A PERSONAL ACCOUNT OF PROGRAMS TO ERADICATE THE SCREWWORM,
Cochliomyia hominivorax, IN THE UNITED STATES AND MEXICO WITH SPECIAL
EMPHASIS ON THE FLORIDA PROGRAM
The great saga
of the eradication of the screwworm first from Curacao and then from all
of North and Central America is recounted with special emphasis on activities
in Curacao and Florida from 1951 through 1957. The author, who worked
as a research scientist on all aspects of laboratory and field research
and operations, brings to light many biological and operational problems
along with corresponding solutions, which are not treated in the published
accounts of USDA administrators involved in these programs.
Curacao served as
a 170 square mile outdoor laboratory for developing the sterile insect
technique. This setting permitted quantitative determination of the dynamics
of the wild population, and the overflooding ratios and dispersal patterns
essential for population suppression. The attack on the wild population
during the time of year when it naturally undergoes decline proved to
be essential in achieving eradication with minimal resources.
The Florida programs
yielded three extremely important findings. The first is that eradication
is greatly facilitated by taking advantage of severe weather events which
reduce the range and density of the target population. Secondly eradication
cannot be readily attained merely by the release of sexually sterile insects,
since it is absolutely essential that producers simultaneously attack
the immature stages by diligent inspection and treatment of wounds. Thirdly
the leadership of the producer clientele is critically important to securing
program resources from livestock owners, the State Legislature and the
Congress. These lessons were corroborated repeatedly as the program dealt
with the southwestern USA, Mexico and the Central American countries.
Key Words: Screwworm, Cochliomyia hominivorax, sterile
insect technique, eradication, Curacao, history
Se hace un recuento
de el primer programa de erradicación del gusano barrenador del ganado
en Curacao y luego en Norteamerica y Centroamerica, con especial enfasis
en las actividades realizadas en Curacao y Florida desde 1951 hasta 1957.
El autor de este artículo, quien trabajó como científico investigador
en todos los aspectos relacionados con investigaciones de laboratorio,
campo y como coordinador de operaciones, da a conocer los pormenores de
los problemas biológicos y de operación de este programa, así como también
de sus soluciones, las cuales no se han sido discutidas en ninguna de
las publicaciones administrativas de estos programas del USDA .
El area de 170 millas cuadradas del territorio
de Curacao sirvió como laboratorio para desarrollar el programa de tecnicas
de insectos esteriles. Esto permitió que se realizara una determinación
cuantitativa de la dinamica poblacional de la población salvaje del insecto,
y del area adecuada para liberar proporciones elevadas de la población
esteril los cuales ayudaron a establecer patrones de dispersión esenciales
para la supresión de la población. Efectuar liberaciones de insectos esteriles
en la epoca del año cuando la poblacion natural está en declive, fue un
factor esencial para lograr la eradicación en un programa que contaba
con recursos mínimos.
Tres resultados muy importantes fueron encontrados
a traves de los programas realizados en Florida. El primer resultado demostró
que la eradicación se facilita enormemente al realizar liberaciones cuando
hay cambios climaticos severos los cuales reducen el rango y densidad
de la población del gusano barrenador. Segundo, la erradicación no puede
ser lograda unicamente por el hecho de liberar insectos sexualmente esteriles,
sino que es esencial que los ganaderos controlen oportunamente los estados
inmaduros del gusano, efectuando un monitoreo y un tratamiento constante
de las heridas causadas por el gusano. Tercero, el liderazgo que tome
la clientela de los productores es esencialmente importante para asegurar
que hayan recursos que provengan de los ganaderos, la Legislatura estatal
y del Congreso de la Nación. Estas lecciones fueron corroboradas repetidamente
cuando el programa tuvo que realizarse en el suroeste de los Estados Unidos
de America, en Mexico y en los países centroamericanos.
The screwworm, Cochliomyia hominivorax (Coquerel),
is an obligatory parasite of living warm blooded animals, including
man. It has a life cycle of about 21 days during periods of warm weather.
The female oviposits on any lacerated or bloody areas caused by fighting,
barbed wire scratches, castration, dehorning, branding, ticks, and on
body openings with fetid odors. Moreover, the tender navel area of newborn
animals is particularly attractive. Unless the infested wound is treated
with an insecticide, flies will continue to oviposit on their host until
near death (Fig. 1). Laake
et al., 1936, has provided a detailed description of the various life
stages, economic importance, distribution, etc. The various life stages
are shown in Figs. 2 - 5.
The screwworm survives
only in mild climates such as those suitable for growing citrus (Fig.
6). Its overwintering area in the USA varied with the severity of
the winter. During a mild winter, it survived as far north as Oklahoma
in the Southwest, and in the Southeast as far north as the lower third
of Georgia and South Carolina. The average overwintering zone was 50,000
square miles each in Texas and Florida, with much smaller areas in California,
Arizona, and New Mexico.
Fortunately, a severe
winter in 1957-1958 eliminated screwworm activity above a line extending
from Tampa, Florida, to Vero Beach, Florida. As will be shown later, screwworms
were eradicated in Florida and the Southeastern United States in less
than one year instead of the two years anticipated if screwworms had survived
in the average overwintering area below Jacksonville, Florida. Indeed
if the program had started following a mild winter, three years may have
been required to eradicate.
male technique involves mass rearing the parasite, irradiating them late
in the pupal stage with gamma rays (Figs.
7 - 19), and releasing the adults usually from aircraft (Fig.
20) to compete with wild males in nature. Wild females mated to released
males oviposit normally, the embryos initiate development, but die before
hatching. Economic losses in Florida prior to eradication were estimated
at $20 million annually.
