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How to Manage PestsPests of Homes, Structures, People, and Pets
Horsehair worms belong to the phylum Nematomorpha (from the Greek 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 created by Gordius in the Greek myth. 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. LIFE CYCLEThere 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 3 weeks to 1 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 are not well understood. Some larvae are ingested directly by the host, where they immediately move into their parasitic stage and develop within that host. For other horsehair worm species, the preparasitic 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 cystlike 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. This type of life cycle is illustrated in Figure 1. Finally, some preparasitic horsehair worm larvae encyst on leaves or other debris when a water source dries up. If this cyst is consumed by a suitable host (such as a millipede) when the host ingests vegetation, the horsehair worm larvae can move onto the parasitic stage. 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. They overwinter in water or mud, and the cycle repeats itself the following spring. IMPORTANCEHorsehair worms only parasitize invertebrates such as insects. To complete their life cycle, they must infect large invertebrates that are relatively long-lived. Generally, they are not considered an effective biological control agent because they parasitize only a small percentage of a host population, and therefore, the effect on the population is minimal. 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. MANAGEMENTControl 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. A fine mesh filter should be used if water is pumped from a surface supply such as a canal or pond. Domestic water supply systems should be filtered and chemically treated and inspected for necessary repairs, especially when the homeowner discovers horsehair worms in wash water, bathtubs, or sinks. Moreover, it is not unusual to find horsehair worms in the home in such places as shower stalls or toilets where crickets may die and the worm emerges into the water. Prevent nuisance insects such as crickets that are known hosts from entering the home by caulking or sealing entryways. WARNING ON THE USE OF CHEMICALS COMPILED FROMLoomis, E. C., and L. L. Dunning. 1981. Horsehair Worms. Univ. Calif. Div. Agric. Nat. Res. Leaflet 21238. PUBLICATION INFORMATION![[UC Peer Reviewed]](/IMAGES/logo-ucpeerreviewed.jpg){kind=logo}
Pest Notes: Horsehair
Worms Author: H. Kaya, Nematology, UC Davis PDF: You need a PDF reader, such as Acrobat Reader version 8 or later, to view or print this PDF. If no reader is installed on your computer, you can download a free copy of Adobe Acrobat Reader. |
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