It may be only their fourth year in Massachusetts, but as many a Harvard gardener can attest, jumping worms really made themselves at home here last summer. And with the high reproduction rate these worm species have, this summer promises to bring many more, and that does not bode well for town gardens.
Unlike earthworms, which aerate and enrich the soil, jumping worms destroy the top layer of soil, depleting nutrients. They leave behind what looks like coffee grounds but is actually excrement (see photo). As if that’s not bad enough, the worms thrash violently when disturbed (which sometimes looks like jumping, hence the name). By August, resident gardeners who once found a sense of calm when digging in the dirt will once again grow weary of being startled every few minutes by the abrupt twitching of one or more jumping worms.
Jumping worms are known for their light-colored ring, which extends around their body, and a tendency to move like a snake. (Photo courtesy Univ. of Wisconsin Arboretum)
Jumping worms leave the soil with a consistency of coffee grounds. (Photo courtesy Wisconsin Department of Natural Resources)
In January, UMass Extension, a unit of the Center for Agriculture, Food, and the Environment at UMass Amherst, held a two-day Zoom conference on jumping worms. Experts discussed the damage that these creatures are doing to the landscape and what measures can be taken to stop them. Speakers included Dr. Justin Richardson, assistant professor of geoscience at the University of Massachusetts Amherst, Olga Kostromytska, extension assistant professor at the Stockbridge School of Agriculture, and Josef Görres and Maryam Houri-Aiin, Department of Plant and Soil Science at the University of Vermont.
Jumping worms are native to Japan and the Korean peninsula, but they have now found their way to 34 states in the U.S., mostly in the eastern half of the country. The Stockbridge School of Agriculture received a few reports of jumping worms in Massachusetts in 2019; the number of reports grew in 2020, and 2021 was dubbed the “season of the jumping worm,” according to Kostromytska. She attributed the huge increase last year to a mild winter.
Jumping worms are found everywhere, from city parks to suburban gardens to forests. They are voracious feeders, and because they are large and abundant in the areas they’ve invaded, they do a lot of damage. In gardens and farms, they destroy plant roots.
In forests, jumping worms do the most damage by consuming the decomposing leaves and roots on the forest floor. This layer of organic matter is necessary for seed germination; when it’s gone, native understory plants struggle to survive. The soil is destabilized, and erosion increases. The loss of the forest floor can also make way for invasive species that get their nutrients from deeper soil, like Japanese knotweed, to move in.
In addition to the damage they actively do, jumping worms can retain toxic metals that might be in the soil they are consuming, such as mercury, cadmium, and lead. These metals can have a harmful effect on anything that eats them, mainly birds.
What to do?
At this time of year, the worms are difficult to spot. Adults die annually after the first hard frost, but each adult leaves behind up to 60 cocoons, each containing one or two eggs. In late spring, a new generation hatches. The worms reach adulthood by mid-July, when they are typically 3 to 7 inches long depending on the species.
The cocoons are nearly impossible to see because they look like small pieces of dirt, so it is futile to go on a search-and-destroy mission for those. But there are a few options to at least reduce an existing infestation. Rototilling between mid-May and early June will kill many juvenile worms because they live in the top 2 inches of soil. In areas where rototilling is not an option, a mustard seed mix will irritate them and drive them to the surface, where they can be hand-picked and destroyed. The mix should consist of 1/3 cup of ground yellow mustard seed and 1 gallon of water. Pour half the liquid slowly over 1 square foot of soil (it won’t harm plants). Worms that come to the surface can be killed by bagging them and placing them in the hot sun for 10 minutes or drowning them in a bucket of soapy water or vinegar. Dead worms should be thrown in the trash.
Organic fertilizer made with tea tree seed meal irritates and slowly exterminates worms. Two applications are recommended, one in late April or early May to destroy hatchlings and one later in the summer to deal with the adult survivors.
Since destroying an existing infestation of jumping worms is difficult, prevention is critical. Cocoons in mulch, soil, compost, or plants from outside sources are mainly responsible for distributing the worms to new homes. Plants should be bought bare root if possible or grown from seedlings or cuttings. Speakers at the conference recommended using certified distributors for mulch, compost, and soil, because they use a heating procedure that kills both cocoons and worms.
The other option is to create a do-it-yourself “solarization package” consisting of 10 to 15 feet of plastic, covered with the material to be decontaminated, and a top layer of plastic tucked under that material. The bottom layer of plastic should be brought up over the top layer and secured to it with duct tape or small stones. That will keep the worms from escaping; they are very fast movers. The package needs to remain sealed, in the sun, for three days. It should reach a temperature of at least 105 degrees, enough to kill the cocoons and any live worms.
Lastly, since jumping worms do the most damage in forests, it’s important not to dump infested yard or garden waste into a natural area. A single worm can start an infestation since they reproduce without mating.
