Horsehair Worms Integrated Pest Management

1. Horsehair worms belong to the phylum Nematomorpha, from the Greek word meaning thread-shaped, class Gordioida (Figure 1). 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 (Figure 2).
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 CYCLE
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
Integrated Pest Management around the Home
Horsehair Worms
Statewide Integrated Pest Management Program March 2013
PEST NOTES Publication 7471
Figure 3. 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.
mayfly adultmayfly nymphmantidadult wormseggsADULTS INWATERPREPARASITIC LARVAENTERS AQUATIC NYMPHEGGS INWATERENCYSTEDLARVAPARASITICLARVA
Figure 2. Horsehair worms often twist into a loose ball-shaped knot, such as these adults.
Figure 1. Adult horsehair worm.

2. March 2013 Horsehair Worms
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weeks to one month later, minute im-mature
larvae hatch. These larvae must
parasitize an invertebrate host to develop.
Suitable hosts for different species of
horsehair worms include larger preda-ceous
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, oth-ers
aren’t well understood. Sometimes
the host directly ingests the larvae,
which immediately move into their par-asitic
stage and develop within that host.
For other horsehair worm species, the
larvae of water-inhabiting insects (may-flies,
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 develop-ment
in the second host (Figure 3).
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.
IMPORTANCE
Horsehair worms parasitize only inverte-brates
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 ver-tebrates,
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.
MANAGEMENT
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 wa-ter
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.
REFERENCES
Loomis, E. C., and L. L. Dunning. 1981.
Horsehair Worms. Oakland: Univ. Calif.
Agric. Nat. Res. Leaflet 21238. v
Produced by UC Statewide
Integrated Pest Management Program
University of California, Davis, CA 95616
AUTHOR: H. K. Kaya, Entomology and
Nematology, UC Davis.
TECHNICAL EDITOR: M. L. Flint
EDITOR: M. L. Fayard
ILLUSTRATIONS: Figs. 1–2, J. K. Clark;
and Fig. 3, C. M. Dewees.
This and other Pest Notes are available at
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