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LEAFLET NO. 450
U.S. DEPARTMENT OF AGRICU

Prevention and treatment of the disorders caused by
horse bots concern thousands of horse owners in this
country. In 1972, the total horse population in the United
States was approximately 7,000,000 animals. Of this num-
ber, about 2,739,500 were farm horses and mules; and
about 4,260,500 were nonfarm or light horses.

Although the number of draft animals has declined
steadily since the advent of the tractor, maintaining the
health of draft animals still in use is vital in many areas.
And the increasing popularity of light horses has created
new demands for information about horse health,

To meet the needs of horsemen for help in controlling
horse bots, the U.S. Department of Agriculture publishes
this leaflet.

This edition replaces all previous editions of this publication. Because of changed
insecticide recommendations, earlier copies should be destroyed.

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HORSE BOTS

How To Control Them

Three species of horse bot flies are
serious pests in this country.

The common bot fly and the
throat bot fly occur throughout the
United States. The nose bot fly is
found in Northwestern and Mid-
western States.

Bot flies can be controlled by
treating infested animals to destroy
larvae and eggs.

HOW BOT FLIES LIVE

Bot flies have four life stages—
adult, egg, larva (bot), and pupa.
These stages are described on this
page and on pages 4 and 5.

Adult flies do not feed. Their sole
purpose in life is to reproduce. They
live from a few days to 3 weeks.

In Northern States, flies appear
about the middle of June; they live
until there is a heavy freeze. In
Southern States, they appear as
early as March and may be seen
until December.

Eggs usually can be found on
horses as soon as flies are seen. Egg
laying continues as long as adults
are present. Eggs of the common
bot fly that can hatch may be found
on the animals as late as December
in Northern States, and as late as
February in Southern States.

During the larval stage, the bots
develop inside the animal; they

grow from minute size to about two-
thirds of an inch in length. The
exact nature of the food they take
is not known, but bots feed on body
fluids.

Mature bots may be found in the
digestive tracts of horses and other
equines at any time of the year, but
they usually pass out of the animals
by October. Practically all the bots
found in animals during the early
part of the winter are young.

When fully developed, bots pass
out of the animal to the ground,
where they seek protection. They
crawl very little. Usually they bur-
row into the ground near the place
where they were dropped. They
burrow only deep enough to escape
the direct rays of the sun.

COMPARISON OF THE
THREE SPECIES

Common Bot Fly
(Gasterophilus intestinalis )

Adult—Largest of the three
species; about the size of a honey
bee. Body is covered with black and
yellow hair. Wings are mottled.

Female hovers about animal;
darts from place to place as she
cements egg after egg to hairs on
the forelegs (a favorite place), the

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mane, the shoulders, the belly, the
neck, and the flanks. Fly takes but
a second to attach each egg. Several
eggs may be fastened to the same
hair; many may be attached to the
mane and flanks.
One female can lay 500 eggs.
Egg—yYellow. Ready to hatch
in about 7 days. Eggs hatch any
time (within 2 or 3 months). The
animal licks or bites itself at the
spot where eggs are attached; heat
and moisture from the animal’s
mouth stimulate hatching.
Larva——Newly hatched larvae
are taken into animal’s mouth
when it licks or bites itself. Larvae
burrow into tongue. They continue
to burrow for 3 to 4 weeks, then pass
into the animal’s stomach, attach
themselves, and remain until ma-
ture. The total period for larval
development is 10 to 11 months.
Mature larvae pass out of animal
with feces.

Throat Bot Fiy

(Gasterophilus nasalis)

Adult—Smaller than common
bot fly and more rapid in flight.
Wings have no markings.

Female poises in midair, usually
near the animal’s forelegs, then
darts at the animal to attach eggs
on hair under the jaws; cements one
to four eggs to a hair during each
attack, flies away after each attack;
returns in a few minutes.

One female can lay 500 eggs.

Egg—yYellowish. Hatches in
about 6 days. Does not require mois-
ture or friction for hatching.

Larva.—Newly hatched larvae
crawl into animal’s mouth. Usually

4

Pupa—Similar for all three
species. The pupal stage begins
1 to 4 days after the bots burrow
into the ground. The outer skin
of the larva hardens to form a
protective coating—the pupal
case—this case 1s brown or black.

The bot changes into a fly within
the pupal case. At the end of the
pupal period, which ranges from
15 to 70 days, the fly emerges
from the pupal case and mates.
Another life cycle begins.

they lodge in the pockets between
the molars.

In 20 to 30 days larvae pass from
the mouth into the stomach, then
into the forward end of the small
intestine, where they attach and re-
main until mature. The total period
for larval development is 10 to 11
months.

Mature larvae pass out of animal
with feces.

Nose Bot Fly
(Gasterophilus haemorrhoidalis )

Adult—Smallest of bot flies;
most rapid in flight. Most hairs on
body are dark. Wings have no
markings.

Female darts at the lips of the
animal; deposits one egg at the base
of the hair, then darts away. A few
seconds later she strikes again. Eggs
are deposited close to the skin,
usually along the front of the upper
and lower lips.

