The spraying area for the Mediterranean fruit fly is being expanded into San Jose because maggots of the fast-breeding insect have been found there, officials said today.

The discovery came as many residents of the Santa Clara Valley prepared to leave their homes Monday rather than risk being sprayed by the pesticide malathion.

In Washington, a spokesman for the Department of Agriculture said the areas currently under federal quarantine probably would be expanded to include any places where new infestation is found. Parts of three counties now are quarantined, meaning that fruit cannot be taken outside unless it has been treated for Medfly or inspected and found free of the insect.

Secretary of Agriculture John R. Block will decide this week, perhaps Monday, whether to quarantine all of California's produce, said spokesman King F. Lovinger. Such a quarantine could cost the state $2 billion in lost sales this year, farm experts said.

A task force of about 50 Agriculture Department experts from around the country flew to California Saturday to work with state officials in setting up any new quaratine and to review trapping of fruit flies to learn more about their traveling patterns, Lovinger said.

Officials of the Medfly eradication project said the maggots were found in three San Jose neighborhoods -- outside the 97-square-mile area that was to be sprayed with pesticide from helicopters starting early Tuesday. Officials then expanded the target area to 120 square miles.

San Jose Has a population of 650,000.

"It's highly unlikely these boundaries will stay the same during the spray program," said project spokesman Dick Thompson. "The insect is an extremely fast breeder and it's very adqptable. It's rewriting the rule book."

A hearing was scheduled for Monday before Superior Court Judge Bruce Allen on a request by several cities south and east of San Francisco for an injunction against aerial spraying of the pesticide, which federal officials contend is safe. The request is similar to one denied Friday on the federal level by U.S. District Court Judge William A. Ingram.

Gov. Edmund G. Brown Jr. ordered the controversial aerial spraying Friday, saying the Reagan administration "put a gun to my head."

Brown had at first rejected the spraying, favoring instead a vast ground assault on the Medfly, which officials say threatens the $14 billion California farming industry. But he reversed his decision Friday after Block said the government was prepared to quarantine all California produce unless aerial spraying begins.

Helcopters, using specially developed night navigation techniques, will begin releasing malathion at about 2 a.m. Tuesday over the enlarged 120-square-mile area from an altitude of 300 feet. The area, containing an estimated 575,000 people, will be sprayed at least six times.

Some local officials urged residents to leave home rather than be sprayed by the pesticide. Douglas Owen, a father of four, plans to remain in Palo Alto but is taking precautions to make sure his family remains inside their house. "It's deplorable. It's like in a gas chamber. It's out of control," he said.