(1997) first conceived the idea of sterile insect technique for suppressing
screwworm flies in 1937. He visualized that the sustained area-wide release
of large numbers of sexually sterile males into the wild population would
eliminate their reproduction and lead to their eradication. As a fledgling
entomologist Knipling's proposal was not seriously considered by his superiors
in the Bureau of Entomology and Plant Quarantine, U.S. Department of Agriculture,
but the idea remained firmly in his mind and was discussed repeatedly
with his close colleagues, Dr. A.W. Lindquist and Dr. R.C. Bushland. During
World War II, Knipling and Bushland both worked on projects to protect
the armed forces from arthropods and arthropod-borne diseases. As a result,
their follow up on this innovative concept was delayed. However after
the War, as Knipling rose rapidly through the Bureau's ranks, he was able
to secure limited funds to research his idea. Bushland (Melvin and Bushland,
1935) had already fulfilled one of the requirements of the sterile insect
technique by developing an artificial diet for the screwworm. The diet
consisted of a mixture of ground lean beef, blood, water and 0.2% formaldehyde
to deter decomposition. Previously screwworms had been reared on rabbits
or baby calves, a very nasty and cruel procedure. To induce sexual sterility
various chemicals were evaluated, but none was found effective. However,
H. J. Muller (1950), reported in the American Scientist that exposure
of Drosophila to high doses of x-rays induces dominant lethal mutations
in the germ cells. These mutations prevent the development of the embryo,
and thereby cause sexual sterility. A.W. Lindquist read this popular article
aimed at swaying public opinion against atmospheric tests of atom bombs
and showed it to Knipling. Knipling corresponded with Muller, and Muller
expressed confidence that ionizing radiation would induce sterility in
the screwworm. Indeed Bushland and Hopkins (1951, 1953) observed this
same effect when screwworm life stages were exposed to x-rays or to gamma
rays from Colbalt60 (1953). They showed
in laboratory cage tests that sterile screwworm males were able to compete
with normal males in mating with untreated females. Since the screwworm
female mates only once, all egg masses laid by a wild female who mated
with a sterile male are non-viable.
In order to
assist in evaluating the performance in the field of irradiated sterile
males, I was reassigned from the USDA Grasshopper Control Division, first
to the USDA's Insects Affecting Man & Animals Laboratory, Kerrville,
Texas, and later to a sub-laboratory at Orlando, Florida. Prior to my
transfer, Bushland had made an attempt to evaluate the field performance
of irradiated screwworm males on the shoals of Texas near Austwell. This
effort had failed apparently because the released flies were carried by
the prevailing winds to the mainland. In any case no egg masses were laid
on wounded sentinel goats. Subsequently during the winter of 1951-1952
efforts were made on Sanibel Island near Ft. Meyers, Florida to evaluate
the capacity of lab-reared sterile males to compete with lab-reared normal
males in mating released females. Because livestock were not on the island
it was assumed that native screwworm flies would be scarce and not produce
sufficient egg masses for evaluation. However, native screwworms were
found to be as numerous as on the mainland, and to infest feral cats,
opossums, and rabbits. This experiment showed that lab-reared flies performed
well in nature, since a high ratio of sterile to fertile egg masses were
collected from the wounds of sentinel animals. Indeed during the following
winter, 1952-1953, when only sterile males were released at the rate of
100 per square mile, egg mass sterility temporarily reached 100% within
the first 8 weeks of releases. However during the 12th week
a fertile egg mass was found. Since Sanibel is only two miles from the
mainland - a fraction of the parasite's flight range - we realized that
eradication could not be demonstrated in this experiment because of migration
across the channel between island and peninsula.
Clearly the feasibility
of using sterile males to achieve eradication would have to be evaluated
on a fully isolated island small enough to accommodate an experiment on
a very limited budget. We believed that these requirements were met by
Vieques, nine miles from Puerto Rico, but we were prevented from working
there by the danger posed by use of half the island as a bombing range
by the U.S. Navy. Fortuitously, B. A. Bitter, focused our attention on
Curacao, Netherlands Antilles, forty miles north of Venezuela, South America.
Bitter, an agricultural officer on Curacao, wrote to the USDA Entomology
Research Division for recommendations on protecting dairy and grazing
animals from the screwworm. I was dispatched to this island in July, 1953,
to investigate its suitability for our test. Indeed, it had a serious
screwworm problem dating back at least ninety years. The island was populated
with about 25,000 goats and 5,000 sheep, 300 deer and numerous rabbits.
Cattle and horses were present but were not considered significant screwworm
hosts because of close surveillance and care by the owners. Because of
mortality caused by the screwworm, many of the nanny goats had only one
kid or none at all.
On March 17, 1953,
eight months after my survey, the way was finally cleared to begin the
eradication test (Baumhover et al, 1955). Egg mass data were collected
from screwworm infested goats maintained in ten pens throughout the entire
island (additional pens were established later). During the first two
weeks prior to the release of flies, all 288 egg masses collected were
100% fertile showing that unmated females did not oviposit and that our
egg collection and incubation techniques were adequate. During the release
of 200 irradiated flies of both sexes (see
Note 1) per square mile per week for six weeks, egg masses collections
increased from 121 in the first week to 277 during the sixth week. However
on average only 15% of these egg masses were sterile. Obviously, the rate
of 200 sterile flies per mile per week was insufficient to suppress the
natural population. Therefore we compared two rates of release. On half
the island we released 200 per square mile, and on the other half we released
800 per square mile. During the 3rd week of this test the higher rate
resulted in 53% sterility, and the lower rate induced only 28% sterility.
Therefore, beginning August 9, 1954, the entire island was treated with
800 sterile flies per square mile per week, although some deviations occurred
because of fluctuating supply of sterile flies from the rearing facility
at Orlando, Florida. During the first four weeks when egg masses averaged
only 4.6 per pen, sterility increased from 69% to 79%, and by seven weeks
sterility had reached 100%. No further egg masses were collected except
for single, small sterile masses during the thirteenth and fourteenth
weeks, respectively (Table 1).
results were in close agreement with projections made with Knipling's
model (1959). Eradication was achieved by increasing the release rate
when the native screwworm population was in a period of natural decline.
Releases were continued
through the 22nd week, when the test was successfully terminated. Through
the news media, radio, and print, we requested livestock owners to report
any larval infestations. Nine cases were investigated from mid-October
to mid-December. All turned out to be C. macellaria the secondary
screwworm fly, a scavenger of minor importance.
No further screwworms
were seen on Curacao until 1971, seventeen years after eradication. We
had outlined procedures for preventing re-infestation, but in spite of
this the screwworm likely was reintroduced by infested livestock shipped
from South America to be slaughtered at the abattoir on Curacao. Screwworms
were again eradicated by October 25, 1977. This was accomplished by first
using SWASS, a combination of a lure and insecticide. Its application
for ten weeks suppressed the population by 65% - 85%. Thereupon eradication
was easily achieved through the release of sterile males (Coppedge et
In 1953 sterile flies
were released from a single engine training plane of World War II vintage.
