Horsehair worms

Quick facts

Horsehair or gordian worms are long, slender worms related to nematodes.
When they are immature, they are parasites of insects, arthropods and other invertebrate animals.
They are harmless to people in all stages of their lives.
They are considered beneficial as they control other insects.
They become a nuisance when found in swimming pools, water tanks or toilets.

Horsehair worms are long, measuring from several inches to over 14 inches.
They are quite thin, ranging from 1/25 inch to 1/16 inch wide (1 mm to 1.5 mm) and are uniform in diameter from front to back.
They vary in color from whitish to yellow/tan to brown/black.
Horsehair worms are found on the ground or on plants, especially near water.

Horsehair worms mate during spring, early summer or fall.
Males coil around females in pools of fresh water or damp soil.
It is common to see a number of worms to be intertwined, forming a loose ball during mating.
Eggs are laid in a long, gelatinous string in fresh water.
Once they hatch, immature horsehair worms try to infect a host.
They can attack a wide variety of insects and related animals: grasshoppers, crickets, cockroaches, beetles, and katydids, as well as dragonflies, caddisflies, millipedes, centipedes, spiders, crustaceans, leeches, snails, slugs and other invertebrates.

It is not clear how immature horsehair worms infect hosts.
When they infect their host, horsehair worms store up fats and food reserves.
When the horsehair worm is mature and near water or damp soil, it emerges from its host.
This process usually kills the host.
Adults do not feed.

Horsehair worms are often seen in puddles and other pools of fresh water, swimming pools, water tanks and on plants.
They are especially noticeable after rainfall.
Horsehair worms may be found inside homes in toilets causing people to be concerned that it is a human parasite.

Horsehair worms are harmless to people, pets and plants.

They should be considered beneficial because they can be effective in controlling certain insects.
No control is necessary for horsehair worms.

Author: Jeffrey Hahn, former Extension entomologist

Reviewed in 2023