A slug gnawing on an earthworm. Photo by Dana Wilde
On one of the three un-raining afternoons last month, Silas and I were deep into our weekly expedition around and around the backyard kidnapping toads, frogs, salamanders, spiders, snakes, sow bugs and whatever else moves within reach, when his dad, Jack, said from across the driveway, “Guys, what the heck is this?”
Jack usually sits out these expeditions, so we knew it was something unusual. We scrambled over to look.
On the ground next to the steps, a yellow-orange slug was apparently crawling over an earthworm.
“It’s a slug, Dad,” Silas declared, scooching down to look.
“Look, though,” Jack said. “Is that slug trying to eat the worm?”
“I thought slugs ate tomato leaves,” I said.
Sure enough, the slug did not seem to be just passing through. Its nose looked like it was gnawing the earthworm. The worm was slowly writhing, as if trying to escape.
I knew who to ask about this, and sent off a text and a video to my entomologist neighbor, Leon.
Meanwhile I discovered that slugs, like their close cousins snails, are mollusks. (In phylum Mollusca, along with, for example, clams and octopuses. Insects, spiders, lobsters and their relatives make up phylum Arthropoda.)
Three kinds of slugs live in Maine, according to Maine Pest Management Bulletin No. 5036. The gray garden slug, usually around three-quarters of an inch long, is most common and most destructive in gardens. The spotted garden or giant slug grows to between 3 and 7 inches, and is found mainly along the coast but has been creeping inland. The tawny garden slug is less commonly found than the others, though my guess is it’s the one we saw interacting with the earthworm. It usually grows to no more than 4 inches long.
When I was trying to find out what the worm was doing in the scene by the steps, I discovered this startling fact: Maine has no native earthworms. Nightcrawlers and most of the other worms you see in your garden soil came here with European settlers, according to a fact sheet of the Maine Department of Agriculture Horticulture Program. They comprise the phylum Annelida (segmented worms), and are mainly good citizens, beneficial to your grounds and garden.
In some spots in Maine is the invasive Amynthas worm, which came to North America from East Asia. It can be pretty destructive in the forest. Its castings (fecal droppings) do not mix well with the decomposition processes in Maine woods, and degrade the soil, making life harder for everybody who lives in there. Amynthas worms — also called jumping worms because they wriggle, writhe and thrash when threatened — were first spotted in Maine around the turn of the last century; in 2014, established populations of them were found in the Augusta area.
Earthworms feed on decomposing matter and microscopic animals. Slugs eat plant parts, decaying matter, fungi … and earthworms!
Leon texted back to say he’s not a worm expert (being an entomologist, not an annelidologist or malacologist), but it was his understanding that slugs do sometimes feed on worms. An interesting footnote to all this is that several cursory Google tries at finding formal reports or studies on slug-worm interactions turned up almost nothing at all.
Silas was fascinated by the slug gnawing away at the poor earthworm, and stayed scooched over the scene for several minutes. His zeal for squirming and unfamiliar things is inspiring.
Every day, he sees something he never saw or knew before. Me too.
Dana Wilde lives in Troy. You can contact him at dwilde.naturalist@gmail.com. His book “A Backyard Book of Spiders in Maine” is available from North Country Press. Backyard Naturalist appears the second and fourth Thursdays each month.
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