European earthworms in Minnesota
What's the big deal about earthworms in Minnesota?
All of the terrestrial earthworms in Minnesota are non-native species from Europe and Asia. (There is a native species that woodcock eat.) At least fifteen non-native terrestrial species have been introduced so far. Studies conducted by the University of Minnesota and forest managers show that at least seven species are invading our hardwood forests and causing the loss of tree seedlings, wildflowers, and ferns.
Why aren't there native earthworms in Minnesota?
We have no evidence that earthworms ever inhabited Minnesota before European settlement. Even if they did, the glaciers killed any native North American earthworms in our region. For the last 11,000 years since the glaciers receded, Minnesota ecosystems developed without earthworms. There are many species of native North American earthworms in such unglaciated areas as the southeastern states 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.
How did the 15 earthworm species get here?
The first earthworms probably arrived with soils and plants brought from Europe. Ships traveling to North America used rocks and soil as ballast which they dumped on shore as they adjusted the ballast weight of the ship. During the late 1800's and early 1900's many European settlers imported European plants that likely had earthworms or earthworm cocoons (egg cases) in their soils. More recently, the widespread use of earthworms as fishing bait has spread them to more remote areas of the state. All common bait worms are non-native species, including those sold as "night crawlers", "Canadian crawlers", "leaf worms", or "angle worms".
Much of the bait that is sold contains a mix of species in it. For example, when you buy night crawlers (Lumbricus terrestris) (Figure 1) you also often get , Lumbricus rubellus (Figure 2), one of the most damaging species to the forest floor of sugar maple forests. There are also many species of earthworms (including L. rubellus) available for sale over the internet. Introducing any non-native species-including earthworms--into the wild in MN is illegal.
What are the harmful effects of nonnative earthworms?
Earthworms are ecosystem engineers. They alter the rate at which nutrients are recycled in the forest ecosystem, and change the physical structure of seedbeds on the forest floor. These are major changes that will cascade through the ecosystem and affect every species of plant and animal in Minnesota's forests. European earthworms will change forest productivity and determine the species of plants and animals that will live in the forest in the future.
Minnesota's hardwood forests developed in the absence of earthworms. 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 (Figure 3). It also provides habitat for ground-dwelling animals and helps prevent soil erosion.
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 (Figure 4). Some species return after the initial invasion, such as Pennsylvania sedge (Carex pennsylvanica) and Jack-in-the-pulpit (Arisaema tryphillum) (Figure 5), but others disappear. During the initial invasion, much of the standing crop of plants is killed. Afterwards, those few plants left are subject to grazing by deer, which can lead to complete extirpation of forest herbs within a given woodland. There is then no seed source for the plants to recover, even if they would otherwise be able to recover.
In areas heavily infested by earthworms, soil erosion and leaching of nutrients may reduce the productivity of forests and ultimately degrade fish habitat.
Do all European earthworms species have the same impacts?
Earthworms fall into 3 ecological groups. The epigeic species, are those that live in leaf litter and do not go deep into the soil. They are small and reddish-brown in color, and eat decomposing leaf litter and fungi. Dendrobaena octaedra is one of the most widely spread species in the state. They break up the leaf litter from a solid chunk (mor) to a looser texture (mull). They do not seem to do much damage to native plant species. Dendrobaena is parthenogenetic, meaning it can reproduce and form a population from one individual. This characteristic probably helps it spread rapidly.
Another common epigeic species, Lumbricus rubellus (Commonly called "red worms" or "leaf worms", Figure 2), actually consumes the duff layer quite fast when it invades. This species has been shown to contribute to the disappearance of native plant species, especially the endangered goblin fern (Botrychium mormo) (Figure 6), but it is likely that other species such as bellwort, trillium, blue and yellow violets, spikenard and bloodroot are greatly reduced in abundance when this species invades (Figures 7-9). Of particular concern are species that rely on associations with fungi, and species that depend on decaying duff, such as coral root orchids.
