Gov. Edmund G. (Jerry) Brown Jr., who less than a week ago refused to order aerial spraying against the Mediterranean fruit fly, today called for federal help in conducting airborne chemical warfare against the steadily spreading invader.

His plea came as maggots were discovered 20 miles outside a state spraying area and California officials fell further behind in their efforts to stop the insect that threatens half the nation's fruit and vegetable crop.

Mechanical trouble this morning grounded one of the two helicopters hired to spray an infected area, along San Francisco Bay, covering nearly 200 square miles. About 6.17 square miles have been sprayed so far.

"The increasing magnitude of the infestation constitutes a disaster which is now beyond the control of the services, personnel, equipment and facilities" of the three counties involved in the spraying, the governor wrote Reagan. "California faces the threat of an economic disaster of unprecedented proportions by the virtual shutdown of the state's $14 billion agricultural industry."

A week ago, Brown had said that aerial spraying would jeopardize "the sense of well-being and the security of 500,000 residents of Santa Clara County." Officials from five jurisdictions in the area had fought the spraying order to the California Supreme Court, and San Jose Airport officials had refused to let chemical-laden helicopters fly from the facility.

But today Brown issued an emergency proclamation opening San Jose and all other airports to the "Medfly helicopters" carrying the pesticide malathion, and members of the state assembly held hearings in Mountain View, one of the affected cities, to complain about the slow start of the spraying.

The U.S. Department of Agriculture has threatened a statewide boycott, which could cripple California's agricultural industry and raise food prices all over the country, if the fly is not eradicated. Thirteen states have instituted quarantines or have taken other action against produce from the area affected by the Medfly.

Gordon Snow, a state food and agriculture official, said today he expected that the federal quarantine would be invoked if the fly managed to break out of the Bay residential areas, where it has been limited so far, and struck fruits and vegetables in the fertil Sacramento and San Joaquin valleys.

Brown said California has spent $20 million trying to bring the pest under contol. Asked how much federal money he is seeking, he said, "We'll take whatever we can get. Helicopters aren't cheap."

State officials said they found maggots or larvae of the fruit fly Tuesday at one spot in Milpitas, north of San Jose. San Mateo County officials said larvae were found today in the city of San Mateo, a 20-mile jump north from the limits of the aerial spraying zone. Ground spraying and tree stripping in the area of the two new finds will begin immediately, and both will be added to the area to undergo aerial spraying, state officials said.

Frank Hubbard, a spokesman for the state food and agricultural department, said he hoped to have seven helicopters ready for spraying duties by Sunday. He said project officials estimated that they would have caught up by the end of the week with their schedule to cover the spraying area once every seven to 10 days.

The spraying is expected to continue for two months, until federal officials are convinced that the insect has been eliminated.

Hubbard said the state had ready access to only two private helicopters when Brown decided Friday to order spraying in the face of a threatened statewide federal quarantine. "We did not have a standby plan to get helicopters ready to go," Hubbard said.

Pilots had to be certified by the Federal Aviation Administration for the unusual night flying. The state had decided to spray only between midnight and 6 a.m. to allay fears of area residents that direct contact with the small droplets of syrup and pesticide might be a health hazard.

Despite the limited extent of the spraying early this morning, the number of people using Red Cross shelters set up for residents who wish to avoid the pesticide increased from 89 Monday night to 170 Tuesday night.

Agricultural officials in North Carolina, Georgia, Texas, Alabama, Louisiana and Florida, where peach and pear crops could be severely damaged by the Medfly, said they are demanding certification that California fruits coming into their areas did not originate in the infested zone.

California food and agriculture department spokesman Dick Thompson said this posed no problem becase all fruit leaving the state contains the necessary certification from the county agricultural commissioner. The state has quarantined the three counties where the Medfly has been discovered and no fruits or vegetables are allowed to leave those counties.