Swatting Mosquitoes with Sex

As insects become more immune to chemical insecticides, scientists are developing other weapons against them. Like sex. By encouraging the mating of mosquitoes that are incapable of producing offspring, a West German scientist has wiped out the disease-bearing mosquito population of a Burmese village. And, the attraction that proved so fatal in Burma, he reported to a recent health conference in Washington, can have similar results elsewhere.

While experimenting in 1948, Geneticist Hannes Laven of the Johannes Gutenberg University in Mainz discovered that common mosquitoes from Paris that were mated with members of the same species from Hamburg would not produce offspring. The reason for this sterility, he determined, was a difference in the cytoplasm (the protoplasm surrounding the cell nucleus) between the Paris and Hamburg strains of mosquitoes. Because of this difference, the egg cells of the females of one strain could not accept the sperm cells from males of the other strain, causing the female to lay infertile eggs. This Franco-German incompatibility was not unique. In succeeding years, Laven discovered 19 additional strains of common mosquitoes that could not produce offspring when mated with other strains.

Aggressive Males. One of these incompatible pairings consisted of a strain from Fresno, Calif., and a Burmese strain that transmitted filariasis, a tropical disease that causes chills, fever and headaches and can lead to elephantiasis. Last March, after breeding a host of strong male mosquitoes from insects caught around Fresno, he flew them to Burma and released 5,000 a day in the isolated village of Okpo. "Those huge males," Laven says, "were quicker and stronger than the indigenous breed." The Fresno males immediately began outdoing their Okpo counterparts in mating with Okpo females--which promptly began laying infertile eggs.

Even if the less virile Okpo males later had a chance to mate with these females, Laven explains, the female would produce no offspring: although the female mosquito can lay three or four sets of eggs during her two-week lifetime, she draws only on the first sperm she receives to fertilize all her eggs. Thus, after only a few generations of vigorous activity by the males from Fresno, there were no female mosquitoes left in Okpo that could lay fertile eggs. By May, Okpo was mosquito free.

Cytoplasmic incompatibility can be widely used to control mosquitoes, Laven says, but he cautions that total eradication of mosquito populations might have unpredictable ecological effects. To fill the gap that his control technique may create, he is attempting to produce a mosquito strain that will not transmit filariasis--and hopes eventually to develop a breed that simply will not bite humans.

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