Biology Reference
In-Depth Information
Box 10.1 Eradication of the “Flesh-Eating Fly”
The genetic-control method used to eradicate the screwworm
Cochliomyia hominivorax
was called the sterile-
insect release method (SIRM) or sterile-insect technique (SIT). The SIRM involves mass rearing, sterilization
of males by chemicals or irradiation, and their subsequent release to mate with wild females. Because
females of the screwworm mate only once, any wild female mating with a sterile male fails to contribute
progeny to the next generation (
Knipling 1955, 1985
). By releasing an excess of sterile males (compared
with the number of wild males), populations decline in a predictable manner, ultimately becoming extinct.
Because absolute population densities of
C. hominivorex
were typically low during the winter in the USA, the
number of sterile males that had to be released could be produced in fly factories.
The screwworm eradication program was initiated in Florida with small-scale trials on Sanibel Island in
1951. The results were promising and the project was geared up to cover the state of Florida and then the
southeastern United States. The screwworm was declared eradicated from the southeastern United States in
1959, one year ahead of schedule. Eradication was achieved in a surprisingly short time due to the combined
effects of a severe winter in Florida during 1957-1958, which greatly reduced the overwintering screwworm
population, and a 17-month eradication program beginning in July 1958 that cost
≈
US$7 million and involved
the release of almost 9 billion sterile screwworm flies over an area of
≈
56,000 square miles (
Meadows 1985
).
Since 1959, the livestock industry of Florida and adjacent states have saved at least US$20 million each year
because the screwworm is no longer present; actual benefits are even greater in today's dollars (
Meadows
1985
). Furthermore, the elimination of losses due to the deaths of livestock and the elimination of labor
and control costs are only part of the benefits; loss of wildlife to screwworm attack also was eliminated.
The success of the SIRM program in the southeastern United States led the cattle growers of Texas
to mount, in collaboration with the state and the U.S. Department of Agriculture, a similar but more
ambitious program in the southwestern United States in the 1960s (
Bushland 1985
). This program
required more time and effort because the area from which the screwworm was to be eradicated bordered
on a front 2400 km long, stretching from the Gulf of Mexico to the Pacific Ocean. Despite this challenge,
and some setbacks with quality control and reinvasion of flies from Mexico, both Texas and New Mexico
were declared “screwworm free” in 1964.
The SIRM program was moved into Arizona and California in 1965, and by 1966 the entire United States
could be declared free of screwworms. To reduce the likelihood that the screwworm would reinvade the
United States from Mexico, the program was expanded into Mexico in 1972, with the goal of eradicating
the screwworm all the way south to the Isthmus of Tehuantepec (
Pineda-Vargas 1985
).
After successfully eliminating the pest in Mexico, the SIRM program was expanded to cover all of Central
America (
Wyss 2000
). Screwworms were eliminated from Guatemala between 1988 and 1994, from
Belize between 1988 and 1994, from El Salvador between 1991 and 1995, from Honduras between 1991
and 1996, from Nicaragua between 1992 and 1998, from Costa Rica between 1995 and 1999, and from
Panama between 1997 and 2000. These eradication programs were carried out so that a barrier zone
could be set up at the 90 km wide Isthmus of Panama, which is only easier to maintain compared to the
2400-km border that the United States and Mexico share (
Snow et al. 1985
). This barrier zone is being
maintained by a combination of quarantines and mass releases of sterile screwworms.
Benefits of this massive, and expensive, screwworm eradication program are large (
Wyss 2000
). In 1996,
the producer benefits in the United States, Mexico, and Central America were estimated to be US$796
million, US$292 million, and US$77.9 million annually, respectively. These benefits were due to decreases
in deaths of livestock, reduced veterinary services, medicines, insecticides, inspections, handling costs, and
increases in meat and milk production (
Richardson and Averhoff 1978
). The estimated benefit to cost
ratios for the eradication programs average 12.2:1 for Central America to 18:1 for the United States and
Mexico (
Wyss 2000
).
