E-520 January 1941
United States Department of Agriculture
Bureau of Entomology and Plant Quarantine
RANCH MANAGEMENT FOR SCREWWORM PREVENTION AND ERADICATION
IN TEXAS AND ADJOINING STATES
By Daniel C. Parman and William L. Barrett, Jr.,
Division of Insects Affecting Man and Animals
Prevention of screwworm losses in livestock and valuable wildlife at
low cost has been one of the investigational goals of the Bureau of Ento-
mology and Plant Quarantine during the last 25 years.
Through research rather complete information has been obtained on the
identities, life, habits, adaptations, and regional and seasonal distribu-
tion and abundance of most of the blowflies of known economic importance in
the United States. Also, the more important factors affecting distribution
and abundance of blowflies have been determined and grouped into such
classes as climate-weather, animals attacked, natural enemies, method and
rate of dissemination, type of terrain, land usage, and ranch-management
Analysis of information obtained has led to recommendations for ranch
practices that will, if followed, materially reduce attacks of these flies
on livestock and game animals. The indicated ranch practices are considered
feasible by many officials and livestockmen and entail less expense than
The investigations have disclosed that 90 percent or more of all
screwworm cases are initially infested by one kind of fly, commonly called
the primary screwworm fly, 1 in all areas where the fly is established and
that very few wounds are infested in any areas where that fly is not present
Known scientifically since 1933 as Cochliomyia americana Cushing
and Patton. Hereafter in this circular the fly is referred to simply as the
2 Several groups of common blowflies rarely initiate worm infesta-
tions in wounds, but they may be destructive to animals as secondary invaders
after an attack has been started by the screwworm. The common blowflies
(Phormia regina Meig., Lucilia spp., Sarcophaga spp., and Cochliomia
macellaria (F.)), unlike the screwworm, develop in carcasses and infest
wool in sheep and goats. These common blowflies have short life cycles and
build up to great numbers in short periods.
This circular describes and recommends a plan of ranch management for
reducing attacks of the screwworm fly on livestock and game animals, and
presents the more pertinent facts upon which the recommendations are based.
ESSENTIAL FACTS ABOUT THE SCREWWORM FLY
Life Cycle and Breeding Habits
The sc,ewworm passes through several distinct stages. Eggs are laid
by .he adlt'1 fly; the eggs hatch into worms; the worms develop and when
mature hey drop from the wound and enter the ground and lie immobile in
their hardened outer coverings. From the ground the flies of the new
generation emerge and lay more eggs.
The young worms of the screwworm fly require a wounded warm-blooded
animal for their first 3 days of development, The early stages of the worms
have never been known to develop in dead animals. Should an infested animal
die after the worms are 3 or more days old, the worms will continue to
develop in the carcass the remaining 1 to 7 days required to attain full
The developmental period from eggs to mature adult flies capable of
laying eggs is about 3 weeks under most favorable conditions, or about twice
the time required ifor development of the most common carcass-breeding blow-
flies. TJider less favorable conditions the developmental period in the
ground iz lengthened; but the fly has no true resting or hibernating stage,
and the flies must emerge from the soil usually in less than 60 days or die.
The pecentage of survival and emergence from the soil is usually low if
cool weather retards development for more than 40 days. No adult flies are
likely to survive 2 months. Cold weather (below approximately 550 F,)
prevents adult fly activity, and temperatures below approximately 20 F.
kill adults in a few hours. Because there is no true hibernating or dormant
edin a of the life gcle of the screwworm fl it cannot survive
a, eriod of 4 months without producin1 at least one generation.
Factors Influencing Screwworm Attack
S ,, worm fly attacks and the resultant loss are proportional to the
nunt)ei of filies present. If fhe f]lics are limited in number, Lhe wounds are
not ail 'es, aiid infested ai r i usUly survive the initial attack
?nd e '3 ,boquent reinfestations without t eatments As the numbers of
the fly inorease to the highest populations encountered, all open unprotected
pounds beco01- infested. Even slight scratches and fly and tick bites may
become infested, and the larger wounds so grossly attacked that animals,
unless treated, may die in 2 or 3 days.
Even under favorable weather conditions, and in the presence of large
numbers of unheated wounds, the fly requires from 2 to 3 months to build up
high populations Populations of the screwworm fly build up and diminish,
other gatorss being constant, in proportion to the continuity and number of
wounded animals available for develupi.ig worulI The 1qiy.h clilu
(fig. 2) depicts the typical annual and seasonal pupulti11 ,i thn
adult fly on the western Balcones Escarpmiie_1t of the Edwai-d6 i dteaU in outhi-
western Texas. This also applies reasonably well to thie ute~naild of the
winter-survival area in southern Texas
In the different regions climatic codition6 produce auinual vairiations
in the population cycle from year to year, and the weather conditions produce
seasonal variations in the numbers of the fly in all areas There the
continuity of fly activity and survival is not broken at some time of the
year; high peak populations occur in the spring and the fall months Fewer
flies are present and much less fly activity occurs in the late summer and
in the late winter months.
