If you’re tired of hearing about new invasive forest pests, I’m right there with you. Seems like they arrive at an ever-increasing pace, and the harm potential ratchets up with each newcomer. At this rate maybe we’ll get some wood-boring beetle whose larvae explode inside the tree. That would put things in perspective. As distasteful as it may be to learn who’s next in the queue, we all realize it’s better to know what we’re up against.
If there’s a bright side to spotted lanternfly, it’s that it has an actual bright side — it stands out. At the other end of the Obvious Spectrum is a new and significant threat to forests, Asian earthworms, which have cleverly disguised themselves as earthworms. The fact that we are used to seeing worms in the landscape makes them a challenge to notice, but it is well worth the effort.
The new pests are three related species of earthworms from East Asia, Amynthas agrestis, A. tokioensis and Metaphire hilgendorfi, all of which are established in New York state. While A. agrestis was the first to be identified, it has come to light that A. tokioensis is the more common. Known variously as Asian jumping worms, Alabama jumpers, snake worms, crazy worms and probably some choice expletives, these super-sized (8 inches when mature) annelid cousins look very similar. In fact they can be accurately separated by species only through dissection, so I think it’s fair to consider them as a group.
Asian worms can be identified by the smooth band called a clitellum, which in European earthworms is dark, close to their middle, and thicker than the rest of the body. In these worms it is milky-gray to white, and much closer to the head. It is generally flush with the body as well.
Behavior is another clue: when disturbed they scatter across the ground, snake-like, rather than disappear into the soil. If touched, they thrash wildly and may shed their tail. They feel drier than other worms, and more turgid. A woman who tried to fish with them told me they “exploded” when she put a hook in them. Incidentally she said fish will not touch Asian worms, which speaks to their toxicity.
One issue with Asian worms in the forest is that they have a high reproductive potential, with two or sometimes three generations per season compared to one for European worms. The latter are hermaphroditic, having both male and female organs, but still needing to find a mate. Invasive Asian worms are parthenogenic, all females who bypass the need to find a date, and spew out cocoons teeming with baby female worms. All it takes is one to start an infestation.
As with any recent pest, Asian worms have not been well-studied. Even the modest research which has been done can conflict at times. It is known that adult Asian worms die off in winter, but that their cocoons are cold-hardy. I have read figures which place the hardiness cutoff around -30, and other references to the limit being -40 degrees or colder. The point is, no part of New York is too cold for these things. And based on research done by Josef Görres at the University of Vermont, cocoons can remain viable in the soil for at least 3 years, analogous to a soil seed bank.
By the end of summer, Asian worm biomass is many times that of other species. This means they eat a lot, primarily on the surface and within the top one inch or 2.4 cm of soil. A September 2016 article in Science Daily cites work done by Jiangxiao Qiu, now a postdoctoral researcher with The Nature Conservancy. His graduate research at the University of Wisconsin-Madison found that Asian worms reduced leaf litter in hardwood forests by 95%. I have seen an infested forest in Cortland County and can attest that it was almost entirely bare soil.
Not only do Asian worms leave soil bare, they leave it barren. Breaking down detritus incredibly fast leads to a big increase in plant-available nitrogen late in the season, when few plants can use it. Nearly all these nitrates leach out of the soil by spring. The soil also loses its healthy structure, becoming granular, somewhat like coffee grounds, much more vulnerable to erosion and compaction.
In addition to their insatiable appetites for organic matter, Asian worms are known to vastly increase lignin-busting enzymes, according to a March 2015 article in Applied Soil Ecology. At this time it is unclear whether the worms secrete the enzyme itself, or a substance which induces white-rot fungi to ramp up production of the compound. As a result of this effect, wood breaks down at least 2.5 times faster in the presence of Asian worms. It is also not known what effect this has on tree seeds.
In the face of an Asian worm infestation, other changes in hardwood forest ecosystems occur. European earthworms disappear, although no one yet knows the mechanism by which this happens. Bruce Allen Snyder of the University of Georgia documented a “significant decrease in millipede abundance and species richness” in the Great Smoky Mountains. Salamander populations fall as well, with researchers finding fewer juvenile and male salamanders at infested sites.
These invaders have few predators. Raccoons, opossums, moles, and centipedes find them tasty. Many amphibians feel otherwise. Wisconsin Department of Natural Resources Invasive Species Specialist Bernadette Williams, who essentially broke the whole Asian worm story, told me in a 2014 phone interview she observed a salamander grab an Asian worm, then release it and spend the next 10 minutes wiping its mouth on the soil.
I doubt anyone needs convincing that Asian worms should be taken seriously. Bernadette Williams put it bluntly: “Their [Asian worm] introduction into our state poses a huge threat to the future of our forests.”
So where to from here? Asian worms are commonly spread through the horticulture trade, both in containerized plants, and especially mulch, which they love. Paw through bulk mulch before purchasing, and maybe think twice about getting fill delivered to your property. And with tree seedlings, it is safer to plant bare-root stock than plugs.
Equipment may pose the biggest threat to forest land, so be cautious. Find out where their last job was before a logging crew shows up at your place. If you go on a woods walk in another woodlot, turn over a log or flat rock. Do the worms dive down, or scoot out? Always clean off your boots before getting in the vehicle to return. Worm cocoons are tan to brown spheres about 2 mm in diameter, and very easy to take home with you. A little prevention goes a long way.
Paul Hetzler is a natural resource and horticultural educator with Cornell Cooperative Extension of St. Lawrence County. Contact him at 315-379-9192 ext. 232.