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The medfly: Future target for a new pest-control strategy.Scott Bauer/USDA-ARS

For decades, scientists have blasted insects with radiation to create sterile males that farmers then released for pest control.

Now, a team based in England suggests a more subtle approach. Two new lines of Drosophila melanogaster prove that it's possible to create insects with dominant genes that make females selfdestruct on cue, report Dean D. Thomas of Oxford University and his colleagues in the March 31 Science.

The next challenge is to use the method on an economically important pest, says Luke S. Alphey, who led the team. "Top of the list is the medfly," he says.

Although male irradiation has had successes—it wiped out screwworm in the United States—that technique is costly. Irradiating the insects to render them sterile leaves them less healthy than wild competitors. Producing healthier males that are still effective for pest control would make smaller numbers sufficient, says Robert T. Staten, director of the Phoenix (Ariz.) Plant Protection Center. "You're talking a huge amount of dollars," he says.

He and his colleagues have demonstrated a pest-control option that uses genetic engineering. In one fruit fly line, the team inserted a three-part genetic time bomb: a lethal gene that disrupts signaling in cells, a switch controlling it that flips only in females, and a control for that switch that keeps it in its off state when the fly consumes the antibiotic tetracycline.

The researchers also created a fruit fly line with a switch that's flipped in both sexes but a gene that's lethal only to females.

In each case, removing tetracycline from the diet eliminates all the females. The males, healthy as natural flies, then would be released into the wild. When they would mate, all their daughters—lacking tetracycline—would die. Many of their sons would inherit the gene, so those flies' daughters would die and the population would dwindle.

Other geneticists have engineered sex-specific gimmicks, but they were designed for identifying female insects, Alphey explains. His approach to avoiding irradiation gets a cautious response from Marianne Frommer of the Fruit Fly Research Centre at the University of Sydney in Australia. "I think that it would be preferable to concentrate on developing systems where the released males are completely sterile, so that we need have no worries about the release of genetically modified organisms," she says.

Alphey predicts that his system will eventually mimic the full effects of sterilizing radiation. He's focusing on a combination of self-destruct genes from two known groups. One gene would kill females in the rearing facility, and another would kill all the offspring in the wild.

The technique should translate well to medfy relatives, but tsetse flies look daunting, Alphey says. Females bear live young, and so far, genetic engineers have worked with insect eggs. Proffering the usual scientific caveats, Alphey speculates, "I would hope that we could engineer a strain suitable for field trials in something like the medfly in 5 years."

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