Last week, I came across several “crazy snake worms” happily wriggling through fallen leaves in the landscape. This was surprising, since the adults are usually gone by this point in the season, killed by hard autumn frost and survived only by their overwintering eggs. However, the unseasonably warm weather this year has prolonged the life cycles of many invertebrates. Unfortunately for the forests of New England, the crazy snake worm is no exception.
“Crazy snake worm” is actually just one of many common names used to refer to three very similar looking earthworm species: Amynthas agrestis, A. tokioensis, and Metaphire hilgendorfi. All three species are native to East Asia and share distinctive traits. For example, rather than slowly crawling like other earthworms, crazy snake worms slither quickly and forcefully and thrash wildly when handled. They are also generally large worms (M. hilgendorfi can reach 8 inches) and have smooth, milky white bands (clitellum) that look like collars. The bands form unbroken circles around the earthworms, as opposed to most European species that have 3/4 circumference “saddle”.
Although crazy snake worms are, like all earthworms, hermaphroditic, they are also parthenogenetic (capable of reproducing without a mate), an uncommon trait that allows them to quickly populate new areas. They also tend to breed faster, produce more young, and live in greater population densities than other earthworm species. And, unlike most European earthworms, crazy snake worms are considered “epi-endogeic” (feeding in both the surface litter and in the first few inches of soil) and are easily transported by humans through the movement of compost, leaf piles, soil, and mulch.
Crazy snake worms are relative newcomers to New England’s entirely non-native earthworm community. While most of the region’s thirty or so earthworm species arrived via European trading vessels or from the horticultural trade during the eighteenth and nineteenth centuries, crazy snake worms came to the United States as fishing bait and reached New England only within the last few decades.
In this short period of time, they have earned their reputation as highly destructive invasive organisms. They produce an enzyme that efficiently breaks down the lignin in leaves, and feast upon the leaf layer or “duff” found on mature forest floors. Many native plants depend on duff to regulate soil temperatures, cycle nutrients, and provide a moist, safe place for seed storage and germination. Acer saccharum (sugar maple), A. rubrum (red maple), and Quercus rubra (red oak) are three common New England trees whose seedlings are less successful in areas where crazy snake worms are present and the duff is destroyed.
Crazy snake worms are also a factor in the decline of native herbaceous plants, including Erythronium americanum (trout lily), Viola sorroria (common blue violet), Trillium spp. (trillium), and Polygonatum spp. (Solomon seal). In general, areas with large crazy snake worm populations have less native plant biodiversity and less native understory vegetation. However, some plants benefit from the changed soil chemistry and nutrient availability that result from rapid leaf decomposition, including non-native species with invasive tendencies like Berberis thunbergii (Japanese barberry), Rhamnus cathartica (common buckthorn), and Rosa multiflora (multiflora rose).
In short, by eliminating duff and altering soil composition, crazy snake worms are changing the plant communities in New England. The soil conditions they create favor non-natives over native species, making it less likely for many native plants to survive even if all invasive plants could eventually be removed. For most trees in the Arboretum landscape, which grow amid organisms far removed from those of their native habitats, the earthworms’ threat may be minimal. However, seeing crazy snake worms active on a warm November afternoon is still a stark reminder of the potential challenges facing all plants in our landscapes—challenges as large as climate change and as small as an earthworm.