Sex and the Screwworm

Hatched in open sores on cattle, their screw-shaped larvae can literally eat their way through a live steer. For years, they were a major scourge of the cattle country in the U.S. Southwest. It was not until the 1960s that screwworm flies were brought under control by a cunning form of biological warfare. Millions of flies, bred in a factory in Mission, Texas, were irradiated with sterilizing doses of gamma rays and released into the wild. When sterile males mated with normal females, which make only one sexual contact during their two or three weeks of life, the unions produced only infertile eggs--and the fly population and cattle losses dropped sharply. Now the number of infested cattle has begun to rise again, and three Texas scientists think they know one major reason why: the irradiated male flies are outperformed sexually by their normal counterparts.

Afternoon Heat. While probing differences between wild-and factory-bred flies, Zoologist Guy L. Bush and Biochemist G. Barrie Kitto of the University of Texas, with Zoologist Raymond W. Neck of the Texas parks and wildlife department, found that the larvae were kept at an unnaturally constant, warm temperature, mainly to speed up growth. Also, young flies were unable to fly around much in their cages. Eventually, the researchers write in Science, a markedly different strain emerged. No longer as vigorous, the male does not become active until the heat of afternoon, whereas his wild brethren are busy impregnating females from early morning until late afternoon, keeping the species alive and well.

U.S. Department of Agriculture scientists concede that the Texas scientists could have a point, but insist that other factors are also at work in the new wave of cattle infestation: warm weather last winter and moist conditions this summer have increased the birth rate of the fly; there are fewer ranch hands to check and treat cattle on the ranges; and a recent proliferation of Gulf Coast ear ticks has resulted in wounds on cattle that provide ideal hatching places for screwworm larvae. In addition, some scientists speculate that because the factory males are smaller and differently colored, the wild females may be finding them less attractive. In any case, future factory-bred males may be more formidable sexual competitors. The Texas factory and a large new breeding plant formally opened last month under a joint U.S.-Mexican commission in Tuxtla Gutierrez, Mexico, are now producing a more aggressive fly strain, tagged 009. Explains a commission spokesman: "He is a macho Mexican fly, and factory breeding should not dilute his sex drive."

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