Several endogeic species of worms, often called "angle worms" (Figure 9), live in the soil and rarely come to the surface unless it is dark or rainy. This group includes the narrow-bodied species Apporectodea rosea, which is pale in color with a bright pink head at one end. It is very common in city lawns and shows up on sidewalks after the first warm rain in spring.
There is one anecic, or deep burrowing species in MN-the night crawler-Lumbricus terrestris (Figure 1). Night crawlers can go up to 6 feet into the soil, and they primarily eat fresh leaf litter that falls to the ground. In many hardwood forests, night crawlers consume most of the leaves that fall in October by mid-summer, leaving the soil exposed to the erosive action of rain drops.
Aren't earthworms good for soil and gardens?
It depends. Earthworms create a soil of a certain consistency. For soils that are compacted due to heavy use by agriculture and urbanization, for example, earthworm tunnels can create "macro-pores" to aid the movement of water through the soil. They also help incorporate organic matter into the mineral soil to make more nutrients available to plants. However, in agricultural settings earthworms can also have harmful effects. For instance, their castings (worm excrement) can increase erosion along irrigation ditches. In the urban setting, earthworm burrows can cause lumpy lawns.
Relative to simplified ecosystems such as agricultural and urban/suburban soils, earthworm-free hardwood forests in Minnesota have a naturally loose soil with a thick duff layer. Most of our native hardwood forest tree seedlings, wildflowers, and ferns grow best in these conditions.
The increased bulk density of soils caused by earthworms can lead to less infiltration of rain-fall water, which hits directly on the hardened crust formed by earthworms as they work the soil. This means drier soils and more surface erosion. There might also be more leaching of nutrients.
We have noticed that wildflowers such as trillium do not grow as large and spread as fast in a garden with earthworms as in another garden without them. The high density of the soil when earthworms are present seems to contribute to this problem. There is also evidence that mycorrhizal fungi are less abundant in areas with earthworms, and many North American plant species require mycorrhizae for optimum growth.
If nonnative earthworms are already here, isn't it already too late?
No. Without humans moving them around, earthworms move slowly, less than a half mile over 100 years. If we stop introducing them we can retain earthworm free areas for a long time. Also, there are many other non-native earthworms available for sale that could have even more harmful effects. Even in areas with earthworms already present, we don't want to risk introducing any of these other species.
What about worms in compost piles?
Non-native "red wiggler" earthworms are sold and shipped all over the country for home compost piles and vermicomposting (worm composting) operations. Thus far, they have not been shown to survive Minnesota winters. However, if they or other species are able to survive Minnesota winters and escape from compost piles they could further harm native forests. If you have a compost pile in a forested area, do not introduce additional non-native earthworms. If you are concerned about spreading non-native worms with your compost, you can kill worms and their eggs by freezing the compost for at least 1 week.
Can earthworms be eliminated from forests?
Currently there are no chemical methods. Chlordane was used to remove earthworms from lawns, but is now banned as a toxic chemical.
Preventing earthworm introductions is the best protection. Birds such as robins eat earthworms, but are unable to control earthworm populations. There are some natural predators, such as the New Zealand flatworm, which is causing extirpation of earthworms in their native habitat in England. However, its introduction here is controversial, because it is not known if it would harm other native species.
The pH of the soil and the palatability of the decaying organic matter on the soil surface does influence the abundance of earthworms in your garden. For instance, soils under spruce or pine trees are more acidic and thus have fewer earthworms. This may be a better location for a northern native flower garden. On the other hand, areas under maple and basswood trees have leaf litter that earthworms consume voraciously, thus exposing the bare mineral soil. Since earthworms cannot consume wood chips, they may be a better mulch for native woodland flower gardens with high earthworm populations. This is an important area for future applied research.
What can I do to help?
Don't transplant plants and trees into remote wooded areas where there are not any earthworms at this time. Dump your extra fishing bait in the trash. Remember, anything that moves dirt can move worms, so use good "hygiene" when recreating and traveling through remote areas. Tell others "the dirt" on invasive earthworms in Minnesota.
For more information on nonnative earthworms and other ways to help, visit Minnesota Worm Watch.
Published in Yard & Garden Line News Brief, March 1, 2003