In any locality the total annual population of the screwwurm fly has
rarely varied more than 50 percent from the average The most destructive
populations have usually built up over 2 or more years with favorable weather
conditions. In almost every year there are short periods in which the fly
becomes persistent in attacking livestock, when weather is most favorable for
activity of the fly and is such that wounds are kept attractive for longer
intervals than at other times. During those periods even a few flies may
cause more than ordinary losses.
Overwintering of the Pest
The fly is exterminated by climatic conditions every winter in that
part of Texas north of the solid line shown on the accompanying map (fig. 1),
and it rarely survives the winters in the area north of the broken line.
Tie fly survives during normal winters in the dotted area on the map. The
density of the dots indicates the winter abundance of the fly. Although the
numbers of flies that survive in all of the overwintering area are small,
two areas of higher winter populations have been found. One of the areas of
higher winter population is in the densely brushy section of the southern
Gulf Plains; the other is on the western Balcones Escarpment There is less
abundance in the intervening region. To Lhe present timta thei- t- bee
observed no relation of winter activity on the southerii Gulf' Plains area to
spring and summer activity in the region to the north.
Under present conditions a w(derately -high pvpuiation ot the- fily
builds up in the southern Gulf Plains. area late in the winter and eaIiy il;
the spring as a result of attack on young Calves, spreads to the Gull' coast
area to the northeast, attains highest populations usually ni April or May,
decreases to slight activity during th summer, and builds up again in the
fall in Gulf coast tick infestations in cattle.
In the area intervening between the Gulf coast and the Balcones
Escarpment a relatively low population su ,Aives the winter, chiefly by being
propagated in surgical wounds and in ccnnection with the birth of young iii
cattle. In the spring there is a slow build-up of flies until a peak of
fly abundance is reached in April and May, after which the fly population
diminishes rapidly as the result of hot, dry weather.
Along the westezn Balcones Escarpment there has been each year a close
rela ion bet'ee the amount of winter activity and survival of the fly and
the spring an. sumer activity and population of the fly in that area. Also,
the spr ead of the fly northward in Texas and to States as far away as Iowa,
Illiiois, and Mississippi is affected by the amount of survival on the
escarpment. The escarpment area from Terrell to Kerr Counties has con-
sistently had populations of the fly that were from 2 to 20 times as great
as in any other area in the United States.
The winte activity diminishes more from fall to spring in the
-sorpkuent area than iin the southernmost Gulf Plains area. Screwworm flies
living iv November ave dead iii February; only their descendants are alive.
Urdt~i present methods of ranch management a few cases of screwworms occur in
Deceiber aid January and these enable the pest to survive the winter. During
the winter months there are fewer natural wounds in livestock, such as those
caused by boils, prickly pear, and needle grass, and virtually no wounds in
wild animals other than a few produced by traps and bullets. No wild animals
that are common hosts of the screwworm have young during the winter. More
than 90 percent of the December and January screwworm cases have been found
to be in wounds caused by birth of vung domestic animals and by surgical
operations. Even in the presence of these man-made wounds, from each 100
flies present in November there results a much smaller number in the mid-
winter generation and only 3 survive until March. From these 3 an average
of 500 flies develop and become active in June. These 500 flies are produced
by successive generations developing in wounds resulting from shearing,
surgical operations, birth of young livestock and deer, and other miscel-
laneous wounds in animals. Some rather high populations of the fly have
ajprently developed from deer during the fawning season in some few sections
where deer are abundant chiefly in the eastern Balones Escarpment and on
the southeastern Edwards Plateau. The dates of fawning in the two last-
e~ioned areas are such that if the arrival of the screwworm there could be
ietarded i or 2 months, most of the fawns would be- saved, dnd the development
o" large numbers of screwworn flies that later attack domestic animals
would be prevented.
