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![photo of screwworm mouth](jpgsgifs/k7576-1i.jpg)
Tusklike mandibles protruding from the screwworm larva's mouth rasp the flesh
of living warm-blooded animals.
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It's one of
the greatest entomological success stories of all times and also one of the
least well-known peaceful uses of atomic energy: perfection of an effective
control for screwworm, Cochliomyia hominivorax (Coquerel), using
x-radiation.
Obnoxious and destructive, the screwworm is
the only insect known to consume the living flesh of warm-blooded animals. It
has caused immeasurable suffering and losses in livestock, wildlife, and even
human populations the world over.
In the 1930's ARS scientists Edward F.
Knipling and Raymond C. Bushland turned their attention to alternative
screwworm control measures. They concluded that reducing or eliminating the
insect population would be a better solution than treating the pests topically
after entry into hosts via skin wounds, as was then commonly done.
Though World War II's pressing entomological
needs intervened, Knipling never gave up thinking about using genetic means to
control screwworms. In 1946, he was named chief of the Insects Affecting Man
and Animals Division of USDA's Bureau of Entomology and Plant Quarantine. Four
years later, colleague Arthur W. Lindquist recommended to Knipling a book by
Nobel laureate H. J. Muller titled "Drosophila," which discussed use
of radiation to alter the genetic material of insects. Knipling immediately
began a correspondence with Muller exploring the possible use of radiation to
sterilize screwworms.
Convinced that the approach could work,
Knipling reordered priorities to provide funding for Bushland to carry out
tests at Kerrville, Texas. Bushland secured the cooperation of a nearby U.S.
Army medical unit with suitable x-ray equipment. In just 6 months, it was
handily demonstrated that 2,500 to 5,000 roentgens of x-rays would sterilize
screwworm pupae without disrupting their adult mating behavior.
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Knipling's
theory was simple: Fertile females would mate with sterilized males mass-reared
in insectaries and released into infested areas. With offspring resulting only
from matings with native, unsterilized males, the screwworm population would
gradually become insignificant and perhaps disappear.
But could it really work? First, a field
test on Sanibel, the 20-square-mile Florida island, confirmed the theory.
Though encouraged, scientists knew that a larger test was needed to verify
those early findings. By chance, a routine request from a veterinarian from
Curacao, Netherlands Antilles, alerted them to the screwworm's presence on the
170-square-mile island. The Dutch government was eager to assist in its
elimination.
So a thousand sterile flies per square mile
were released each week by airplane. After just three weeks--the length of one
screwworm reproductive cycle--about 70 percent of new egg cases found were
sterile. After the next 3 weeks of releases, sterility was 84 percent. And by
the end of the third 3-week period, very few egg cases were to be found and all
were sterile!
The speed with which screwworm eradication
was achieved on Curacao demonstrated the great potential of this control
method. Since then, strategic deployment of sterile flies has been used
effectively in many locales--most recently, northern Africa--to protect vast
areas from the horrific screwworm's predations.
Ever the visionary, Knipling believes that
the sterile fly technique can be a successful management tool for many other
insects of economic importance. "It's just a matter of working out a few
more details," he says. He and Bushland were recently honored for their
screwworm research by the United Nations' Food and Agriculture Organization in
Rome, Italy.--By Linda R. Tokarz, ARS Information Staff.
[NOTE: The The sterile-insect approach
eradicated the screwworm from the U.S. in 1966. Since 1991, Mexico and several
countries in Central America have been declared free of screwworm: Belize,
Guatemala, El Salvador, Honduras and Nicaragua.]
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