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University of Minnesota Extension

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.

How to identify horsehair worms

Horsehair worm
  • 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 worm
  • 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.

Where they live

  • 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.

No control is necessary

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.

Reviewed in 2024

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