Flowers and Garden Home Horsehair worms are slender (1/25 to 1/16 inch wide), very long (4 to 14 inches) and colored yellowish-tan to brownish-black.  
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Browse Articles: Insects and Pests
Horsehair Worms
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Reference: Ohio State University Extension

Common NameScientific Name
Horsehair, Cabbagehair, Gordiacea(Nematomorphora)
Gordiid or Gordian Worm
Horsehair Worm

These active, long, threadlike roundworms resembling the "hair of a horse's tail or mane" are occasionally found in water troughs and domestic water supplies such as bird baths, swimming pools, sinks, bathtubs, toilets, pet dishes, on cabbage plants, and garden soil and in the body cavity of various pests such as grasshoppers, katydids, crickets, beetles, cockroaches, millipedes, centipedes, snails, slugs, etc. These insect parasites do not harm humans, animals or plants.

Horsehair worms are slender (1/25 to 1/16 inch wide), very long (4 to 14 inches) and colored yellowish-tan to brownish-black. The worm is white inside the host's body, turning brown after emergence. They often squirm and twist, knotting themselves into a loose, ball-like shape, resembling the so called "Gordian Knot" in freshwater pools. These worms may be found in masses of 100 or more, especially after a rainfall. In water troughs and puddles, they resemble horsehairs actively moving in the water. Some people still believe that these worms develop from the long, thin hairs of a horse's mane or tail that had fallen into the water trough as a horse drank and later came to life.

Life Cycle and Habits
Adult worms of both sexes are free-living and do not feed. They live for over 6 months in ponds, mud or moist earth. However, juveniles or immature stages are internal parasites of many insects.

Both male and female adult worms mate in the freshwater and damp soil. The female lays up to several million eggs in fresh water in long strings or slender broken cords that wrap around water plants. Eggs hatch in two weeks to three months, with the 0.01-inch larvae not resembling the adults. Some believe that within 24 hours after hatching, the larvae encyst on vegetation near the water's edge. After the water level drops, the exposed vegetation is eaten by a grasshopper or cricket. The cyst covering dissolves, permitting juvenile worms to bore through the gut wall and into the body cavity of the host. All nutrients are absorbed across the body wall of the worm, as no alimentary system is present.

As larvae develop fully or nearly so (several weeks or months), they break through the body wall of the host (in moist habitats) and become free-living. Other people believe that young worm larvae bore into or are swallowed by immature stages of water-inhabiting insects such as mayflies, dragonflies or beetles. When the pest host emerges from the water as a free-flying adult, the mature horsehair worm breaks out of the body cavity. Grasshoppers or crickets may be a second host if they eat the dead, infested mayfly adults. It appears that the pest host must first come in contact with water to enable horsehair worms to escape the body cavity.

Control Measures
Horsehair worms are sometimes seen after crushing pests (crickets, millipedes, centipedes, etc.), which have invaded the house. When crushed, worms are released and crawl indoors. Some are found in toilets where infested pests have been discarded, in pet dishes, where an infested cricket has crawled or in gardens on vegetable plants. Humans sometimes fear worms may have come from a household pet or child, when found indoors. Fears are not warranted since horsehair worms can only parasitize suitable insect hosts.

No control measures are recommended as horsehair worms do not injure humans, animals or plants (cause no economic damage). Instead, they are considered beneficial since they kill many harmful pests such as grasshoppers, crickets, cockroaches, beetles, millipedes, centipedes, snails, slugs, etc.

Control of horsehair worms in natural water is impractical. However, one can install a fine mesh filter or screen to keep out tangled masses of worms from water pumped from a surface supply such as a farm pond or canal. Should the homeowner find nuisance worms in the wash water, bathtub and sinks; domestic water supply systems can be filtered and chemically treated under supervision of the local health department. Remove and discard individual worms. Prevent nuisance insects, such as crickets, from entering the house by caulking or sealing entry sites. If an infested cricket dies on a shower floor or is killed and thrown into a toilet, the horsehair worm will escape from the cricket's body. (This is just another wonder of Mother Nature.) Livestock water troughs can be kept free of horsehair worms by routine flushing.

An insecticide barrier around the house foundation might be helpful to kill any arthropod pests infested with these worms, if large numbers of horsehair worms occur.

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  Data Source: Ohio State University Extension. Articles and resource may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide and it is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used.