Two scientists planned to wipe out a killer fly by mating it out of existence
A female screwworm mates only once, so one mating with a sterile male means no offspring.
Construction Physics · The Fall and Rise of ScrewwormRaymond Bushland was working nights and weekends to find a way to sterilize flies. Bushland, a U.S. Department of Agriculture researcher, could not afford the local hospital's X-ray machine. It wanted $200 per test, so he called on his Army contacts to get free access to an Army hospital X-ray unit.
A few years earlier, Bushland and his USDA colleague Edward Knipling had talked themselves into an idea that sounded absurd. Their colleagues told them so. "Who ever heard of castrating flies?" they asked.
The idea was aimed at the New World screwworm, a fly whose larvae feed on living tissue. A female lays her eggs on an open wound in livestock, the eggs hatch into worms that burrow deeper into the flesh, and an untreated infestation is typically fatal. Ranchers hired hands whose entire job was inspecting animals for worms.
What fascinated Knipling was the math. In the laboratory, Bushland could raise huge numbers of flies. In the wild, Knipling had found, the population was surprisingly thin. And in the lab, Knipling confirmed a hunch from his fieldwork: a female screwworm mates only once in her life.
That one fact suggested the whole campaign. If they flooded an area with sterile males, especially where cold weather had already thinned the wild flies, most females would mate with a sterile male. Those matings would produce no viable offspring. Keep flooding the area, generation after generation, and the population would breed itself out of existence.
For nearly a decade, it was only talk. Professional skepticism was intense, World War II intervened, and a 1947 assignment to find a chemical sterilizer went nowhere. Then, in 1950, Knipling read an article by Hermann Muller, the Nobel Prize-winning scientist who had shown that radiation could cause mutations in fruit flies.
Muller's article was a warning about nuclear war and radioactive fallout creating "a world of sterile human beings." Knipling wrote to ask whether radiation could sterilize screwworm males on a mass scale. Muller replied, "I know nothing of screwworms but your theory is sound."
Encouraged, Bushland tested X-rays, then persuaded Oak Ridge National Laboratory to lend him highly radioactive cobalt-60, which produces powerful gamma rays. Knipling secured $20,000 from the Atomic Energy Commission, which was interested in radiation and sterility.
The dose had to be high, but it worked. The males were completely sterile, yet they survived and still mated. The females still laid eggs, but the males' chromosomes were so damaged that the eggs never hatched. Knipling and Bushland finally had a way to make sterile males by the million. All that remained was to try it.
Questions people ask
Did the sterile-male idea actually work in the field?
Yes. After proving that radiation sterilized males without stopping them from mating, the USDA first tested live fly drops on Sanibel Island in Florida and then on the island of Curacao, where raising the release rate eliminated viable offspring within 14 weeks. Those trials led to fly factories and the eventual eradication of screwworm from the United States, Mexico, and Central America.
Why does it matter that the female mates only once?
It is the linchpin. If a female mated many times, a single sterile mating would only reduce her fertility, because a later mating with a fertile male could still produce offspring. Because she mates once, one wrong choice ends her reproductive line.
Were the sterilized flies radioactive when they were released?
No, not in the way it was used. The flies were exposed to X-rays or gamma rays in the laboratory, which damaged their chromosomes but did not make them radioactive. The released males carried no radiation themselves and were harmless except that their eggs did not hatch.