Wild Side: Non-native earthworms

These soil critters are not always beneficial in a garden.

An Asian jumping worm is efficient at breaking down compost, but it may quickly become invasive. — Matt Pelikan

A down-Island gardener just brought me a surprise: a pail of compost containing worms the gardener had correctly identified as Asian jumping worms, one of several species in the genus Amynthas that have become broadly established in the Eastern United States. Significantly, this conscientious gardener had no idea how the worms turned up in her compost; they arrived, somehow, on their own.

Smaller than your familiar nightcrawler, firmer bodied, and prone to spasmodic thrashing when poked, Asian jumping worms turn out to be the latest depressing demonstration of the ecological Law of Unintended Consequences.

What’s the concern? It’s important to note that our region did not originally have any earthworms at all. New England was covered with ice and largely scraped down to bedrock by the continental glaciers of the last Ice Age. What is now Martha’s Vineyard began as a vast pile of crushed rock pumped here by flowing ice, mixed with rock and gravel snowplowed here by the leading edge of the ice sheet. The Vineyard started as a sterile heap of sand and rocky debris. That is not worm habitat.

As the climate warmed and successive waves of vegetation spread onto the Vineyard, the Island-to-be was colonized by a range of critters that live in leaf litter — think millipedes, centipedes, and mites — helping break down organic matter into more basic forms that could be re-used by plants. Originally, our ecosystems evolved in response to the particular patterns of breakdown that these native “reducers” created.

But with the introduction, both intentionally and haphazardly, of species like the European nightcrawler (New England has about 20 introduced earthworm species), things changed. Gradually infiltrating natural habitats beyond their original agricultural settings, such worms proved to be highly efficient at breaking down organics. That’s a good thing in some circumstances, such as a garden where rapidly recycling nutrients is desirable. But it dramatically altered the condition of natural areas, producing a depleted top organic layer on top of the soil and altering the understory composition in woodlands in ways that biologists can’t even completely reconstruct. A boon for horticulture, introduced worms were a slow-moving catastrophe for our woodlands.

Asian jumping worms are, then, another chapter in an old book. The concern is that they are even more efficient at breaking down organics than nightcrawlers are. This is great for composting and garden soil health! But if these worms become abundant in a natural area, their impacts on soil composition are extreme and fast, giving wildlife little time to adapt to the changes and driving the system toward lower diversity.

There’s no question in my mind that small-scale agriculture is a good thing, and that composting and other measures to manage nutrients and promote soil health are essential. And given the crop species and growing methods our food production system has settled on, it appears that non-native worms (like the honeybee, another sometimes invasive species essential to agriculture) are here to stay.

Several invasive species experts I spoke with about “crazy worms” expressed pessimism that this new invader can be eliminated, or even meaningfully controlled. These worms are already widespread in the East, with adults and the much more resilient eggs traveling readily in potted plants and other horticultural products. These worms, along with other insufficiently studied worm species, are mostly unregulated and hence are readily available over the internet. The horse, this is to say, is out of the barn.

If you compost, be careful about adding new organisms to accelerate your composting process. A little poking around on the web shows that multiple species of non-native worms are readily available by mail order, and — more disturbingly — that the actual identity of what is being sold often can’t be determined, perhaps even by the people doing the selling. Various ecological constraints, particularly the cold temperatures the Vineyard experiences during winter, provide some protection against the establishment and spread of exotic worms. But like all organisms, non-native worms are resourceful in finding ways to flourish.

Particularly if you’ve deliberately introduced non-native worms into your compost, and especially if a demonstrably harmful worm like Amynthas has arrived there by any means, I’d urge you to keep your compost on-site, apply it only to cultivated areas, and not sell or give it away to others. Achieving high temperatures while composting may kill Amynthas cocoons. Asian jumping worm adults can be collected and killed by dropping them into soapy water or putting them in the freezer. Doing so consistently might allow you to eradicate your population, if you have one.

Want to learn more? Good for you! Here’s a University of Massachusetts website with information on the larger issue of earthworms generally: bit.ly/massworms. And here’s one specifically on Asian jumping worms: bit.ly/asianworms. Read up, pay attention, and do what you can to avoid making a bad situation worse.