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Horsehair Worms

Sue Mosbacher, UCCE Program Representative

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Recently in Amador, a man brought in a glass jar with two long stringy wormlike creatures he’d found in a puddle in his horse pasture. He wanted to know what they were and if they were harmful to him or his horses.

They are horsehair worms, and they are not harmful to humans or livestock.  But they are deadly to some unfortunate insects.

Horsehair worms occur in knotted masses or as single worms in water sources such as ponds, rain puddles, swimming pools, animal drinking troughs, and even domestic water supplies. Adult worms measure 1/25 inch in diameter and may reach 1 foot or more in length.  They will often twist into a loose, ball-shaped knot.

Horsehair worm larvae are parasitic and must have a host in which to develop. Insects are their target hosts. This happens in a variety of ways.

  • Some larvae are ingested directly by the host, where they immediately move into their parasitic stage and develop within the host.
  • Some larvae are ingested by larvae of water-inhabiting insects (mayflies, mosquitoes, chironomids) or tadpoles. When they enter these organisms, they encyst (enclose themselves in a cyst-like structure) in the host’s body cavity and remain encysted as this initial host develops into an adult. If the parasitized adult is eaten by an insect such as a mantid, cricket, or carabid beetle, the worm emerges from the cyst and completes its development in the second host.
  • When a water source dries up, some larvae encyst on leaves or other debris which are later eaten by a suitable host (such as a millipede).

That’s when trouble begins for the host insect. The horsehair worm larva develops within the host and slowly eats it from the inside. About 3 months after the horsehair worm parasitizes a host, the host is impelled to seek out water. When the host enters the water, the mature worm emerges.

Adult worms are free-living in water and do not feed, but they can live many months.  (That’s why after several weeks the two worms pictured here are still moving in the jar sitting on my desk.  It’s kind of creepy, but is a great conversation starter.) They overwinter in water or mud, and the cycle repeats itself the following spring.

Horsehair worms only parasitize invertebrates such as insects. Horsehair worms are harmless to vertebrates because they cannot parasitize people, livestock, pets, and birds. They also do not infect plants. If humans ingest the worms, they may encounter some mild discomfort of the intestinal tract but infection never occurs.