Initially, pilots of the Royal Dutch Airline, KLM, agreed to fly for us
when not flying for KLM, but surprisingly some became airsick and others
found their wives had better things in mind for them than dispersing screwworms
on their days off. Fortunately I was able to enlist the services of Peter
Mijs, an adventurous former pilot for the British Royal Air Force. He
was ready on a moment's notice, but I had to agree to detour from the
flight lanes to photograph incoming oil tankers so his partner could later
board the ships to sell photos to the crew.
The old Texas strain was
used on Curacao because the overworked crew at Orlando, Florida (Jack
Graham, Don Hopkins, and Frank Dudley) did not have time to colonize a
strain from Curacao.
Part of the difficulty
in mass rearing arose from the failure to recognize that the first third
of the flies to emerge are predominantly females and the last third are
predominantly males. Since the early part of each weekly batch was irradiated
and sent to Curacao the latest pupae were reserved for egg production.
As a result the colony cages contained a high proportion of males. Males
are highly aggressive sexually, so that when males outnumber females in
a cage, the females die prematurely as a result of incessant sexual harassment.
Under such circumstances egg production is reduced greatly (Baumhover,
was afraid we would eradicate the screwworm from Curacao before we gained
detailed scientific information about population dynamics and other factors
contributing to the success or failure of the sterile insect technique.
On the other hand Knipling and Lindquist were confident that the technique
was scientifically valid, but they were afraid we might fail because of
operational difficulties. I felt we had a least a 50-50 chance of success.
When sterility of
egg mass collections reached 100%, and later when collections decreased
toward zero egg masses, Wes New and I experienced great suspense each
evening as we tabulated the egg mass data. I wired the results daily to
Lindquist who - along with Knipling - eagerly awaited the report.
The last two small
sterile egg masses were collected after four weeks of zero collections.
Probably these were second or third ovipositions by long-lived screwworm
females. I was intrigued. Were these really screwworm eggs or the eggs
from a rare species overshadowed by the screwworm? However, I verified
that these were screwworms, since several of eggs in each mass had developed
to the spined stage, characteristic of the sterilizing dosage delivered
to our released males.
seemed imminent, I requested the local authorities to build a security
pen completely enclosed with fly proof screen. Goats to be used as sentinel
animals were infested with screwworms in this enclosure and held for three
or four days. Infested wounds emit an odor produced by bacterial action
which makes these wounds far more attractive to screwworms than uninfested
wounds. Before the goats were removed from this secure compound, all larvae
were removed from the wounds and killed with benzol. Subsequently these
wounded goats were transported to the ten or more pens used to collect
egg mass data. Fortunately, we never lost a goat through escape or theft,
since fertile flies produced from the wounds of lost infested goats could
seriously prolong the eradication program.
To increase our
surveillance during the final weeks of the test we established additional
pens in the lower southern portion of the island. Although a few sterile
masses were collected during the first two weeks, the number quickly dropped
We are deeply indebted
to the Curacao Administration, not only for allowing us to conduct the
experiment but for assisting in carrying it out. Bitter was assigned to
us full time and helped us immeasurably by procuring goats, securing the
cooperation of land owners, solving problems as they arose, and assisting
in routine fly releases and egg mass collections. KLM, the Royal Dutch
Airlines gave priority to our fly shipments, which always arrived on time,
even during the Christmas Holiday rush. The local government also furnished
a caretaker who fed and watered our goats.
FLORIDA 2000 SQUARE MILE TEST
The success on
Curacao induced the Florida Livestock Board to insist that the USDA draft
a proposal for a Florida and Southeastern US Eradication Program. The
proposed program would require a two year all-out effort on an enormous
scale requiring the weekly production, irradiation and release of ca.
50 million sterile flies throughout 50,000 square miles. By contrast the
Curacao program had involved only 170 square miles - not even 1 percent
as large as required to cope with the parasite in the southeastern US
- and had required less than 200,000 flies weekly.
Therefore an intermediate
step was taken to test the sterile insect technique in a 2000 mi2
area southeast of Orlando, Florida, bordering on the Atlantic Coast (Baumhover,
1958; Baumhover et al, 1959; Graham & Dudley, 1959). This was a cooperative
effort of the Florida Livestock Board, USDA Animal Disease Eradication
Division, and the USDA Entomology Research Division.
Because of the foul
odors associated with larval rearing, a temporary rearing facility was
constructed near Bithlo, an uninhabited area 20 miles east of Orlando,
Florida. Several carloads of discarded temporary building sections were
sent from Beltsville, MD and used by the ARS research team to construct
a rearing facility. Since I had tinkered with radios and knew a little
about electricity, I was appointed chief electrician. It was disconcerting
to me and to other professional scientists to be removed from exciting
on-going research projects, and required to work as laborers in construction.
However - because adequate funds were rarely available - this was standard
practice during the early years of research, development and implementation
of screwworm sterile insect technique programs. Only after the successful
eradication of the screwworm from Florida and the Southeast United States
were adequate funds (ca. $500,000) appropriated annually for research
to support the program in Texas, the Southwest U.S., and Northern Mexico.
Beginning May 2,
1957, one thousand flies per square mile per week (2 million) were released
against a dense wild population which was infesting 80 to 100% of new
born calves (Meadows, 1985). In spite of the severity of this screwworm
outbreak , the number of egg masses in the release area declined from
a high of 575 per week to only 17 the 16th week ending August
24, 1957, and egg mass sterility had risen to ca. 70% sterility by August
10 (Table 2). There was also a
decline in egg masses in check pens south and west of the release zone,
but collected egg masses remained numerous in the north.
Since the test area
was not isolated, eradication could not be achieved. Nevertheless, since
trends in the egg mass collections mimicked those on Curacao, the test
was considered successful and it convinced livestock owners, various -
but not all - government officials, and Florida legislators that eradication
in Florida was feasible. Indeed Knipling (1985) stated if egg mass sterility
under these circumstances had reached only 50%, he would have deemed the
test to be a success.
