California's tenacious Mediterranean fruit fly has taken a large leap down the Pacific Coast, forcing state officials today to widen significantly their fruit and vegetable quarantine.

A spokesman for the anti-Medfly campaign said the quarantine had been expanded to six counties and increased by about 457 square miles because of two fertile Medflies found south of the quarantine line between San Jose and Santa Cruz.

The new flies were found on the west side of the Diablo mountain range, away from the agricultural land of the San Joaquin Valley, so today's quarantine expansion does not pose as serious a blow to the state's $16 billion agriculture industry as it might have, although much of the state's bell pepper industry is located in San Benito County and is threatened. But the discovery of the new flies demonstrates the continued difficulty of catching all fruits leaving the quarantined area.

Medfly-infested fruits apparently brought out by motorists evading roadblocks have already given the insect a small foothold in the San Joaquin Valley, near Westley in Stanislaus County. A total of 2,831 square miles of northern California is now under quarantine, but state and federal officials still are having difficulty persuading other states and foreign countries that a ban on all California produce is unnecessary.

U.S. Department of Agriculture official William Helms said today that negotiations were continuing in Tokyo to persuade Japan, the California fruit industry's No. 1 foreign customer, to limit its proposed total embargo. Some importers have been told they may not bring California produce into Japan, despite a preliminary agreement announced last week to delay Japan's ban. Helms and California orange growers said today the situation remains unclear.

The marked expansion of the Medfly eradication effort today followed the discovery over the weekend of four fertile flies and one larvae-infested pear. The two flies trapped south of the quarantine line were in Hollister in San Benito County and Ben Lomond in Santa Cruz County. A quarantine was ordered on all of Santa Cruz county, where only 81 square miles had previously been restricted, and on the northern half of San Benito County.

Workers also found two flies in traps in Mountain View, inside a current spraying area, and a pear with Medfly larvae in Woodside. A Medfly campaign spokesman said 16 square miles of Woodside, which is inside the old quarantine zone, would be added to the approximately 800 square miles now designated for repeated aerial sprayings of the pesticide Malathion.

The Hollister fly was trapped in a walnut orchard about 10 miles south of the quarantine line. The crops grown in the area include tomatoes for canning, bell peppers, apricots, walnuts, sugar beets and grapes. Apricots are not in season and canned produce can be taken out of the quarantined area. A USDA official said today that fresh peppers could be sold if they had been sprayed with malathion, which some farmers in the area have been doing on their own in anticipation of a Medfly discovery.

The insect's increasing impact on California politics became even more apparent yesterday as Los Angeles Mayor Tom Bradley was quoted as saying he would have ordered prompt aerial spraying of infected areas when the fly was first detected.

Bradley appears to be the leading Democratic candidate in the 1982 governor's race and his remarks seemed indirectly critical of current governor Edmund G. (Jerry) Brown Jr., a fellow Democrat who hopes to run for the U.S. Senate next year.

Brown hestitated before ordering aerial spraying in July and has been severely criticized by the state's farmers. Although some environmentalists have supported Brown, his standing in polls at the moment is very low.