[ 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
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.