The screwwoim fly is miigratozy iind uny spread at a rate of approxi-
ate3y 5 miles a week. Mass migrations are not apparent, but there is a
Ji..persion of some :lies in all directions. The rate of spread appears to be
about the same from high or low fly population centers, but the number of
flies reaching adjoining uninfest ed territory is proportionalto the size of
the infested area and to the number of flies it contains. The first dis-
per'sion n th spring appears to take place in the most northern areas where
)VerWiIteri~g occurs These iclude river valleys in the escarpment, and
teritory north and west if the escarpment. The migrating flies usually
iv iIVC t ce~tral Texas by May i, in southern Oklahoma about June 1, in
utheia Kansas I ]y th< middle Ji iast of July, aid in Mississippi by Septe-
fav1e 1C th fly may reach IJoa by September. Few flies
emirate into the northern areas, and serious outbreaks would not occur if
there were no multiplication of the migrating flies. As th f li reacb the
Northern States weather conditions are favorable for fly breediig, and in the
presence of wounds that are untreated large fly populations resulT_ Later in
the summer hot dry weather tends to retard increase in fly abundiLce.
Many records have been made of shipments of screww ;rm-infested animals
to distant points from the southern Texas area, and such shipments, made
early in the season, have established infestations that have become destruc-
SUMMARY OF IMPORTANT FACTS UPON WHICH THIS PROGRAM OF SCR7WOR/
PREVENTION IS BASED
Worms must develop more than 3 days in a wound before they cai become
mature enough to transform later into flies. If all worms in all wounds
were killed before they were 4 davZ old., the screwworm fly would be ex-
terminated. It is almost impossible to find all cases of worms in livestock
in the rugged brushy area of the Balcones Escarpment, and the high wild
animal population makes complete wound treatment impractical during some
seasons of the year. Since there are few wounds in wild a.iimals in the
fall and winter months, complete treatment of all infested wounds may be
approached. In leafless brush, animals are more easily found; and as the
weather is cooler, animals can be handled to better advantage
When a program of ranch management for screwworm prevention is con-
sidered, the essential facts about the screwworm fly may be summarized as
1. The young worms require a wounded warm-blooded animal for develop-
ment and have never been observed to develop in carcasses.
2. The severity of attacks on livestock and wild animals is propor-
tional to the number of flies in an area:.
3. The fly has no true dormant stage in its life cycle and cannot
survive a period of 4 months without producing at least one new generation.
4. The population of the fly rises or falls, other actors remaining
constant, in proportion to the number and continuity of live, warm-blooded,
wounded animals available.
5. The fly is exterminated each winter by low temperatures in all
areas except the extreme southern portions of the United States. In Texas
the area of continued winter activity and survival is generally confined to
the southern part of the State shown on the accompanying map below the heavy
broken line, and is usually only in the dotted area,
6. The breeding of the fly under the prevailing ranch-management
conditions persists mainly because of wintertime surgical operations, the
birth of young livestock, the wounding of game animals, and the lack of
treatment. The winter season in the overwintering area is the critical
period in the breeding cycle and subsequent population of the fly. Condi-
tions are most critical for the pest in the northern portions of the winter-
survival area. In this area the insect is most destructive and from it the
early advance of the pest northward takes place.
7. By natural migration of the fly, which may be as rapid as approxi-
mately 35 miles a week. and by the shipment of infested livestock, the pest
spreads each year over the vast region to the north and east from the
relatively small overwintering area in southern Texas. When this rate of
dispersion continues for a number of months, sections as much as 1,000 miles
from the overwintering area may be infested naturally.
8. The importation of infested animals into uninfested areas may
establish early infestations that will have time to build up destructive
populations before the beginning of cold weather,
9. Although southern Texas furnishes the flies that each year
reinfest the remainder of Texas and adjoining States, each area suffers only
if the insect is permitted to breed up after arrival of the few flies by
migration or through introduction in infested livestock.
RANCH-MANAGEMENT PROCEDURES RECOMMENDED
TO AVOID SCREWWORM LOSSES
The screwworm has originated mainly from an abnormal number of wounds
in livestock during the winter months in southern Texas. The customary
system of ranch management in that region provides wounds at the time of
year when it is difficult for the fly to survive. If this system is changed
so that the birth of young and all wound-oreating operations are suspended
during this critical period, much ground would be gained in keeping the
initial population low. In some of the more northerly areas of Texas and in
adjacent States the necessary wound-creating surgical operations could be
performed and the birth of young could occur during periods of complete
absence of the fly Ranchmen in all areas outside of the winter-survival
area should instruct shippers not to transport animals infested with screw-
worMS. Animals imported should be examined, and, if infested, treated
On tho accompanying map are presented the suggested dates for carrying
out ranch-managemet procedures in each of the areas outlined. The chart
gives further details of the practices recommended for the area below the
heavy broken line on the map.
Briefly, Texas has been divided into 4 zones, r-unning roughly from
west to east, in which are indicated perioc' when surgical operations,
shearing, and the dropping of young should be avoided or carried out. These
zones are as follows:
Zone 1 includes counties south of a line running north of Terrell,
Kimble, Blanco, Lavaca, and Matagorda Counties. In this zone, surgical
opera ons should be performed during the periods February 1 to April 30
ind August 1 to September 0. All wounds should be avoided during the
periods October I to January 31 and May 1 to July 31.