One female can lay 160 eggs.

Egg-—Black. Hatches in about 2
days. Moisture is necessary for

hatching, but apparently friction is
not required.

Larva—Newly hatched larvae
burrow through the lips to the in-
side of the lips just in front of the
teeth. Here they remain for 6 weeks
or more.

Larvae then pass to various parts
of the stomach, and attach them-
selves to the lining. The greatest
number attach near the stomach
exit.

Mature larvae release their at-
tachment in the stomach, pass to the
rectum, and attach again. They re-
main here 2 or 3 days, then release
their attachment and drop to the
ground. The total period for larval
development is similar to that for
the other two species (10 to 11
months).

DAMAGE

Horse bot flies damage animals
indirectly and directly.

Animals under attack may inflict
damage on themselves or on anyone
trying to handle them. Fright and
irritation caused by egg-laying
adults or newly hatched bots may
result in animals going out of
control.

Direct damage is produced by lar-
vae feeding on the tissues of the
animal.

Indirect Damage

By flies—Animals fear the flies
and are annoyed by them. The com-
mon bot fly is the least annoying
of the three species, probably be-
cause it does not fly toward the
animal’s head as do the throat and
nose bot flies.

On warm, sunny days, when bot
flies are most numerous, horses and
mules on pasture fight the flies from
morning until late afternoon. They
walk about trying to get rid of the
flies, constantly bobbing their heads
as they walk. If this fails to prevent
attack, they run. They may gather
in the shade on high ground when
the wind blows, or they may enter
barns or other shelters. Colts which
are heavily attacked may lie down
near the older animals.

While the horses are fighting the
flies, they are unable to graze; after
days of bot fly attack, they may lose
weight and suffer from lack of
proper nourishment.

-Men who go near horses being at-
tacked by flies may be injured se-
verely. Horses attacked in harness
or when saddled often become un-
controllable. Runaways commonly
result.

By larvae—Newly hatched bots
produce a severe irritation as they
burrow into the animal’s tongue,
gums, or lips. The irritation and
itching caused by the young nose
bots is particularly severe.

To relieve the irritation, animals
may stand at watering troughs, dip
their lips into water, and rub their
lips violently against the wall of the
tank. They may injure themselves
by rubbing their lips and noses on
the ground, fences, stones, or other
objects.

Direct Damage

The bots attach themselves with
spiny mouth hooks to the lining of
the animal’s stomach or small intes-
tine. These attachments cause 1n-

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On eae

flammation, which interferes with
digestion.

Bots attached to the lower part of
the stomach interfere with the pas-
sage of food. Infested animals often
suffer from colic or other gastric
disturbances.

The degree of damage done by the
feeding of the larvae is roughly in
proportion to the number present.
Several hundred larvae may be
found in one animal; more than
1,000 have been found in_ the
stomach of a colt.

CONTROL

Bots control in horses should con-
sist of procedures for prevention of
infection, as well as treatment of in-
fections with effective drugs.

Preventing Infection

Infections of bots can be pre-
vented significantly by carrying out
both the following simple proce-
dures during the fly season:

@ Remove bot fly eggs from hairs
by chpping. .

@ Apply warm water rinses (at
120° F.) to induce hatching of the
eggs and the subsequent death of the
young fly larvae.

_ Drug Treatment

The available drugs that will re-
move a high percentage of bots from
the digestive tract are carbon disul-
fide, piperazinecarbodithioic acid
(piperazine and carbon disulfide),
trichlorfon, and dichlorvos.

M & A 4283

Part of a horse’s stomach heavily infested with bots. Note the lesions that these

bots have caused.

Because bots may reach the
digestive tract of horses throughout
the fly season, and for several
months afterward, the best control
of bots by drugs is by giving treat-
ments at intervals during the fly
season and at least once during the
following winter.

COMMUNITY CONTROL
PROGRAMS

A number of communities have
attempted bot control by a system-
atic treatment of all donkeys, mules,
horses, and colts. In only 1 year,
such treatment has greatly de-
creased the annoyance from _ bot
flies and has improved the health of
the animals.

It appears possible to achieve a
high degree of control of bot flies
by 2 or 3 years of systematic com-
munity effort.

To arrange for a community con-
trol program, consult your county
agent, farm advisor, or veterinarian.

PRECAUTION

Insecticides and drugs used im-
properly can be injurious to man,
animals, and plants. Follow the di-
rections and heed all precautions on
the labels.

In general, drugs should be ad-
ministered to horses under veteri-
nary supervision. However,.if pro-
prietary products are used, the
manufacturer’s directions on the
labels should be followed carefully.

Prepared by

Northeastern Region

Agricultural Research Service

Washington, D.C.

Revised April 1973
Slightly revised February 1976

For sale by the Superintendent of Documents,
U.S. Government Printing Office, Washington, D.C. 20402

U.S. GOVERNMENT PRINTING OFFICE : 1976—O-213-043 7