Invasive Earthworms in Northern Hardwoods Forests

There are no native terrestrial earthworms in most of the northern U.S. and Canada. During the glaciation of the last Ice Age, any earthworms that might have lived in this region died.[1] Thus, the earthworms that we see here today can all be classified as exotic species. We have recently learned that many of these exotic earthworms are invasive species, that is, non-native species that cause environmental harm.

These findings come as a surprise to many gardeners, who have usually been told that earthworms are beneficial to the soil. In the early 2000s, however, research started to come out about the harmful consequences of exotic earthworms in northern hardwood forests.[2] Where earthworms are present, tree seedlings and native forbs are scarcer. The biggest “problem worms” are epigeic and anecic earthworms that eat leaf litter, which native plants in northern North America depend upon for nutrients and protection from harsh winters. These earthworms turn spongy humus into denser, mineral soil.[3] Soil compaction then increases surface runoff and erosion.[4] Earthworm invasions are also associated with changes in soil chemistry.[5]

Some examples of native plants that are particularly sensitive to earthworm invasions are Aralia nudicaulis, Uvularia grandiflora, Polygonatum spp., Hepatica nobilis var. obtusa, Trientalis borealis, Thalictrum dioicum, Lycopodium obscurum, Acer saccharum, Quercus rubra, Tilia americana, and Amelanchier spp.[6] A few native plants appear to benefit from earthworm invasions, possibly due to reduced competition: Arisaema triphyllum, Maianthemum racemosum, Carex pensylvanica, and Fraxinus spp. My personal impression of forest ecology in western New York is that these latter species are certainly more likely to be found in disturbed and fragmented sites, while the former species – apart from some of the trees – are less common and more likely to be found in relatively undisturbed sites.

Two of the most problematic invaders are Lumbricus rubellus and Lumbricus terrestris, both anecic earthworms (they eat leaf litter but reside below the surface), which also eat fungi around the roots of forest plants. As most native wildflower enthusiasts know, many native species depend on soil fungi for germination and growth. These reddish-hued worms are now quite common, having been used as bait for decades. Since earthworms move only very gradually, researchers infer that present-day populations of these invaders in native forests are largely due to agricultural introductions, ballast dumping, and discarding of bait by fishermen.[7]

Recent research suggests that the problem is even worse than previously thought. According to a recent article in Conservation Biology, earthworm invasion might be the driving factor behind floral invasions of northeastern forests.[8] In woodlands with both “invaded” and “uninvaded” sections (defined by invasive plant species Alliaria petiolata, Berberis thunbergii, and Microstegium vimineum), earthworm biomass was negatively associated with native plant cover and positively associated with non-native plant cover, while (when the previous relationship is fully controlled) native and non-native plant cover were not associated. If this result generalizes to other plant invaders of northeastern forests, it implies that controlling exotic earthworms not only helps native plants directly but could prevent future invasions of exotic plants. It also implies that controlling the invasive plants without controlling earthworms may accomplish little in the long run.

What can we do about invasive earthworms? The first and most obvious solution is to avoid introducing earthworms to the soil. Earthworm-free areas still exist and should be kept that way. Many gardeners practice “vermicomposting,” using worm castings to fertilize garden plants. To avoid introducing worms or their eggs into the garden, vermicompost can be frozen solid for a month before use. We should remember to wash our shoes and tire treads before we walk or drive into a forest. We should avoid introducing soil by planting in natural areas unless we have verified that the soil is earthworm-free (better to use bare-root stock).

Is there anything that can be done to control existing earthworm populations? Earthworms cannot tolerate highly acidic soils. Of course, amending soil in natural areas is not a wise solution. In a small garden patch, it may be viable to use a hot mustard solution to force worms to the surface and collect them.[9] Another possibility is the introduction of a biocontrol. New Zealand flatworms eat earthworms and have been associated with rapid declines in native earthworms in Europe, where the flatworms are a pest.[10] Introducing another exotic species is always fraught with potential downsides, and thus research continues on the costs, benefits, and risks of this approach. In the mean time, avoiding earthworm introductions and reducing other stresses on native plants, such as deer herbivory, appears to be extremely important.

[1] Minnesota Department of Natural Resources, “Invasive earthworms,” Invasive earthworms - Invasive species: Minnesota DNR, accessed January 24, 2010.

[2] Niall Dunne (2004), “Invasive Earthworms—A Threat to North American Forests,” Plants and Gardens News, Brooklyn Botanic Garden: Brooklyn Botanic Garden: How Exotic Earthworms Are Wreaking Havoc in North American Forests, accessed January 24, 2010.

[3] Great Lakes Worm Watch, “How Do Earthworms Affect Forest Soils?,” Great Lakes Worm Watch :: Soil and the forest floor, accessed January 24, 2010.

[4] Minnesota DNR, “Invasive earthworms.”

[5] S.E. Crow, T.R. Filley, M. Mccormick, K. Szlavecz, D.E. Stott, D. Gamblin, and G. Conyers (2009), “Invasive Earthworms and Forest Successional Stage Interact to Impact Plant Litter Inputs and Particulate Matter Organic Chemistry,” Biogeochemistry 92: 61-82.

[6] Great Lakes Worm Watch, “Forest Ecology and Worms: Plants,” Great Lakes Worm Watch :: Plants, accessed January 24, 2010.

[7] Minnesota DNR, “Invasive earthworms.”

[8] V.A. Nuzzo, J.C. Maerz, & B. Blossey (2009), “Earthworm Invasion as the Driving Force Behind Plant Invasion and Community Change in Northeastern North American Forests,” Conservation Biology 24 (3): 966-974.

[9] A.P. Lawrence & M.A. Bowers (2002), “A Test of the ‘Hot’ Mustard Extraction Method of Sampling Earthworms,” Soil Biology and Biochemistry 34 (4): 549-552.

[10] Great Lakes Worm Watch, “What can I do?,” Great Lakes Worm Watch :: What can I do?, accessed January 24, 2010.