Invasive Earthworms in Vermont
There are no indigenous earthworms in Vermont. Native earthworms became extinct during the last ice age as mile-high glaciers rested on the New England landscape. Generally, earthworms present in agricultural settings are thought to have a positive effect on soil health. However, in the forested ecosystems that developed in the absence of earthworms since the ice age, earthworms alter soil and ecosystem structure. They remove the top, organic soil layer and thus the seed bank and germination medium for many understory plants. As the understory diversity is decreased, deer will begin to consume tree saplings so that at advanced stages of earthworm invasion, the forest understory is absent or depleted or invaded by invasive plants. And, as a result forest regeneration slows or even ceases. As such earthworms should be placed on a par with other forest pests. The reason why this has not happened yet is because earthworms are regarded as positive contributors to ecosystem processes and damage done may not be attributed to earthworms; earthworms do not directly affect tree species but cause damage to silviculture through influencing ecosystem processes; damage does not leave an immediate impression because changes are subtle in the beginning.
Types of Earthworms Found in Vermont
Most earthworms found in the Green Mountain State are from Europe, but newer arrivals include earthworms from Asia. The first post-glacial earthworms in the New England arrived with early European settlers either in ship ballast or with plant material. Settlers that colonized the American continent spread European earthworms, e.g. the Common Nightcrawler or Dew Worm (Lumbricus terrestris) from the early coastal colonies to many locations in the New World. Nowadays, fishing and horticulture are probably the most active vectors of earthworm invasions into New England forests although hiking and off-roading have also been mentioned as significant pathways of invasion.
Not all exotic earthworms are invasive. For example, gardeners who compost have argued that the Red Wiggler (Eisenia foetida) is likely not invasive because it is not often found in the Wild. It is argued that Red Wigglers cannot withstand the cold Vermont climate. However, some populations of this species are found in cold climates, e.g., in the Himalayans and around the White Sea in northern Russia. There are a couple of other composting worms that are regarded as non-invasive.
The Malaysian Blue (Perionyx excavatus) (see photo) is the vermicomposter of choice for tropical and sub-tropical regions. The adult Malaysian Blues die quickly in cold weather.
Likewise Crazy Snakeworm, a.k.a. jumper worms, Jersey Wrigglers (Amynthas spp.) die in the wintertime but there are actually significant populations in the Northeastern USA. In Vermont, Amynthas agrestis (see photo) are associated with deciduous remnant forests, mulched horticultural installations, compost heaps and community gardens. In Georgia they are reaching into the remote forests of the Blue Ridge Mountain National Park. The example of the Crazy Snakeworm shows that the cocoons (egg casings) of earthworms can survive cold temperatures and ensure the persistence of a population of invasive earthworms from one year to the next.
What You Can Do About Them
Fact is that there is not much evidence that supports statements about invasiveness of particular earthworm species. Erring on the side of caution may be important when looking at sustainable woodland management. One of the most effective ways of protecting your woodland from earthworm invasions is to prevent the invasions. Control food and off-road traffic through them. Make sure that horticultural products are not stored or dumped near or in the forest.
Invasive Earthworms in Vermont- A PowerPoint presentation by Josef Gorres, UVM Assistant Professor