ERADICATION IN FLORIDA AND SOUTHEASTERN UNITED STATES
Prior to 1933
the dreaded screwworm fly was not present in Florida, but when introduced
with infested cattle from the drought-stricken Southwest into southern
Georgia, it spread southward to overwinter in Florida. Moreover, within
three years the parasite had infested the entire state. Losses were catastrophic
until ranchers were given assistance by the USDA (Bruce and Sheely, 1944).
An estimated 75,000 screwworm cases occurred in south Georgia during 1933,
and by the end of 1934 an estimated 1,300,000 cases had occurred in Florida.
Control programs sponsored by the State and the USDA reduced the annual
cases to only 48,737 during 1936, and animal mortality of 12% during 1934
was reduced to only 0.71%. This type of control is essential to screwworm
eradication by means of the sterile insect technique when conditions are
ideal for an increase in the native population. Indeed this fact was demonstrated
unequivocally in attempting to eradicate the persistent infestation in
Broward and adjoining counties.
owners were particularly anxious to be rid of this intruder. However,
the official USDA policy was that in spite of the success of 2000 mi2
test, more research was needed. Consequently, the print media criticized
the USDA "for dragging its feet". In a conference with Governor
Collins of Florida, Knipling told him that two years of additional research
would result in $2 million a year in savings. Collins replied, "why
wait two years to save $2 million when losses are $ 10 million per year?"
Indeed, these losses were estimated at $20 million by some ranchers and
at $40 million by Knipling (1959). Thus Governor Collins astute reckoning
made a deep impression on Knipling's philosophy regarding wide area insect
control. Florida livestock owners, under the leadership of Okeechobee
rancher, J. O. Pierce, soon convinced their legislature to appropriate
$3 million as their share of a proposed $6 million program.
1957, after the successful conclusion of the 2000 mi2
test about thirty individuals interested in the program met to design
a mass rearing facility. Each time an additional, prominent participant
entered the room the plan changed dramatically. However, in the end the
final design was left to the genius of USDA engineer, C.N. Husman (Baumhover
et al, 1966). Facilities at a World War II airbase, 7 miles east of Sebring
had been selected as the site for rearing. Husman consulted frequently
with us to determine optimum holding and handling conditions for the reared
insects. Previously Husman had worked many years supplying equipment
and facilities for entomological research. Frequently, we informally urged
Husman to enlarge the dimensions of the requested facilities. As a result,
he surreptitiously, but wisely, increased production capability by 50%.
Indeed Husman was right in taking this action. After the program got underway
as many as 80 million flies were produced weekly to cover 85,000 square
miles during peak activity, when widely scattered cases required releases
as far north as Montgomery Co., Alabama.
program was initiated in an unexpected manner. However, luck was with
us. The coldest winter ever recorded occurred in 1957-1958, killing screwworms
southward to a line running from Tampa to Vero Beach, Florida. To take
advantage of this break in the weather, USDA officials in December, 1957,
decided to expand the research facilities at Bithlo to produce up to 13
million sterilized flies per week (Bushland, 1960). The intention was
to use these sterile flies to prevent the reinfestation of the northern
half of Florida and Georgia.
18 to April 1, 1958, weekly production averaged only three million sterile
flies weekly. These were released primarily in a band across the peninsula
between Gainesville and Orlando at the 200 per square mile per week rate.
However beginning April 1, 1958, when production had been increased to
14 million sterile flies per week, they were released at this rate across
the northern half of Florida, and north to Savannah, Georgia, to combat
scattered outbreaks above the overwintering line. Also some were released
as far south as Miami.
quarantine lines in Florida and along the Mississippi River had not been
established in time to prevent shipment of infested animals into the screwworm
free areas from the Southwest or from Southern Florida. It is conceivable
that with adequate quarantines that flies from the Bithlo facility alone
would have eradicated screwworms from the much reduced overwintering zone.
Thus the large expensive plant at Sebring may not have been needed.
one have explained this to the administrators and the legislators who
approved the $6 million dollar program? Simple: coldest winter ever recorded.
Only 865 screwworm cases were confirmed in Florida from April 1, 1958
to February 19, 1959 (Table 3).
Of this number only 31 occurred more than 50 miles above the overwintering
line (Baumhover, 1966). However, there were 1901 cases reported but not
confirmed of which 1711 (90%) were probably screwworms as found by Knipling
& Rainwater (1937). Thus, the total number of cases probably was ca.
2575, or 0.08% of a possible 3 million cases if an eradication program
had not been in effect. In less than one year, April 1, 1958 to February
19, 1959, screwworms were no longer detected in the entire Southeast.
Broward County and the adjacent counties of Palm Beach and Miami-Dade,
screwworms were only a minor problem throughout the remainder of the southern
half of Florida during the entire program. Only 19 confirmed and 16 reported
cases were recorded in these counties from January 1, 1958 through August,
1958 (35 weeks) for a weekly average of 1.0. However, during September,
1958 the average increased to 4.75 (Table
4) and peaked at 33.0 between mid December to mid January, 1959. The
percent-infested wounds peaked at 5.86% during this same period. Finally,
by February 19, 1959 the last infestation was recorded as a fertile egg
mass (40 eggs) taken from a 1 day-old calf. This does not include a spurious
case recorded June 17, 1959, and discussed below. An additional 23 sterile
egg masses were collected up to March 13, 1959: 4 of these contained 6
eggs or less, and 2 had malformed eggs, indicating they were oviposited
by released females. Similar data were obtained from adjacent Palm Beach
and Miami-Dade counties but are summarized for brevity. Palm Beach had
32,434 wounds observed with only 35 cases (0.15%); Miami-Dade had only
3,321 wounds with 30 cases (0.90%). Even though egg mass sterility ranged
from 74.5 to 76.3% from November 23, 1958 to January 17, 1959 (8 weeks),
time for almost 3 generations, a substantial downturn in cases did not
occur until mid February, 1959, when localized release rates had been
increased from 400 to 10,600. In November 1959, eradication was declared,
and the program was terminated.
