Fatal Monogamy

The female screw worm, a serious warm-country cattle pest, mates only once. Dr. A. W. Lindquist of the U.S. Department of Agriculture told a Tampa meeting of entomologists how this determined monogamy may be the screw worm's undoing.

The adult females, which are ordinary-looking flies, lay eggs on wounds or scratches in the hide of cattle; the larvae that hatch burrow into the flesh, sometimes eating the poor beasts alive. Since screw worms breed in wild animals as well as tame ones, they are almost immune to extermination.

After pondering the problem, Entomologist E.F. Knipling, U.S.D.A., had an idea. The male flies, he mused, hunt and catch the females with singleminded enthusiasm. Why not draft the males into the extermination service?

So Knipling and his assistants raised male screw worm flies and exposed them to X rays. The flies suffered no obvious ill effects and were as successful as unexposed males in the pursuit of females. The difference was that the X-rayed males were sterile. This meant that every female with which one of them mated would lay infertile eggs for the rest of her life.

Knipling's Machiavellian scheme was given a preliminary tryout in Florida with good results. Then last year an entomological task force invaded the Dutch island of Curacao in the Carribbean, where screw worms were strongly in possession. Supplied by air with male flies raised in Florida (on blood and horse meat) and sterilized by gamma rays from Cobalt 60, the experimenters released them on the island at the rate of 400 males a week for each square mile.

Results came quickly. Wounded goats exposed to the flies showed clusters of screw worm eggs, but many proved infertile. The females that laid the eggs had mated with sterilized males from Florida. After seven weeks, all eggs were infertile, and the screw worm population dropped toward the vanishing point. No eggs at all were collected after mid-October, and since November there have been no signs of screw worms on the island.

Now the U.S.D.A. entomologists are figuring out how many sterilized males wil be needed to hunt down all the females in the state of Florida. They are even hoping to use the same method to exterminate other insect pests whose females are equally monogamous.

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