What Are Horsehair Worms?

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.

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 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.

Horsehair worms, also known as Gordian worms, are parasitic nematodes that belong to the phylum Nematomorpha. They are named for their long, thin appearance resembling a horse's hair. These worms are notable for their parasitic life cycle, where they infect insects and other arthropods during their larval stage. Adult worms are free-living and non-parasitic, often found in water bodies.

Horsehair worms begin their life cycle as larvae inside the bodies of insects like crickets and grasshoppers. According to research, the infection occurs when a host ingests water or vegetation contaminated with horsehair worm eggs. Once inside, the larvae penetrate the gut wall and develop, eventually taking over the host's body to complete their growth.

Despite their unsettling appearance and parasitic nature, horsehair worms are not harmful to humans or pets. They are highly specialized parasites that cannot live inside mammals. According to the University of California Statewide Integrated Pest Management Program, there are no known cases of horsehair worms infecting humans or domestic animals.

Horsehair worms can vary greatly in size, with adults typically ranging from 50 centimeters to about 1 meter in length, though some species can reach up to 2 meters. Their diameter is usually about 1 to 3 millimeters, contributing to their hair-like appearance. The size can depend on the species and the environment in which they live.

Yes, horsehair worms can be considered beneficial to the environment. By parasitizing pest insects such as crickets and grasshoppers, they can naturally control these populations, which might otherwise damage crops and gardens. This biological control is an important ecological service that helps maintain a balanced ecosystem.

If you find horsehair worms in your home or garden, there is generally no need for concern or control measures. These worms do not pose a threat to humans, pets, or plants. However, if you wish to remove them, you can simply pick them up (using gloves if preferred) and place them in a nearby water body, where they will continue their life cycle.