PROBLEMS IN THE FLORIDA AND SOUTHEASTERN PROGRAM AND SOLUTIONS
A. Persistence of screwworms in Broward and adjacent Palm
Beach and Miami-Dade Counties
1. The Problem:
a. Sharman (1960) attributed the screwworm outbreak in the Broward County
area to "poor" quality flies. However, there is no evidence
that the effectiveness of the sterile males had diminished. In fact it
may have increased due to the reduced irradiation effects discussed below.
Based on a review of the data available I am convinced that the primary
reason for the outbreak was complacency of the ranchers. As shown above,
screwworm cases averaged only 1.0 per week in Broward County, and less
than this in Palm Beach and Miami-Dade from January 1, 1958 to August
31, 1959. Too many ranchers decided prematurely that eradication had been
achieved, and they had abandoned the animal husbandry practices needed
to avoid a population build up and serious losses.
b. Hundreds of new-born calves were not treated. Infested calves were
not treated until after many of the larvae had left the wounds to pupate
and later emerge as fertile flies. Calf navels accounted for 415 out of
491 (84.5%) collections made, including both egg masses and larvae from
November 17, 1958 to March, 13, 1959. Over 36% of the wounds had mature
3rd instar larvae ready to drop out and pupate. One rancher had inherited
an additional ranch and unsuccessfully attempted to maintain screwworm
control without employing additional laborers. As a result he was one
to two weeks late in treating his newborn calves. Another rancher, even
though he had been advised by a livestock inspector that many of his herd
were infested, delayed two weeks before rounding up his animals for treatment.
A third rancher had no personnel to treat infested animals.
c. A wildlife refuge within the problem area contained feral hogs notoriously
susceptible to screwworm infestation because of fighting and udder wounds
created by suckling pigs. Feral hogs were also implicated in persistent
screwworm populations in Hardee, Desoto, and Lee counties.
d. Korlansmear, a successor to EQ-335, proved to be ineffective in treating
screwworms in wounds.
e. Release rates of 400 were not increased until late September, 1958.
This was too late to prevent the build up of the native population.
f. Many fertile egg masses collected were small, indicating that long-lived
females had mated to fertile males prior to substantial increases in the
g. In this southern portion of the state mild temperatures and rainfall
were ideal for screwworm development and survival throughout the entire
h. We failed to realize that historically, screwworms had been most abundant
in Broward County during each November, when most calves are born. Flytraps
had been operated throughout the state prior to the eradication program,
but the data did not reflect the potential for screwworm buildup in Broward
County during November 1958.
i. The supervisor of airplane release operations decided not to disperse
flies over metropolitan areas until it was apparent that screwworms were
out of control.
j. Due primarily to the delay in establishing quarantine lines in northern
Florida and along the Mississippi River, isolated screwworm infestations
were found in Alabama and Georgia hundreds of miles from known infestations.
As a result, 48 million sterile screwworms were released in Alabama and
66 million in Georgia. Had these outlying cases been prevented, these
sterile flies could have been deployed in southern Florida to shorten
the eradication effort.
2. The Solution:
a. Livestock inspectors were increased from three (one per county) to
an average of six from October 4, 1958 to December 20, 1958, and then
to an average of twelve through January 31, 1959 in an attempt to cover
all the livestock owners in the infested area. Up to fifteen inspectors
were active during the remainder of the campaign through mid-March, 1959.
b. Ground releases of flies were made on the most heavily infested premises
to ensure an abundance of sterile males in the vicinity.
c. From November 18, 1958, until May 15, 1959, up to 193 liver-baited
traps (Fig. 21) were operated
in the infested area (4 per mi2). During
the first 13 days, 8 sampling traps caught an average of 6,037 released
and 61 wild females for a ratio of 99: 1. From November 18, 1958 to December
14, 1958, 216 wild females were caught. During the same period only 73
infestations were found by livestock inspectors.
d. Egg mass collections were given priority since they provided the best
measure of progress. As shown on Curacao, and in the 2000 square mile
test in Florida, to achieve success egg mass sterility must increase as
the number of egg masses collected decreases. From November 17, 1958 through
January 23, 1959, (10 weeks) egg mass sterility averaged 76.5% in 230
masses collected but in the final week of this period 32 masses were collected,
9 more than the weekly average of 23 for this period. As Knipling's models
show, this percentage of sterility is still not high enough to bring down
the population when the potential for increase from one generation to
the next is quite high, as it undoubtedly was in Broward and adjacent
counties. Unfortunately, egg mass data after January 23, 1959 were of
little value because of oviposition by sub-sterile females resulting from
anoxia (Baumhover, 1963) in the irradiation canisters (see "D"
below). However, only 2 fertile masses were found after this date. Although
the scheduled dose rate of 8000r may have been reduced through anoxia
to 4000r, this remained more than the 2500r required to sterilize males.
As a result little if any hatch would be expected because of the preponderance
of sterile males in the population. However according to LaChance (1963),
if the released females had mated with fertile males, a substantial number
of viable eggs would have been produced. In LaChance's two tests the females
- which had been mated to fertile males and which had received 3500r -
produced an average of 45.9 to 53.4 eggs each, of which 21.7 to 49.5%
e. All infested herds except two, reported by the livestock inspectors,
were sprayed with CO-RAL, an effective insecticide with several weeks
residual action (Fig. 22).
One owner refused to round up his stock for treatment; and another had
no one to round up her herd. Baby calves were treated only with insecticide
"smears" because of their sensitivity to CO-RAL.
f. Heavy rains during January 1959 may have reduced emergence of pupae
in soil in areas submerged for more than four or five days.
g. Special treatment ("hot spotting") was begun as early as
June 6, 1958 in Osceola, Co. and later in Lee, Desoto, Hardee, Palm Beach,
Broward and Dade counties. By October 1, 1958 we decided to move a mobile
laboratory into counties with persistent infestations. However, by the
time the lab was ready, all of the "hot spots" were free of
the screwworm except Palm Beach, Broward and Miami-Dade. As a result,
the mobile lab was set up November 17, 1958 in Broward County to develop
the information given above.
may never know how many of the above measures represented "overkill",
in this "all out" assault the use of all possible weapons was
justified since the entire program could not be terminated until the Broward
County infestation had been eliminated. Needless to say the Herculean
measures taken in this area caused much alarm to those contemplating an
eradication program in Southwestern United States. However, these results
clearly indicate that a critical requirement of an effective eradication
program is extensive cooperation from ranchers in promptly treating their
susceptible and infested animals, particularly during periods of optimum
weather when attractive wounds and navels are abundant. Without this cooperation,
the cost of producing and releasing enough sterile males could be prohibitive.