- 7 -
Zone 2 lies north of zone 1 and south of a line running north of Pecos,
Upton, Green, Lampasas, Cherokee, and Shelby Counties. In this zone surgical
operations should be performed from December 1 to April 30, and all wounds
should be avoided between May 1 and November 30. Fall shearing may cause
some wounds during this period. These should be carefully treated.
Zone 3 includes the extreme western part of Texas from Brewster to
El Paso Counties and northward to a line running along the northern edge of
Bailey, Floyd, and Hardeman Counties, thence eastward along theRedRiver to
the Arkansas line. In this zone all surgical operations should be performed
between October 1 and April 30. All wounds, except those incidental to fall
shearing, should be avoided from May 1 to September 30.
Zone 4 includes all of the region north of zone 3. In this zone all
surgical operations should be performed between October 1 and May 31, and
all wounds should be avoided between June 1 and September 30.
Stock owners in adjoining States should make their ranch operations
conform in general to the periods indicated by extending the zone boundary
lines eastward or westward as the case may be.
If the suggested practices are followed by all ranchmen, the fly
should be eradicated in most or all of the present winter-survival area.
If the fly were eradicated in the winter-survival area the natural initial
infestations in more northerly areas would occur later or not at all. The
treatment of the first spring and early summer cases in all areas prevents
increase of screwworm fly abundance while weather is favorable for fly
The individual or community acting alone could profit by following
the procedures outlined. The individual ranchman could avoid screw-worm
cases in essential wounds and in birth of young by restricting such ssen-
tials to the most nearly fly-free period; and the community, by preventing
the development of worms in essential wounds and birth of young, would
eliminate the development of flies that could later initially infest wounds
or more grossly reinfest wounds.
Where conditions are such that the wounding and birth-management
practices outlined are not possible, the desired break in winter survival
of the fly can be obtained by careful treatment of all wounds every 3 days.
However, the elimination of wounds is simpler and more economical than wound
treatment, and prevention of screwworm breeding is more certain in complete
absence of wounds.
By managing to have young born and by making all necessary wounds in
certain short periods of time, long seasons of treating worm cases will be
avoided and more uniform herds of animals will be produced. The greatest
protection to livestock and game will be derived from a coordination of the
program in large areas, and the results of such procedures will be cumulative
from year to year as the annual population cycle of the fly is broken down.
STA-rF, ?L- O
The elimination of any cause of screwworm infestations does more than
prevent screwworms in those wounds, for by so doing the production of flies
which would infest other wounds is stopped. Clearing of useless types of
brush, killing cacti of various kinds, use of bloodless emasculators, removal
of projecting nails or other sharp objects in pens, reduction of needle grass,
and removal of any cause of accidental and unnecessary wounds will all
contribute to screwworm control.
The Bureau of Entomology and Plant Quarantine is developing wound
protectors that give longer and more efficient protection.
The purpose of this paper has been to present to the ranchmen the
more significant results of the research work that has been done on the
ranch-management phase of screwworm prevention. The information will
enable ranchmen individually, or, better, on an area basis, to begin work
toward abatement of the pest. The Bureau of Entomology and Plant Quarantine
realizes, however, that the ranching business changes over a period of years;
and as new changes become necessary and as new information is obtained from
research work, additional suggestions for improved and more adequate control
will be made available to livestock men.
ZONES AND DATES RECONWMENDED FO,
Oct. 1 May 31
June 1 Sept. 30
Oct. 1 Apri 30
May 1 Sept, 30
(Except fall shearing)
Perform surgical operations
Feb. 1 April 30
Aug.i Sept. 30 -LON,
Oct. 1 Jan. 31
May July 31
Normal winter screwworm
survival area dotted.
@ "L@-54TM "LS
0 2o 40 s 80 W
,NO WOUNDS DURING THIS PERIOD MEANS
DEATH TO THIS CROP OF FLIES.-.
TIME OF YEAR.
ONE WORM DESTROY-
ED WILL MEAN
ALL YOUNG, PERFORM
HOULD BE SURGICAL OPERATIONS,<
HEATED AT AND SHEAR AS SOON
SEASONS AS POSSIBLE AFTER
OlD WHEN FLIES z
OF SCREWWORM FLIES
FIG. 2-SEASONAL ABUNDANCE OF SCREWWORM FLY
AND ITS RELATION TO RECOMMENDED RANCH MAN-
AGEMENT FOR CONTROL, SOUTHERN TEXAS.(ZONE I -SEE MAP)
UNIVERSITY OF FLORIDA
3 1262 09224 7799