Sterile Fifth Column

At Oak Ridge National Laboratory last week, Entomologist R. C. Bushland of the U.S. Department of Agriculture was planning a dirty trick on an unpleasant insect: the screwworm fly of Texas and Florida. The female flies lay their eggs in open wounds (even scratches or tick bites) in the hides of cattle. From each clutch hatch about 200 maggots, which eat a hole in a cow as big as a lemon. Often other flies attack the same wound. Unless an outside agency (i.e., a cowpoke with anti-fly dressings) comes to the cow's rescue, she may be eaten alive.

Screwworm damage costs $15 million year in Texas and about $10 million in Florida, but in spite of the harm they do, the flies are not very numerous. Even in the warmest parts of the country, comparatively few survive the winter. Bushland decided that if he could slow their reproduction he might reduce their population or even wipe them out.

Following suggestions from earlier experimenters, he focused on a weakness of the flies: the females are strictly monogamous. They mate only once with one male. Then they reject other suitors and concentrate on laying eggs. So Bushland raised a flock of flies on a mixture of blood and hamburger and irradiated them heavily with X rays. This treatment made them sterile. When the X-rayed males mated with fertile wild females, the females laid infertile eggs.

Since the X-ray process was expensive, Bushland is now experimenting at Oak Ridge with radioactive cobalt, a cheap source of sterilizing radiation. Soon he plans to go south and hatch clouds of flies out of washtubs full of hamburger. One pound of hamburger, he figures, is good for 500 flies.

While they are still in the helpless pupal stage, the flies will be irradiated with cobalt. When they emerge as adults, they will be liberated in heavily infested districts. The females among them will do m damage to the cattle: they can lay no normal eggs. The males, sterile but still ambitious, will scour the country for fertile wild mates. But the females that they win will never lay fertile eggs. Bushland believes that "extensive use of this method will have a profoundly depressing effect on the screwworm fly population."

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