B. Pupal Mortality
the entire period of production at Sebring, pupal mortality averaged 25%.
I found this to be caused by desiccation of prepupal and early pupal stages
(Baumhover, 1963), which are highly susceptible to water loss until the
prepupal membrane has formed. Larvae were reared on the second floor and
dropped into huge funnels extending almost to the ground floor. They were
collected in sand trays on a moving belt. Since the flow of larvae was
erratic some trays were too heavily loaded and many of the young pupae
remained on top of the sand, subjected to ambient temperature and humidity,
until their transfer to a room controlled at 80°F and 85% relative humidity.
Another disadvantage was the accumulation of spent medium in the funnels
carried there by the larvae as they left the rearing media. This provided
the traction needed by larvae to crawl up the funnel, and periodic air
blasts were required to force them into the collection trays below. Surprisingly,
the full-grown larvae were able to squeeze through the riveted seams of
the aluminum funnels. These escapees would fall to the ground floor to
create a fly escape hazard, since the lower floor was not fly proof.
and protection from desiccation was greatly improved by installation of
new collection system devised by Husman (Fig.
13) (Baumhover et al, 1966). It consisted of a water sluice onto which
the larvae fell after leaving the rearing vats. I was not concerned about
drowning the larvae since they were in the sluice only a few minutes and
in my laboratory tests five days under water were required to produce
100% mortality. The sluice transported the larvae to a separator where
they collected in measured quantities for placement in optimum numbers
into pupation trays. Within minutes, the trays were transferred to a controlled
room environment of 80°F. and 80% relative humidity. Under these conditions
and improvements noted in "pupal separation" below, emergence
increased to 95% or more.
C. Pupal Separation
At Sebring, the
pupation trays were emptied every four hours onto a moving screen belt.
Larvae crawled through the screen and were collected in sand trays to
be returned to the pupation room. However, pupae remained on the screen
and were collected at the end of the belt, and then poured into screen-bottomed
trays to a depth of two inches. Pupae piled higher than two inches were
subject to uneven development because of metabolic heat. Indeed some pupae
piled more than three or four inches deep actually died from the resulting
high temperatures. Large larvae and those partially immobilized as they
entered the prepupal stage became stuck in the screen belt, and they were
crushed by the roller. (This operation was improved in the Southwestern
program by replacing the screen wire belt with a solid rubber, ribbed
belt (Baumhover, ibid) (Fig.
15)). A bright light was mounted above the belt, and it caused the
mobile larvae to crawl off. Subsequently the latter were returned to trays
and transported back to the holding room. Another improvement was replacement
of the sand with sawdust which could be burned when spent. When sand was
used it had to be cleaned periodically with a steam generator, which caused
delays occasionally when it had to be repaired. On such occasions, larvae
were subjected to desiccation in the empty pupation trays or they were
killed when hot sand was used.
D. Anoxia In Irradiation Canisters
heat of the Broward County campaign, pupae were irradiated at six days
of age instead of five in an effort to improve the vigor of the released
flies. As a result, the concentration of oxygen was reduced through increased
metabolism. Consequently the effectiveness of irradiation was reduced,
so that released females were able to oviposit large numbers of sterile
eggs, (Baumhover, 1963). It was highly embarrassing to have to explain
the rings of white eggs around animal wounds to ranchers. The answer
was: "sterile screwworm eggs". To further exacerbate the difficulty
one technician loaded pupae into many canisters far ahead of schedule.
As a result of anoxia the scheduled dose of 8000r was reduced in effectiveness
to that of 4000r. This was corrected by replacing solid portions of the
canister walls and bottoms with screens (Fig.
17). Air could also be pumped through a drainage port connected to
the bottom of the irradiation chamber to exit outside the irradiator.
Needless to say, these egg masses from improperly irradiated females confounded
the critically important egg mass. However, small egg masses from the
released females could be recognized by their odd shapes, being blunt
on the ends or being banana-shaped.
E. Fail-Safe Irradiation Procedure
the irradiation attendant attached the radiation canister to a moving
carrier. The carrier transferred the canister into and out of the irradiation
chamber, and then returned it to the attendant who passed it to the packaging
department. There was no mechanism to prevent the attendant from passing
the canister to the packaging unit without first irradiating the pupae.
Although there was no evidence that this ever occurred, it seemed prudent
to forestall this possibility. This was done by changing the carrier to
transfer the canister directly to packaging after being irradiated (Fig.
18). Once the canister left the attendant's hands it remained in a
restricted area accessible only to the radiation supervisor (Baumhover
et al, 1966).
For the Mexico
program a new irradiator design by Husman was used. The irradiator was
installed in the wall separating the radiation area from the packaging
area. The only way the canister of pupae could be transferred to the packaging
area was to pass it through the irradiation chamber. This unit contained
Cesium137 with a longer half-life allowing
longer intervals before adjusting exposure time for decay and recharging
the unit than required for the Cobalt60
ERADICATION IN TEXAS, THE SOUTHWEST, AND MEXICO (see
it was not considered feasible to attempt screwworm eradication in Texas
and the other Southwestern states because of the 2000 mile long barrier
required to keep screwworms from migrating back from Mexico into the Southwestern
states (Bushland, 1952). However, when Texas ranchers learned of the success
in the Southeast, a delegation of livestock owners headed by Dolph Briscoe,
Jr. (later to be Governor of Texas) visited the Sebring plant to review
the operation. They were told that the Southwest program would be more
difficult but that it might be feasible. They were willing to take the
gamble and returned to Texas to raise $3 million from ranchers who contributed
$0.50 per cow and horse and $0.10 per sheep, goat and pig. The Texas Legislature
appropriated $2.8 million and the Federal government appropriated $6 million
to begin the program (Scruggs, 1975). A cold winter in 1961-62 killed
screwworms deep into the normal overwintering zone in Texas, much like
it had in Florida in 1957-58. A standby screwworm colony had been maintained
in rearing facilities at Kerrville, Texas for use in the event that screwworms
should reappear in the Southeast. Now this rearing operation was expanded
to produce sterile flies needed to prevent flies from migrating northward
from Mexico. The results of this huge undertaking were somewhat erratic
(Bushland, 1985), but the program progressed aided by sterile fly production
in a major rearing facility at Mission, TX. Thus by 1966 the entire United
States was declared free of screwworms, even though occasional cases occurred
due to the infiltration from Mexico. (For an early detailed history of
the screwworm eradication program in the United States and Mexico, see
Meyer and Simpson (1996), as well as Scruggs (1977) for in depth coverage
of the Southwestern U.S. program.) It became obvious that because the
2000 mile barrier between Mexico and the United States was not only ineffective
but costly, that screwworms had to be eradicated deep into Mexico to protect
the United States.
