Invasive Earthworms A Threat to North American Forests

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Invasive Earthworms A Threat to North American Forests Plants & Gardens News Volume 19, Number 1 Spring 2004 by Niall Dunne Charles Darwin was an earthworm freak.

1 Invasive Earthworms A Threat to North American Forests Plants & Gardens News Volume 19, Number 1 Spring 2004 by Niall Dunne Charles Darwin was an earthworm freak. He spent more than 40 years, on and off, observing, experimenting on, and thinking about these artful annelids. It got a little weird sometimes, like when he had his son play the bassoon to an audience of worms in the billiard room. Darwin published his findings in a slim volume entitled The Formation of Vegetable Mould through the Action of Worms with Observations on their Habits (1881). In it, he outlined the monumental importance of earthworms to the development of soils describing how they help plow, aerate, hydrate, and fertilize the earth. "They mingle the whole intimately together," he wrote, "like a gardener who prepares fine soil for his choicest plants." Darwin's "vermiphilia" lives on today in the hearts of gardeners and farmers who embrace a biologically integrated approach to plant cultivation. But, regrettably, his idea that all earthworms are fundamentally beneficial is a bust. Some species, when placed in the wrong context, can become outright pests. Indeed, a number of nonnative earthworm species have established themselves in North America and been implicated in a range of undesirable activities, including the extirpation of at least one rare plant species. Gardeners need to pay special attention, as they have been identified as one possible source of introduction for some of these troublesome interlopers. An earthworm-invaded site (left) and a non-earthworm-invaded site (right) in the Chippewa National Forest in Minnesota. Earthworms are linked to the absence of tree seedlings in parts of the forest. American Natives In Darwin's defense, he was aware that earthworm activity could cause problems such as soil erosion due to washed-away castings. Moreover, his study was limited to mainly pastoral lands in England and probably to a narrow range of earthworm species. There are, however, thousands of earthworm species worldwide, adapted to many different environments. North America has roughly 100 native species. While this doesn't seem like a mind-blowing number, it's pretty impressive when you consider that Pleistocene glaciation killed most worm species in the northern half of the continent. Because earthworms are extremely slow moving, colonization of the formerly frozen areas has been negligible. Our native earthworm fauna is still confined mostly to the Southeast and Pacific Northwest.

2 Foreign Arrivals Since Colonial times, though, about 45 exotic earthworm species have made a home for themselves in North America. These include hardy European and Asian species that can survive the cold, unforgiving winters of our higher latitudes, many of them introduced through the importation of potted plant material for horticulture and land management. Today, there are strict laws prohibiting soil imports, but exotic earthworms are still gaining access to the country via the vermiculture industry, which imports worms for fish bait, bioremediation, and composting. (See "What's In My Worm Bin?") Exotic earthworm populations are now established in many of our urban, suburban, and agricultural soils. They've also managed to hunker down in wild areas thanks in large part to the misguided dumping of bait by fishermen. And this is the crux of the problem: the addition of worms to forests and grasslands that have evolved for millennia without them. The Diet of Worms Hardwood forests in the North are being hardest hit by the pest-earthworm invasions. The most immediate threat is from "epigeic" earthworms, which inhabit and feed on leaf litter on the soil's surface. In healthy, undisturbed forests, a rich layer of litter covers the forest floor and is held together by webs of fungal filaments. The fungi, along with other decomposers, slowly break down the litter and release nutrients to forest plants the key word here being "slowly." Soil ph is low, and the native plants are generally adapted to acidic conditions. The leaf litter is not just a nutrient bank; it's the medium in which forest plant seeds germinate. It also acts as a mulch, insulating the soil and protecting plants from disease and competition with weeds. In addition, it provides habitat for many small forest animals. Invading earthworms feed voraciously on the leaf litter, breaking it down too fast and flooding the soil with nutrients, especially nitrogen. (Much of this valuable stored nitrogen may eventually be lost via runoff.) Some worm species can also neutralize the soil ph with special calciferous glands to create a more favorable environment for themselves. Both actions dramatically change the soil chemistry in the forest and interfere with plant growth. Also, as the leaf litter is consumed, bare patches appear on the forest floor, making it vulnerable to erosion and to invasion by nitrogen-craving weed plants. Leaf-litter animals are deprived of their habitat. If burrowing worms are present, a harmful mixing of soil strata can occur. Case Studies Some of the most compelling evidence of this phenomenon has come out of the Chippewa National Forest in Minnesota. Back in 1996, researchers discovered a link between the absence of tree seedlings and spring wildflowers in the forest and the presence of six or seven exotic earthworm species in the soil, and they've been studying it ever since. Cindy Hale of the University of Minnesota-Duluth has spearheaded the research and even set up a web site called Minnesota Worm Watch ( to raise public awareness about the invasion problem. The site contains dramatic images of the damaging effects that European worms like Dendrobaena octaedra and Lumbricus terrestris can have on the leaf litter, soil profile, and plant health of the forest.

