Sterile Medflies Begin Invading Infested Area : Agriculture: Planes release 38 million over Granada Hills. Officials hope the crop pests die out after mating with the insects.

What a long, strange trip it was Tuesday for 38 million sterile Medflies.

First, they were immobilized by being chilled to 38 degrees. Then they were packed into twin-engine airplanes and flown to a point 1,000 feet above the San Fernando Valley.

Once positioned over a 36-square-mile area centered on Granada Hills, the flies were released from the aircraft through specially designed dispensers. Caught in the warmer air from the plane’s slipstream, “they literally thawed out and flew off,” said Larry Hawkins, a spokesman for the Cooperative Medfly Project.

The 38 million were the first wave of an invasion of sterile Medflies intended to help wipe out an infestation of the crop-destroying pests in Granada Hills, the first such outbreak in the Valley since 1990. Another 4 million were released on the ground Tuesday.

Agricultural officials, who have also imposed a fruit and vegetable quarantine on a 76-square-mile area, expect to release 42 million sterile flies a week for at least the next eight months. The sterile flies’ mission is to overwhelm their sexual competitors in mating with wild flies. Because matings with the sterile flies produce no offspring, the wild flies are supposed to die out.

Considering that only 36 wild flies have so far been found in Granada Hills and Mission Hills, the first batch of 42 million sterile flies might seem sufficient to do the job. But Hawkins explained that there is no guarantee that all the wild flies have been trapped or that they will all mate with the first few million sterile flies.

“We have to be guided by the biology of the pest,” he said. “You have to have a sustained release for a long period of time to make sure the program is effective. All you have to miss is two wild flies and you have a population of new flies building.”

The area being aerially inundated by sterile flies includes all or part of the communities of Porter Ranch, San Fernando, Sylmar, Mission Hills, Granada Hills and Northridge.

The Medfly is a major threat to California agriculture. Although eradication efforts are expensive--the one conducted in 1989-90 cost $66 million--Hawkins stressed that they are “definitely cheaper than jeopardizing an $18-billion industry.”