Biology: Instant Mosquito Control

The rice fields of California’s Sacramento Valley are flooded every year from spring to fall. For five months, they are perfect breeding grounds for vast swarms of mosquitoes that have largely become resistant to chemical insecticides. For all their immunity to man-made controls, though, the insects may yet meet their match — all because an imaginative University of California scientist has gone back to nature and enlisted the aid of a voracious and prolific South American fish.

Entomologist Ernest Bay has pinned his hopes on the little Cynolebias bellottii, or Argentine pearlfish. Almost alone it keeps large areas of Argentina and Brazil relatively free of mosquitoes. Immediately after hatching in the waters of low-lying flood plains the tiny fish begins eating mosquito larvae. By the time it reaches its mature length of 1½ in. to 3 in. a few weeks later, it is able to consume about 50 larvae per day.

The pearlfish is impressive in numbers as well as appetite. Females begin spawning some three weeks after they are hatched; every week they produce from 30 to 300 eggs, which dry out and are preserved in the baked mud after floodwaters recede. The next year, when the plains are inundated again, the eggs hatch in as little as half an hour after they are moistened, producing a new generation of fish to sweep the waters clear of mosquito larvae.

Bay obtained a small number of pearlfish two years ago and kept some in cages in a water-covered rice field, others in test tanks. Their breeding habits were unaffected; during one two-month test, three pairs of caged pearlfish produced a total of almost 2,000 eggs. While 60% hatched under ideal laboratory conditions, as few as one-tenth of 1% of those left to dry in a simulated ricefield environment later produced fish. But even this rate, Bay calculated, is enough for a yearly population of about 38,000 fish per acre.

Bay is now concentrating on mass-breeding pearlfish in his laboratory, attempting to produce enough to “seed” California’s rice fields and, eventually, the flood plains in California and other states. He envisons the day when many of the intermittently flooded fields in the U.S. will be rich in pearlfish eggs. Then as spring rains herald another flood season, millions of eggs will hatch, providing nature’s own form of instant mosquito control.