In order to
eradicate the screwworm in Mexico, an agreement was negotiated, which
included a cost-sharing arrangement. Also the Mexico-USA Commission for
Screwworm Eradication was established to manage the effort. The Mexican
program began in 1976 with construction of a rearing plant at Tuxtla Gutierrez,
Chiapas, Mexico. The Mission, Texas plant continued to operate along with
the Chiapas plant until January, 1981 by when screwworms had been eliminated
from the Northern states of Mexico. By 1987, the Mexican government declared
the area north of the sterile-fly barrier at the Isthmus of Tehuantepec
to be screwworm free (Meyer and Simpson, 1996). Moreover as a result
of continuing effort, Mexico was officially declared free of screwworms
in 1991, Belize and Guatemala in 1994, and El Salvador in 1995. In addition,
Honduras, Nicaragua, Costa Rica and Panama are largely free of the parasite.
The new barrier is being established at the narrow Darien Gap in the Isthmus
screwworm eradication program in Puerto Rico completed in 1975, the western
1/4 of the island remained screwworm free for eighteen months even though
the eastern 3/4 was infested. Apparently, the ideal conditions for screwworm
survival reduced movement. If similar conditions exist in the Isthmus
of Panama, we may be pleasantly surprised at the reduced cost of maintaining
EDEN - LINCOLN REPORT
a screwworm outbreak in Texas, in 1972, when 95,625 screwworm cases were
confirmed with many others not reported (Bushland, ibid) Congress ordered
an investigation of the program by outside specialists. The study was
conducted by Dr. Charles Lincoln, Entomologist, Department of Entomology,
University of Arkansas and Dr. W.G. Eden, Chairman, Entomology and Nematology
Department, University of Florida.
that, "in our combined 78 years of professional work, we have not
seen an area program of any kind that has the popular grower support of
the Southwestern Eradication Program." They further stated, "we
unhesitatingly recommend that the work of the Mexican-American Commission
for Screwworm Eradication proceed on schedule." They rated individual
years' results as "seven good, three fair, and one terrible, not
a bad track record" (Eden and Lincoln, 1974). In a survey contracted
by these authors in Florida, six prominent ranchers stated that return
of the screwworm would "put them out of business". They related
that screwworm infestations had required all their waking hours to reduce
losses. All of these ranchers had up-graded their herds from "range
cattle" to purebred stock after the screwworm had been eradicated.
soundness of methods used in the process of establishing, adapting, evaluating
and selecting screwworm strains to be mass reared irradiated and released
has always been of great concern to screwworm program personnel, particularly
when field results were below expectations. Attempts were made to colonize
a Curacao strain for release on that island, however, due to a lack of
resources and personnel this was not accomplished. Wild strains are difficult
to colonize because of their refusal to mate or produce eggs under laboratory
conditions. Nevertheless, larvae from eggs of wild flies adapt readily
to the artificial medium. However, we rationalized that conditions for
screwworm survival on Curacao were similar to Texas, and that the Texas
strain could be expected to perform adequately on this island.
we have seen above, the old Texas strain used on Curacao was eminently
successful in eradicating the native population when the latter was undergoing
a natural seasonal decline. Nevertheless, we took a significant risk by
not establishing a Curacao strain of screwworms for use in the event that
the Texas strain had proved unsatisfactory.
For the Florida
campaign we developed a local strain (FLA) and selected it for sexually
active and long-lived males. C.C. Skipper, our able survey supervisor
and liaison with local ranchers collected fully-grown larvae from infested
cattle throughout Florida and in Southern Georgia. To obtain sufficient
egg masses in the first generation we had to use infested goats to induce
oviposition. To select a vigorous strain my assistant, Wes New, caged
single males with 25 virgin females. Progeny from males impregnating the
most females - ten or more - and living the longest were pooled for subsequent
selection. By the 5th generation longevity had improved and successful
matings per male (based on egg hatch) had increased from 5 to 17. George
Spates and I later observed that individual males actually mated up to
72 times over a 7 day period when an abundance of virgin females was present.
However after the 6th or 7th mating on a particular day they no longer
were able to transfer viable sperm. Nevertheless on the following day,
the males again transferred sperm during matings.
for long-lived sexually active FLA males was completed in time for use
in the 2000 mi2 test in Florida. This
strain was also used during the entire Florida campaign and in the Southwest
from April 1962 to December 1966 when it was replaced by the Mexico strain.
This action was taken without field testing the performance of the FLA
strain, and was decided merely on the assumption that after ten years
the FLA strain may have deteriorated, and that it no longer was competitive
with Southwestern U.S. and Mexican screwworms (Eden-Lincoln, ibid.). Eleven
additional strain changes were made from November, 1971 to May, 1985,
based on screwworm outbreaks or inadequate adaptation to mass rearing
conditions. However, strain changes made after January, 1974 were based
on field tests. In Mexico strain changes are made on a periodic "programmed
basis" using field tests (Marroquin, 1985).
were made to colonize a Puerto Rican strain of screwworms. These island
flies were smaller in size and had a shorter life cycle than the mainland
screwworms. On one of my trips to Puerto Rico I collected several egg
masses and placed the hatched larvae on a rearing medium. Since I couldn't
wait for pupae to form (a much more feasible method of transporting live
insects) I carried them in a rearing container enclosed in a plastic bag
to avoid escape of the fetid odors associated with rearing. However, whenever
the plane landed I removed the package from the plane and opened it to
provide air exchange. Although my specimens arrived at the Mission, Texas,
plant in good condition, attempts to colonize them were unsuccessful.
