Institute of Ecosystem Studies. View Here
European nightcrawler move slowly and dig deep tunnels. In heavily compacted soil, nightcrawlers may help with aeration and transportation of water/nutrients. However, in some cases, worm tunnels and castings (excrement) increase erosion, compact the soil, or deliver the wrong mix of nutrients.
As if that isn’t enough of a problem, several species of Asian earthworms nicknamed “Asian jumping worms,” have arrived in this country and just been found within our watershed in Southfield. They are known to slither rapidly like a high-speed snake, to thrash violently about, and to jump off the ground (or out of containers), thus the moniker “Asian jumping worms.” There are a great many species of Asian worms, but three specific ones (Amynthas agrestis and Amynthas tokioensis, or Metaphire hilgendorfi) have been making their way across the continent and colonizing our northern forests.
Native to the grasslands of Japan, these jumping worms eat faster, grow faster, and reproduce faster than their European counterparts. They can also move faster and travel farther as they spread through new regions including being washed downstream in floods along the river.
Unlike tunnel digging European nightcrawlers, Asian jumping worms live at the surface of the ground and leave their castings on the surface. In their native grasslands, where the dominant plants are Miscanthus grasses and fast-growing dwarf bamboos, the voracious eating habits of the jumping worms play a natural role.
Here in the Great Lakes region, Brad Herrick (University of Wisconsin-Madison Arboretum), specifically studies the effects of Asian jumping worms on northern forest communities. He indicates that the insatiable jumping worms consume leaf litter and damage plant roots at such an excessively rapid speed that castings build up on the ground faster than they can be utilized by plants. He believes the castings are often washed away by rain leading to a net loss of nutrients from the forest and a possible overdose of nutrients to stream communities.
There is little visual difference between Asian jumping worms and common European nightcrawlers. On close inspection, Asian jumpers have a lighter colored clitellum band than nightcrawlers, and the band fits flush with the body surface. On nightcrawlers, the clitellum band is slightly larger (raised from) the rest of the body. (The clitellum band is where earthworms store the very, very, tiny cocoons of new earthworms.) However, the best way to identify Asian jumping worms is by observing their crazy movements when disturbed. See a short, 1 minute, video here.
KEY TO INVASIVE SUCCESS
The most obvious factor in the invasive success of earthworms is that people want earthworms and willingly transport them everywhere. We buy them for gardens and for fishing bait. We grow them for compost piles and gift them in plant exchanges.
Yet, even without our willing assistance, earthworms are very adept at transporting themselves. They reproduce via tiny, egg filled cocoons that are made and stored in their clitellum band. (Depending on the specific species, reproduction may occur after mating or happen asexually.) One earthworm may produce as many as 75 or so cocoons per summer, and each cocoon carries at least one fertilized egg that will develop into a new worm. Since Asian jumping worms are believed to be able to reproduce asexually, the introduction of even one adult worm – or one tiny cocoon – can generate an army of new worms.
Worm cocoons are very tiny (about 1/8th inch long). They start out light in color but darken as they mature until they resemble actual dirt. Cocoons are also very hardy and travel well on the soles of shoes, the tires of off-road vehicles, and the gear of fishermen/campers. Additionally, worm cocoons cling to the roots of potted plants and bits of dirt, mulch, and compost in garden supplies. It is, therefore, quite difficult to avoid transporting and spreading the cocoons.
Once earthworms have colonized an area, it is almost impossible to eradicate them. Consequently, it is important to limit accidental spreading of both worms and cocoons to new environments by taking extra care when transporting equipment and gardening supplies. In order to slow the spread of worms please:
- Do not intentionally introduce worms to new areas.
- Do not dump bait worms in the water or on the ground.
- Do not transport firewood between areas.
- Do not trade plants between different geographic areas and limit local plant trades to bare root plants.
- Carefully clean boating, camping, and hiking gear before moving it between locations.
Overall, the spread of invasive species is directly tied to human activity. We move about continually – here, there, and everywhere – and, in so doing, we transport the tag-a-longs to places they don’t belong. If we are ever to stop the “invasions,” we must learn to slow down and take a bit more care in our travels. Hopefully then we may reclaim the many distinctive natural environments of our world.
CBC News Article, Aug 29, 2021: Invasive earthworms are remaking our forests, and climate scientists are worried
Main photo credit: Asian Jumping Worm with smooth, (Identifiable by the light colored clitellum band) – photo by Donald Drife