3 In 2002, Michael Gundale of Michigan Technological University published a report detailing how the epigeic bait and compost worm Lumbricus rubellus may be wiping out populations of the rare goblin fern, Botrychium mormo, and possibly other rare native plants too, in the Chippewa National Forest. Gundale credits the epigeic worm's destruction of mycorrhizal fungi in the soil as a reason for the goblin fern's decline. John C. Maerz and colleagues from Cornell University have found strong evidence linking salamander decline in the hardwood forests of central New York and southeastern Pennsylvania to invasions by L. rubellus and Asian Amynthas species, among others. Maerz says that by eroding forest leaf litter and humus layers, the earthworms are driving down arthropod numbers and depriving juvenile salamanders of a key food supply. The Cornell team has also found strong links between exotic earthworm infestations and the invasion of forests by pest plants such as Japanese barberry (Berberis thunbergii). This possibly symbiotic connection has also been noted in New Jersey by researchers from Rutgers University. Possible Solutions Scientists, land managers, and conservationists are in a quandary over how to address the invasive earthworm issue. Experiments to find viable means of controlling earthworm invasions are under way. Dennis Burton, director of land restoration at the Schuylkill Center for Environmental Education in Pennsylvania, has had some success excluding Amynthas species from test plots using sulphur and pine needle mulch (which lower soil ph) and black walnuts (which are high in phytotoxins). But it remains to be seen if any control methods can be developed for viable large-scale forest restoration, restoration that doesn't threaten collateral damage to other organisms. Given the slow migration of earthworms in soil, however, most experts agree that containing them is of lower priority than preventing new introductions. This means developing stricter laws regulating the importation of worms (which the USDA is considering) and getting the word out to the public. "If you fish, do not dump your extra worms out on the ground," urges Burton. "Throw them entirely in the water or take them home. If you plant new trees and shrubs on your property, examine the root balls for worms and destroy any you find." His advice to gardeners makes sense (especially to those who live near forested areas) because, although there might be controls on the importation of contaminated soils from outside the U.S., there are few regulations addressing the movement of soils within the country. It's conceivable that potted ornamental plants from nurseries in earthworm-infested areas are acting as vectors for new invasions. What's in My Worm Bin? With invasive earthworms wriggling amok in our forest soils, gardeners who use worms to decompose kitchen scraps and plant waste may want to take a closer look at what they ve got growing in their compost piles. Some of the traits that make worms ideal for vermicomposting such as high reproductive rate and adaptability may also make them potentially successful invaders. The worm predominantly sold for composting is the red wiggler or red tiger worm, Eisenia fetida. It has a rusty brown color with alternating yellow and maroon bands down the length of its body; a pigmentless membrane separates each segment. It grows up to three inches long and is highly prolific. Though the worm has established itself in the wild here, so far it has not been identified as a problem species.

Another popular compost species, the red worm, Lumbricus rubellus, is causing trouble, however, and should be avoided. It also grows up to three inches long and has a history of being confused with E.

4 Another popular compost species, the red worm, Lumbricus rubellus, is causing trouble, however, and should be avoided. It also grows up to three inches long and has a history of being confused with E. fetida. This worm is dark red to maroon, has a light yellow underside, and lacks striping between segments. In The Earth Moved (Algonquin Books, 2004), a wonderful new book on earthworms by Amy Stewart, forest ecologist Cindy Hale advises worm composters to freeze their castings in air-tight bags for a least a week before adding them to garden soil, no matter what worms species they use. "It won't hurt the soil microbes, but it will kill all the worms." Niall Dunne is the associate editor of Plants & Gardens News Washington Avenue, Brooklyn NY Copyright 2008 BBG Contact BBG Directions Conditions of Use Privacy Policy Accessibility Statement Site Map Earthworms Contain Those Crawlers. This poster explains the harmful effects of earthworms on hardwood forests. Available in print and as a PDF (1 Mb). What's the big deal about earthworms in Minnesota? All of the terrestrial earthworms in Minnesota are non-native, invasive species from Europe and Asia (There is a native aquatic 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. See "What are the harmful effects of non-native earthworms" below for more information. 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.

The first earthworms probably arrived with soils and plants brought from Europe.

5 There are over 100 species of native North American earthworms in unglaciated 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. 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." What are the harmful effects of non-native earthworms? 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. 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. Some species return after the initial invasion, but others disappear. In areas heavily infested by earthworms, soil erosion and leaching of nutrients may reduce the productivity of forests and ultimately degrade fish habitat. 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.

6 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. However, when earthworms invade they actually increase the compaction of hardwood forest soils. Compaction decreases water infiltration. Less infiltration combined with the removal of the duff and fallen tree leaves results in increased surface runoff and erosion. If non-native 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 are not known to survive Minnesota winters. However, if they or other species are able to survive winter 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 economically feasible methods. Preventing earthworm introductions is the best protection. What can I do to help?!" # $$%% For more information on invasive, non-native earthworms and other ways to help, visit Minnesota Worm Watch Written by Andy Holdsworth, Cindy Hale, and Lee Frelich (University of Minnesota Center for Hardwood Ecology) and reviewed by the Minnesota Interagency Exotic Earthworm Team - March &'(!()* **

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Invasive Earthworms in Northeastern Sugarbushes

Invasive Earthworms in Northeastern Sugarbushes Cheryl Frank Sullivan, Bruce L. Parker, Margaret Skinner & Josef Gӧrres, Univ. of Vermont, Entomology Research Laboratory & Plant & Soil Science Dept. Chittenden