Menacing Mosquitoes

DDT is now widely indicted for killing wildlife and endangering man, but the latest charge has a reverse twist. In California, which is afflicted with 43 kinds of mosquitoes, two species have become virtually immune to DDT plus all other available insecticides.

The worst of the pests is the female pasture mosquito (Aedes nigromaculus). Though it does not transmit diseases to man, the creature is a vicious stinger and travels in swarms as dense as 2,000,000 per acre in Southern California. In parts of the San Joaquin Valley, the pests are so thick at dawn and dusk--their feeding times--that people hardly dare step outdoors. Because of the insects, schools at times have been closed, farm workers have refused to tend crops, and dairy cows, stung on their udders, have produced no milk.

Narrow Escape. Research on pasture mosquitoes can be grueling. Charles H. Schaefer, director of the University of California Mosquito Control Research Laboratory, recalls a recent field trip he took with a coworker. "As we stepped into the pasture, black clouds of mosquitoes swarmed into the air. They landed on us by the thousands. When we tried to run back to the car, we got caught in the barbed-wire fence and they flew into our ears, noses and mouths." The scientists finally escaped the choking swarm by closing themselves in their car.

For a brief period after DDT was first put to use In 1945, the pasture mosquito seemed under control in California. But within seven years, the insects had become so resistant to the new chemical that researchers had to develop another organic compound, ethyl parathion. That failed in 1961--as did methyl parathion in 1963, and fenthion in 1968. Today, California has no chemical able to kill the pasture mosquito in safe dosages.

To compound the problem, aerial spraying used to be so effective that mosquito-control agencies permitted farmers to forgo installing drainage facilities to reduce the insect's breeding grounds. As a result, the Central Valley's heavily irrigated crop lands have become huge hatcheries for Aedes nigromaculus. Now that insecticides are useless, farmers are being ordered to drain their fields--a costly process that may force many small operators out of business.

Until drainage is adequate, California may have increasing trouble with its Anopheles mosquito, which can transmit malaria from infected to healthy humans. Though not immune to insecticides, that bug proliferates in stagnant water and may spread more and more disease. Reason: about one percent of returning Southeast Asian veterans are infected with malaria. What really worries health officials, though, is Culex tarsalis, the second mosquito species that has foiled all insecticides in California. Culex transmits encephalitis, a disease that attacks the human brain. California's most recent encephalitis outbreak occurred in 1952, the year DDT failed, and a major flood turned much of the Great Central Valley into mosquito breeding grounds. The outbreak felled 757 people, and 50 died. Now the problem may be more serious. In pre-DDT days, 40% to 60% of the population developed an immunity to encephalitis through "inapparent infections." Because DDT then reduced the Culex mosquito, few people today have immunity to the disease.

Biological Controls. To combat California's rampant mosquitoes, researchers hope to develop new pesticides, but mainly for "emergencies." They now prefer to use the bugs' natural enemies. Culex, for example, can be controlled by the mosquito fish (gambusia) and the common guppy which eat mosquito larvae in water. Certain bacilli, when applied to pasture land, also kill mosquito larvae. Another method includes releasing large numbers of male mosquitoes of the same species but of different strains. Because of genetic incompatibilities, the eggs of the females fail to hatch.

Fortunately, there is no lack of natural enemies. One researcher lists more than 200 insects alone that prey on mosquitoes, to say nothing of birds, fish and other creatures. The problem is how to spot the one most likely to exterminate California's menacing mosquitoes while also ensuring that the new parasite will not upset the ecology of the region.

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