It’s hard to believe that something as creepy-looking and diabolic as the horsehair worm is harmless to humans, pets, plants and livestock.  But the long, thin worms only are after arthropods like grasshoppers that they trick into drowning.

Horsehair worms are a parasite most often found in late spring and early summer anywhere there is still or slow-moving water.

That means they can pop up in puddles, streams, birdbaths, water troughs, swimming pools, wet sidewalks, moist soil and even in toilets.

Gardeners around Maine have posted photos and videos on social media showing the inches-long worm wriggling out of compost bins and along paths.

Because the worms often clump themselves together in slowly writing tangled knots, they are also known as Gordian worms. This name is inspired by the ancient Greek legend of the Gordian knot. This tangle of rope was so complicated that whomever could untangle it would then rule all of Asia.

“There are several species of horsehair worms and their lifecycle is not completely understood,” said Jim Dill, pest management specialist with University of Maine Cooperative Extension. “No one knows why, but they need to get [insects] into water so they can reproduce.”

Horsehair worms lay their eggs in water — often on water plants. They enter their hosts when the insects eat either the eggs or larvae of the worm. The horsehair worm — which can grow to more than a foot long — then lives out its life inside the host absorbing nutrients from it.

Then things get really interesting.

“Once they have parasitized the host and are mature, the horsehair worm tricks it to seek out water and jump in,” Dill said. “The grasshopper or whatever bug is the host drowns and the worm comes out.”

This sets the worm up perfectly to complete its life cycle and lay eggs in the water before it dies.

“All it’s doing is growing inside the host and taking in nutrients,” Dill said. “The [host] seems to be fine right up until it jumps into the water and drowns.”

A horsehair worm emerges from its insect host. The worm lives inside the host arthropod for most of its life before tricking the host into jumping in water and drowning. Credit: Courtesy of Soni Cochran, University of Nebraska

Once that happens, Dill said, the worm emerges in a matter of minutes.

“They come out of one of the bug’s orifices,” Dill said. “Usually out of the back end and not the mouth.”

Despite gardeners finding the worms in their soil and compost, Dill said that is not a common occurrence.

“It’s probably because it has been wet,” he said. “The worms themselves overwinter in water or mud.”

The most likely scenario is the areas they are currently being spotted were, at one time, puddles.

“It may have been an area so wet that when the host was tricked into going to water, it saw the puddle and just jumped right in,” Dill said.

If a pet manages to actually ingest a worm or the eggs of a worm, it can’t hurt them as neither stage can survive in a mammal’s gut.

The only issue they could cause is if enough of them gather in water that needs to circulate such as a pool or rain gutters. In those cases they will need to be cleaned out as they can clog up drains or pumps.

“It’s not going to hurt you or your soil,” Dill said. “It’s just kind of disturbing when you see one.”

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Julia Bayly

Julia Bayly is a reporter at the Bangor Daily News with a regular bi-weekly column. Julia has been a freelance travel writer/photographer since 2000.