How to Manage Pests

Pests of Homes, Structures, People, and Pets

Horsehair Worms

Revised 3/13

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Adult horsehair worms are long and slender.

Adult horsehair worm.

Adult horsehair worms are long and slender.

Horsehair worms often twist into a loose ball-shaped knot, such as these adults.

An example of a horsehair worm life cycle.

An example of a horsehair worm life cycle. After the egg of the horsehair worm hatches, an aquatic insect, such as a mayfly nymph, eats the preparasitic larva. Inside the mayfly, the larva encysts but doesn’t begin to develop until a host such as a mantid that the horsehair worm can parasitize eats the mayfly. Once in the mantid, the horsehair worm grows to an adult and emerges when the mantid seeks water. Many variations of this cycle occur.

Horsehair worms belong to the phylum Nematomorpha, from the Greek word meaning thread-shaped, class Gordioida. They are also called Gordian worms, because they will often twist into a loose ball-shaped knot resembling the baffling one Gordius created in the Greek myth and that is referred to as the Gordian knot.

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. An old and still common misconception is that these long, thin, brown to blackish worms develop from horsehairs that fall into water. Because horsehair worms are parasites of invertebrates, especially certain insects, they are commonly encountered in agricultural areas, particularly those having water-impoundment and irrigation facilities.


There are four stages in the life of a horsehair worm: the egg, the preparasitic larva that hatches from the egg, the parasitic larva that develops within an invertebrate (its host), and the free-living aquatic adult. The worms spend the winter in water. After mating in spring, the female worm deposits a string of eggs 12 to 24 inches long in the water. About three weeks to one month later, minute immature larvae hatch. These larvae must parasitize an invertebrate host to develop. Suitable hosts for different species of horsehair worms include larger predaceous arthropods (often mantids, water beetles, carabid beetles, or dragonflies) or omnivores (such as crickets and other closely related insects, or millipedes).

There are several ways that horsehair worms parasitize hosts and complete their development. Although some of these life cycles have been studied, others aren't well understood. Sometimes the host directly ingests the larvae, which immediately move into their parasitic stage and develop within that host.

For other horsehair worm species, the larvae of water-inhabiting insects (mayflies, mosquitoes, and chironomids) or tadpoles ingest the preparasitic larvae. When horsehair larvae are ingested by these organisms, they encyst (enclose themselves in a cystlike structure) in the host's body cavity and remain encysted as this initial host develops into an adult. If an insect such as a mantid, cricket, or carabid beetle consumes an adult with an encysted worm, the worm emerges from the cyst and completes its development in the second host.

Finally, some preparasitic horsehair worm larvae encyst on leaves or other debris when a water source dries up. If a suitable host, such as a millipede, eats this cyst when ingesting vegetation, the horsehair worm larvae can move into the parasitic stage.

About three 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 don't feed, but they can live many months. They overwinter in water or mud, and the cycle repeats itself the following spring.


Horsehair worms parasitize only invertebrates such as insects. To complete their life cycle, the worms must infect large invertebrates that are relatively long lived. Generally, horsehair worms aren't considered an effective biological control agent, because they parasitize only a small percentage of a host population.

Horsehair worms are harmless to vertebrates, because they can't parasitize people, livestock, pets, or birds. They also don't infect plants. If humans ingest the worms, they may encounter some mild discomfort of the intestinal tract, but infection never occurs.


Control of horsehair worms in natural water sources is impractical. Furthermore, the worms can be beneficial, because they will parasitize a few pest insect species, although their effect on natural invertebrate populations is minimal.

If the worms are found in livestock water troughs, the water can be kept clean with routine flushing. Use a fine mesh filter if pumping water from a surface supply such as a canal or pond. If the worms occur in  swimming pools, they can be removed by hand or with a net.

Domestic water supply systems should be filtered, chemically treated, and inspected for necessary repairs, especially when the homeowner discovers horsehair worms in wash water, bathtubs, or sinks. Moreover, it isn't unusual to find horsehair worms in the home in such places as shower stalls or toilets where crickets may die and worms emerge into the water. Prevent nuisance insects such as crickets, which are known hosts, from entering the home by caulking or sealing entryways.



Loomis, E. C., and L. L. Dunning. 1981. Horsehair Worms. Oakland: Univ. Calif. Agric. Nat. Res. Leaflet 21238.


[UC Peer Reviewed]

Pest Notes: Horsehair Worms

UC ANR Publication 7471         PDF to Print

Author: H. K. Kaya, Entomology and Nematology, UC Davis.

Editor: M. Fayard

Technical Editor: M. L. Flint

Produced by University of California Statewide IPM Program

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