Mysteries of Invasive Worms Revealed

Composting of all types is finally catching on, and that includes vermicomposting — employing worms to turn your kitchen scraps into black gold.  Seattle and other progressive jurisdictions are even distributing worm composters to their residents.  But mention of these industrious recyclers increasingly leads to the question: But aren’t they invasive?  Well, some are, in some situations, but confusion abounds and unearthing the 411 about which ones and where is easier said than done.  Not that that stopped me from trying.

First, what’s not to love about a critter who returns organic waste to the earth?  And not just kitchen scraps, either.  Rachel Carson wrote about worms being used to remediate pollution by removing toxins from the soil.  Municipal sewage systems are using worms to remove harmful bacteria in human waste and turn it into clean biosolids — a great substitute for synthetic fertilizers on farmland.  Some ranchers are using worms to compost the tons of animal waste that would otherwise be polluting our waterways.   

On organic farms, the castings of another type of worm — the earthworm — not only increase soil fertility but have been shown to reduce plant disease, without the use of chemicals.  Studies show yields increasing by 20 percent after earthworms are added to growing fields.  Even for the home organic gardener, worm castings provide essential nutrients and have fungicidal properties that can fight mildew and other diseases.  Then there are the soil-aerating benefits from earthworms’ constant burrowing, which helps improve both water retention and drainage.

Invasive Worms Damaging Northern Forests

Yet there ARE destructive worms in the U.S. During the last ice age, glaciers covered about half of North America and wiped out the earthworms in their path. Above a certain latitude the forests evolved without earthworms until the introduction of worms from elsewhere, brought there in cargo or carelessly discarded by fishermen, and these invaders are changing the forests. Authorities in Minnesota, the leader in fighting to protect Northern forests, explain that these nonnative earthworms are changing the structure of the soil and therefore the ecology of forests. Worms eat the thick layers of leaves called “duff” that covers the ground, turning it into richer soil, which affects what plants can grow in it, what insects that can live in it, and right up the food chain. Ultimately these worms may could cause some plants and animals to go extinct.

(Here in Maryland, below the glacier line, native earthworms were left intact.  In fact, across North America there are today about 120 native earthworms, none of which seem to be sold as composting worms because they act so slowly.)

Night Crawlers are Bait 

According to Minnesota’s top worm expert, Lee Frelich, seven types of worms have been found in their hardwood forests, including the European Lumbricus rubellus or night crawler. The evidence against this popular bait worm mounts, with both Cornell and Maryland having fingered it as the primary culprit. It and the other invaders thrive in unimproved soil and can survive severe winters. Maryland’s Extension Service goes on to say they’re NOT suited to compost bins and will die in them — as will Maryland’s native worms.

Red Wigglers are Composters

The red wiggler or Eisenia fetida, another European import, is the primary species sold for composting purposes.  Unlike the night crawler, the red wiggler thrives in organic waste, lives close to the surface, and can’t survive temperatures below 40 degrees or above 90 (the ideal is 70-75).  Where winters are mild they still only survive outdoors in compost bins or heavily mulched gardens that are rich in organic matter.   But as composters they’re ideal — they eat, excrete and breed quickly.  I learned that the red wiggler “has not been found outside of compost in Minnesota. They can’t survive being frozen.”  The Brooklyn Botanic Garden website agrees that they’re not a “problem species.”  Maryland’s Extension Service says these worms “do poorly” in average soil, and Virginia reminds us that composting worms like the red wiggler are not an “earthworm” at all.  Now can we relax hire these guys to do some composting for us?

Weird or Confusing Advice about Worms

Unfortunately, a recent New York Times article on invasive worms cited night crawlers and an Asian species as the harmful species, then concluded by telling readers not to “toss out fishing worms or red wrigglers by throwing them on the ground or in a pond.”  Then the author described for urban readers how to capture and kill worms in their yards…but didn’t mention any evidence that worms are actually doing harm in cities (and I couldn’t find any).  Other authors recommend using only “native species of compost worm,” which is totally unhelpful, since there are no native worms suitable for composting, according to the University of Minnesota.  No wonder there’s rampant confusion.

The public is often advised to consult their local Extension Service to see if earthworms pose problems in their geographic area and which species are causing the difficulty, though many of them simply don’t have an answer.  And one online source even suggests that readers “consult relevant scientific studies”.  Again, good luck with that.

Despite state officials in Minnesota having cleared red wigglers as a potential menace there, Minnesota ecologist Cindy Hale tells composters to freeze their worm castings in air-tight bags for a least a week before adding them to garden soil, no matter what worms species they use. “It won’t hurt the soil microbes, but it will kill all the worms.”" But composters tell me this turns castings into hard chunks that are impossible to spread.  She’s not the only authority recommending the freezer treatment so we can all compost “without fear of spreading invasive worms” — apparently not aware that “composting worms” aren’t the problem.

For outdoor composting, the advice is even weirder.   Sources warn against adding “extra earthworms” to outdoor bins… but does anyone buy worms to add to their regular compost bin, anyway?  Minnesota advises that “If you have a compost pile in a forested area, do not introduce additional non-native earthworms.”  Makes sense but really, people do that?