[ FOREST PESTS ]

When Worms Go Rogue

It’s a hot day in early May, and I’m poking through a tray of wet worm castings in a Plant and Soil Science lab at the University of Vermont. I’m looking for worm cocoons – mud-colored spheres about two to three millimeters in diameter – for Dr. Josef Görres, associate professor in the department of plant and soil science. Fingers muddy from crushed castings, I carefully squeeze a possible cocoon. It is hard and round, but has a rubbery resilience between my fingers. We find six cocoons in a soil sample collected earlier that day near Shelburne Farms in southern Chittenden County, Vermont. Görres’s research focuses on soil quality and agriculture, but he’s become particularly interested in earthworms due to the negative effect they’re having on Northeastern forests.

Growing up, I spent a lot of time poking around the woods and never questioned finding earthworms under every rock. I was out of college before I learned that all earthworms in the Northeast came from Europe or Asia; the natives were wiped out by the last glacial period. Dr. Tim Fahey, a professor in the Department of Natural Resources at Cornell University, tells me it’s assumed that the first worm invaders tagged along with early European settlers some 300 years ago.

Of the more than two dozen earthworms in New England, two species are especially interesting for those monitoring forest health. The most familiar is Lumbricus terrestris, better known as the nightcrawler because these large worms come to the soil surface more often than other species. Nightcrawlers dig vertical burrows but feed from the surface by pulling leaf litter down into their burrows. They cap their burrows with small piles of leaves and debris called middens. The other worm being closely watched is Amynthas agrestis, called the crazy snake worm. These worms live and feed in the leaf litter, do not make burrows, and are very active (for worms). When Görres shows me Amynthas, he places it on his palm and it twists and flips like it’s, well, crazy.

Lumbricus and Amynthas worms live and feed differently, but they have similar net effects on the soil. When they move into a forest, they eat away the thick duff layer while churning up the top soil horizons. According to Fahey, it takes several years to a decade for these changes to occur. The end result is a compacted surface soil horizon with little organic matter, below a layer of last year’s leaves. If Lumbricus worms are responsible, there will be middens scattered underneath the leaves. If Amynthas worms are responsible, there will be an extra layer of loose and crumbly worm castings up to four inches deep between the leaves and the actual soil surface.

In any forest, worm-induced changes to the soil structure can have far-reaching consequences. Without a good duff layer, trees have a harder time absorbing rain water. The rain instead washes away important nutrients like nitrogen and phosphorus, and the subsequent erosion exposes the roots of established trees.

It turns out that earthworms have good taste: they like maple leaves. The worms are less interested in coniferous leaf litter, although both Lumbricus and Amynthas worms have been documented in woodlands with little deciduous leaf litter. Görres worries that maple forests could eventually shift to being dominated by trees the earthworms like less, such as conifers. While this remains speculation, it’s a troubling thought for anyone managing a sugarbush.

Earthworm activity also takes a toll on native understory plants. The loss of the duff layer, which acts as a seed bank for ephemerals, and the altered soil structure limits the potential for forest floor plants to germinate and survive.

The worm-invaded sites I visited were all bare below the trees, allowing us to stroll along like we were walking through a manicured park. Görres tells me these sites should have all required serious bushwhacking.

I did see many maple seedlings at the Shelburne Farms site, but without other understory plants to accompany them, Görres fears they will take the brunt of deer browsing. Fahey adds that it’s possible the worms themselves are eating enough of the seedlings’ roots to harm them. Studies have shown that worms can eat up to 20 percent of a mature sugar maple’s feeder roots. He doubts this is a problem for an established tree, but it could prove too stressful for a seedling to survive.

There’s also evidence that earthworms affect the amount of carbon a forest can sequester, something to consider as atmospheric carbon dioxide levels continue to rise. Fahey says that in one recent study, wormy forest soil contained about a third less carbon than worm-free forest soil.

So does all this mean that our forests are about to be overrun by alien destroyers? Fahey sees the earthworm invasion as more of a limited occupation.

In the central New York forests that he’s been monitoring for 13 years, he hasn’t observed much advancement by the earthworm front. Görres says that on their own, earthworms, even Amynthas, move only 10 meters in a year at best, but not uphill and only if they don’t like their current location. Görres’s opinion is that horticultural activities are primarily responsible for encouraging the spread of Amynthas, while bait-dumping by those out fishing spreads Lumbricus.

As for their current distribution, earthworms hang around the same places people do, though forests above 2,500 feet seem to be safe from worms. On Fahey’s hiking trips throughout the Northeast, he makes a point of checking for worms. He doesn’t find them along the ridge lines; they aren’t in the Berkshires, the White Mountains, or the Green Mountains. He does find them when the trails descend back toward civilization. Fahey attributes these observations to low soil fertility and reduced human activity in mountainous areas.

Görres is concerned about the future spread of Amynthas worms and the potential danger they pose to forests. Fahey is less concerned, at least about Lumbricus worms, because he doesn’t see the worms moving on their own, or very fast. The European worms at his study sites in New York have been in place for more than a century. He acknowledges that the Amynthas worms are expanding into new habitats faster than the European species, but he sees worms, in general, as less of a threat to forests than deer, climate change, and exotic pathogens.

If you want to prevent earthworms from gaining a foothold in your forest, Görres has some suggestions: Plant only ornamentals with bare roots or check the potting soil thoroughly for worms; limit the worms’ food options by not using mulch and by raking up fallen leaves; and only use vermicompost from a trusted source. He’s skeptical about pesticide control options.

As with many complex ecological questions, it’s difficult to come to a definite conclusion about the Northeast’s invasive earthworms. There is enough evidence to say that they have the potential to profoundly alter the forests, if they haven’t already. Yet it is also possible that this invasion is more or less contained by the very nature of the invaders. Either way, the worms are here and only time will tell just how they shape our forests.