What Are Horsehair Worms?

Marlene Garcia

Horsehair worms are round worms appearing as threads similar to the mane and tail hair on a horse. These worms might grow more than 1 foot long (0.3 meter), with hundreds intertwined into a loose ball resembling a Gordian knot. Horsehair worms get their scientific name from this twisting characteristic, often called Gordian worms, but also called cabbage hair worms in some regions.

Horsehair worms help keep the cockroach population in check.
Horsehair worms help keep the cockroach population in check.

These worms are harmless to people, pets, and plants. Horsehair worms might be prevalent in animal water troughs, ponds, and lakes. They are also frequently seen in pet water dishes, swimming pools, birdbaths, and sidewalk puddles, especially after heavy rain. Sometimes, horsehair worms are found on wet garden plants or in saturated soil.

Horsehair worms may be frequently seen in swimming pools.
Horsehair worms may be frequently seen in swimming pools.

Horsehair worms mate in spring or early summer in water or wet dirt. The female might lay millions of eggs connected by a thin string. The eggs hatch between two weeks and three months later as minute larvae. Two theories explain how these larvae mature into adult worms, which first appear white but quickly turn yellowish or dark brown.

Larvae might attach to plants at the edge of water waiting for insects to appear. When water levels recede, grasshoppers, roaches, crickets, or other insects might seek exposed plants as food. Larvae could discard their outer shells and bore into these insects to obtain nutrients. Once mature, horsehair worms escape the hosts’ bodies while in a pool of water.

The second theory proposes that larvae enter immature insects, such as dragonflies, mayflies, or other bugs that feed on water surfaces. Once these insects reach adulthood and begin to fly, the horsehair worm larvae emerge from hosts as adults, according to this theory. In both scenarios, the hosts die once worms leave.

Horsehair worms are considered beneficial to keep down populations of cockroaches, grasshoppers, centipedes, and other pests that damage crops or garden plants. Some people use screens to control tangled balls of worms in ponds. If these worms appear in household toilets, sinks, or swimming pools, they might be controlled by chemical treatments or filters. Animal water troughs should be frequently flushed to eliminate these worms.

People sometimes believe horsehair worms found inside the home come from internal parasites, especially when they appear in toilets. Actually, they can emerge when large insects infected with the parasites are crushed inside the home. Caulking or sealing cracks might help keep insects outside and decrease the number of worms inside a residence. One myth that led to this worm’s name contends hair from a horse falls into water and mysteriously comes alive.

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