Pest profile

Horsehair Worms

Nematomorpha (horsehair or Gordian worms) · Gordioida: several families[2]

Horsehair worms look alarming, but they parasitize only insects and other invertebrates and can't infect people, pets, livestock or plants.[1]

Long, thin brown horsehair worm lying in loose coils on a pale surface
Long, thin brown horsehair worm lying in loose coils on a pale surface. (c) Joseph McPhail, some rights reserved (CC BY) — via iNaturalist, CC BY 4.0

How to identify it

  • Adults are long, thin, brown to blackish worms about 1/25 inch wide and up to a foot or more long.[1]
  • They often twist into a loose knot, which earned them the name Gordian worms.[1]
  • They're found singly or in knotted masses in ponds, puddles, pools, troughs and sometimes household water.[1]

Where it lives

They turn up wherever their insect hosts live near water, especially farm areas with ponds and irrigation.[1]

US map: public records from 23 states since 1990 (GBIF).

Biology and life cycle

Females lay strings of eggs 12 to 24 inches long in water in spring, and larvae hatch three to four weeks later.[1]

Larvae develop inside insects such as crickets, mantids, ground beetles or millipedes, sometimes after riding in an aquatic insect.[1]

About three months later the host is driven to water, where the adult worm emerges; adults don't feed and overwinter in water or mud.[1]

Damage and signs

  • They don't infect plants and parasitize too small a share of insects to matter much as biological control.[1]

Health risk

They're harmless to people, pets, livestock and birds. Swallowing one may cause mild intestinal discomfort, but infection never occurs.[1]

Control

Keep them out

Seal and caulk entry points so crickets, a known host, can't get indoors; worms in showers and toilets usually come from dead crickets.[1]

Traps and physical control

Remove worms from swimming pools by hand or with a net, and flush livestock troughs routinely.[1]

When to call a professional

If worms appear in household water, have the supply filtered, treated and inspected for repairs.[1]

Common questions

Are horsehair worms dangerous to my dog or kids?
No. They can't parasitize people, pets, livestock or birds.[1]
Why is there a worm in my toilet?
Crickets that carry horsehair worms seek water; the worm emerges when an infected cricket ends up in a toilet or shower.[1]
Do they come from horse hairs?
No. That's an old misconception; they develop as parasites inside insects.[1]

Sources

  1. University of California Statewide IPM Program, “Pest Notes: Horsehair Worms (Publication 7471).” ipm.ucanr.edu/PMG/PESTNOTES/pn7471.html (accessed 2026-10-06)
  2. iNaturalist, “Nematomorpha taxon page (order and family).” www.inaturalist.org/taxa/63142 (accessed 2026-10-06)

Updated 2026-10-14. Built from sourced records. Spot an error? Tell us.