The eradication effort in Puerto Rico and surrounding islands, using sterile
screwworms produced at the Mission, Texas plant, was hampered by a ten
day shipment delay between radiation and release. Pupae were held at 60°F
to prevent emergence, and this greatly reduced vigor of the flies. However,
when the delay was reduced to only three days, eradication proceeded on
We were able to eradicate screwworms
from Florida and the Southeast in less than a year, well before critics
could marshal their forces against the program. This was not the case
in Texas. In 1966 the entire United States was declared free of screwworms
and it became a federal responsibility to maintain the barrier zone from
the Gulf of Mexico to the Pacific. Thus the States most affected by the
parasite did not have to cost-share the program. Since the program could
not proceed deep into Mexico during the 9 years required to negotiate
an agreement, the strategic character of the program changed from eradication
to area-wide population management. However USDA Officials and State and
Federal politicians continued to refer to this holding action as an eradication
program. Therefore critics had a field day when screwworm outbreaks occurred
in 1968 and 1972. Several critics branded the program a failure and recommended
that it be abandoned. Most of the speculation revolved around incompatibility
of the wild females with the released, sterile males. If this were the
case, the appropriate counter measure would be to colonize, sterilize
and release the wild strain. Graham (1985) lists six articles purporting
to explain the reasons why the sterile male technique may not work, none
of which proved to be valid.
Other Items Of Interest
A. Early in the Florida program a screwworm sample was received
from a South Carolina veterinarian, prior to any collections from the
overwintering area in Florida. This concerned us deeply until the sender
admitted that he collected larvae of fly species other than the screwworm
from a wound, but inadvertently he had interchanged this non-screwworm
collection with a sample of screwworms.
B. After several months of negative screwworm reports in the Southeast,
a livestock inspector in northern Florida reported a navel infestation
in a new born calf. This induced the epitome of consternation for we had
never seen larvae other than screwworms in an infested calf navel. However
if confirmed this development would have gladdened the hearts of critics
who predicted that the screwworm, if eradicated, would be replaced by
another pest. I was flown to the area to investigate. As I approached
the calf, I detected a sweet odor in contrast to the foul smell of a true
screwworm infestation. To our great relief the navel area contained a
neat pocket of a Lucilia species, behaving like screwworms, and
rarely seen previously. Indeed, if the infestation had been that of the
screwworm, it would have caused the extension of the expensive eradication
program for additional months.
C. In late 1959, an inspector had reported the possible infestation
of 20 steers in south Florida. They had been castrated with a knife, the
preferred method in screwworm free areas, instead of the bloodless method
using constricting "O" rings. At first glance the scrotal areas
were swollen and bleeding, similar to wounds with screwworm infestations.
Fortunately, no screwworms were found, and this was additional evidence
that screwworms were scarce or non-existent.
D. I had the opportunity to solve a toxicity problem with the release
cartons at Mission, Texas, site of the Southwestern screwworm plant. One
of the investigators released flies from the cartons into a cage, removing
the carton shortly after the flies had left. These flies lived normal
life spans. I decided to conduct the same test leaving the cartons in
the cage overnight. The following morning most of the flies were dead.
Chemical tests detected a light residue of the widely used insecticide,
Lindane, in the cardboard. Since that time, release cartons and partitions
have been made only from virgin materials rather than recycled products.
E. On another occasion I was asked to head a team investigating
an unusual number of screwworm cases near the Mission, Texas plant. Wild
type screwworms had an interruption in the circle of spines on the 11th
segment (Fletcher, 1966); for an unexplained reason the plant strain had
complete banding. Since larval collections near the plant contained complete
banding, I concluded that the screwworm infestations were originating
from the plant. Although my conclusion was not readily accepted, later
inspection of the fly colony room revealed cracks in the walls. Since
the fly colony room was kept in the dark to prevent flies from congregating
toward the brightest light source, the daylight visible through the cracks
was highly attractive to flies in the room. Further evidence that flies
were escaping was obtained when larvae from collections near the plant
quickly adapted to plant rearing, whereas wild screwworms are usually
colonized with much difficulty.
F. No screwworm infestations were found in Florida from February
19, 1959 to June 17, 1959. However, on June 18 a confirmed sample was
received from a ranch within ten miles of the Sebring plant. The true
origin of the specimens was never determined, however, one of the following
possibilities could have occurred:
(i) A load of 100 Texas cattle transported by train had been delayed on
a ranch near the screwworm plant at Sebring, Florida, several weeks before
the infestation was reported. Screwworms may have escaped from infested
(ii) Screwworms may have escaped from the plant, an ever present concern.
(iii) Sabotage may have been carried out by an employee hoping to extend
his tenure in a well paying job by the unlikely opportunity to remove
fertile screwworms from the plant or by preparing a bogus report accompanied
by a sample of screwworm larvae.
(iv). Ranchers have been known to retain screwworm samples for later use
to "test surveillance by program personnel" or to obtain insecticide
treatment of the herd. This is tempting, since the CO-RAL spray used on
screwworm infested herds also controls other insect pests and ticks.
(v) The rancher may have inadvertently delayed submission of the sample.
of the screwworm from Curacao, followed by a successful 2000 mi2
test in Florida led to the eradication of the parasite from the entire
Southeast. This successful program served as a pilot test for the Southwest
with improvements in mass rearing, irradiation and understanding of factors
in problem areas. At the present time screwworms have been eradicated
from more than 90% of the previously infested areas in North America.
The cost benefit ratio in Texas has been calculated as high as 1:113.
Since 1958 the savings in North America are undoubtedly in the billions
of dollars (Knipling, 1997), and will continue at a half billion annually
so long as screwworms are prevented from re-infesting the screwworm-free
areas. Eradication from the Southwest protects the Southeast; eradication
from Mexico protects the Southwest and the barrier in Panama will protect
all of Central and North America.
I was very
fortunate to have played an official role in screwworm eradication from
1951 through 1965, when Dr. Knipling asked me to head the USDA Tobacco
Insects Investigations. However, I remained in close touch with the program
and attended many of the quarterly staff meetings as well as emergency
sessions until my retirement, February 4, 1984.
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