Horsehair worms are round worms that appear as threads similar to the mane hair and tail of a horse. These worms can measure more than 1 foot long (0.3 meters), with hundreds intertwined in a loose ball that resembles a Gordian knot. Horsehair worms get their scientific name from this twisting feature, often called a gordian worm, but it is also known as a cabbage hair worm in some areas.
These worms are harmless to humans, pets and plants. Horsehair worms can be found in animal waterers, ponds and lakes. They are also commonly seen in pet water bowls, swimming pools, bird baths and puddles on sidewalks, especially after heavy rains. Horsehair worms are sometimes found in moist garden plants or in saturated soil.
Horsehair worms mate in spring or early summer in water or moist soil. The female can lay millions of eggs connected by a thin rope. The eggs hatch between two weeks and three months later as tiny larvae. Two theories explain how these larvae develop into adult worms, which first appear white but quickly turn yellowish or dark brown.
The larvae can attach to plants on the water’s edge while waiting for insects to appear. When water levels drop, lobsters, cockroaches, crickets or other insects can look for exposed plants to eat. The larvae could remove their outer shells and bury themselves in these insects for nutrients. Once mature, horsehair worms escape the body of their guests when they are in a pool of water.
The second theory proposes that larvae enter immature insects, such as dragonflies, ephemerals, or other insects that feed on water surfaces. Once these insects reach adulthood and begin to fly, horsehair worm larvae emerge from hosts as adults, according to this theory. In both scenarios, guests die after the worms are gone.
Horsehair worms are believed to be beneficial in reducing populations of cockroaches, locusts, centipedes and other pests that damage crops or garden plants. Some people use screens to control tangled worm balls in ponds. If these worms appear in toilets, sinks or domestic pools, they can be controlled with chemical treatments or filters. Animal drinkers should wash frequently to remove these worms.
People sometimes believe that horsehair worms found inside the home are internal parasites, especially when they appear in the toilet. In fact, they can arise when large insects infected with pests are crushed inside the house. Caulking or sealing cracks can help keep insects out and reduce the number of worms inside a home. A myth that has given rise to the name of this worm states that the hair of a horse falls into the water and mysteriously comes to life.