COOPERATIVE EXTENSION WORK IN
AGRICULTURE AND HOME ECONOMICS
(Acts of May 8 and June 30, 1914)
AGRICULTURAL EXTENSION SERVICE, UNIVERSITY OF FLORIDA
FLORIDA STATE UNIVERSITY
AND UNITED STATES DEPARTMENT OF AGRICULTURE
H. G. CLAYTON, DIRECTOR
Screwworms and Their Control
By LEONARD E. SWANSON AND OLIVER F. GOEN
Veterinary Parasitologist, Agricultural Experiment Station, and Animal Husbandman,
Agricultural Extension Service
Fig. 1.-Navels of calves, particularly calves dropped late in the season,
often are infested with screwworms.
AGRICULTURAL EXTENSION SERVICE
Screwworms and Their Control
Screwworm flies, Callitroga americana (C. and P.), are para-
sites that must develop in flesh. The females are attracted to
and deposit their eggs only in fresh wounds. The screwworms
are the larvae form of this fly. This pest has been a serious
handicap to the Florida livestock industry since 1933, causing
loss of flesh and even deaths in cattle, hogs and other animals.
Very cold winters may cut down on the number of infestations.
However, the flies usually survive short cold waves. Cold will
hardly kill the larvae in wounds and pupae are protected by
leaves, dirt and other debris. Following a severe winter, cases
of screwworms are light for a short period but the pests soon
re-establish themselves upon resumption of warm weather.
All warm-blooded animals (cattle, sheep, goats, deer, swine,
horses, etc.) are hosts, with an occasional infestation in man.
Any type of open wound, such as dehorning, castration, ear
marking, nail or wire cuts, dog bites or tears, hooking by other
animals, cutting of teeth in pigs, tusk injuries and wounds
created by external parasites, especially ticks and horseflies, are
the usual sites of infestation. When calving coincides with
screwworm activity the navels of newborn animals and injuries
to the dam are the most frequent sites of infestation.
To decrease the incidence of screwworm infestations, dehorn-
ing, castration and ear marking should be done when the flies
are inactive. Cold winter mornings are usually safe times for
these operations. Tip the horns of all cattle. Keep a close watch
of corrals, chutes and fences for nails, sharp boards, poles, etc.,
and remove them when found. Do not let dogs run animals,
especially vicious dogs that have a tendency to grab the ears or
noses of these animals. Avoid all unnecessary rough handling
of animals. Cut the tusks of old boars and pull the eye teeth of
Watch animals closely for any wounds, whether they be made
by man, insect or animal, and apply a recognized screwworm
preparation to them promptly. After a preventive treatment
is applied, observe the animal daily until the wound is healed
and, if necessary, re-treat. Spray or dip all animals in a
recognized insecticide for the control of ticks. Horseflies can-
not be controlled by the use of insecticides and, unfortunately,
there is no other control for these pests. Watch all animals for
screwworm signs swellings with a watery, blood-tinged
exudate, and matted hair. Foul odors may be noted for some
distance in severe infestations. Brands and other injuries
should be watched closely.
All newborn animals should be treated immediately after
birth. Where navel ill is a problem, apply tincture of iodine to
the navel region prior to applying the screwworm preparation.
There are several good screwworm preparations on the mar-
ket to prevent infestation and to destroy the larvae after they
have become established in the wounds. Smears or liquids con-
taining pine tar oil, benzol, diphenylamine or lindane can be
used effectively when applied liberally as a preventive or treat-
ment. Smear EQ 335, containing lindane and white mineral
oil, is a very effective treatment and prevention. Other similar
preparations may be used. Usually one treatment is sufficient.
LIVES UP TO 65 DAYS
FLIES MATE PUPAL STAGE
FEMALE FLIES ATTRACTED 3 TO 14 DAYS UNDER
TO FRESH WOUNDS FAVORABLE CONDITIONS
FLY DEPOSITS EGGS PUPAL STAGE IN
IN SHINGLED BATCHES LOOSE SOIL MAY LIC
(10 TO 400 EGGS) DORMANT 2 MONTHS
EGGS ON WOUND LARVAE CRAWL OUT OF
EDGES HATCH IN WOUND IN 5 TO 6 DAYS.
6 TO 21 HOURS BURROW INTO LOOSE SOIL
SCREWWORM LARVAE FEED
IN CLUSTERS- HEAD DOWN
TAIL OUT OF EACH WOUND
LIFE CYCLE OF SCREWWORMS
However, if excessive bleeding occurs, several treatments may
be necessary. In treating newborn calves use caution, as re-
peated applications and extensive smearing may be dangerous.
An effective control of the pests is primarily dependent upon
close observation of all animals and immediate treatment of