Some insect and agriculture experts have concluded that regular, future outbreaks of the Mediterranean fruit fly and other destructive pests have become inevitable throughout the country, even if the latest California infestation is wiped out.

According to the latest available figures, undermanned U.S. inspection staions have found that the number of insect-infested fruits and vegetables and other items brought into the country has increased 36 percent in a year, from 14,002 finds in fiscal 1979 to 18,978 in fiscal 1980.

Efforts to stop importation of pests, and their movement about the country, have fallen prey, agricultural inspectors say, to a sudden and unexpected revolution in international commerce.

So much produce is shipped now in large, difficult-to-inspect containers and air travel has increased so rapidly that inspectors are hard-pressed to keep up with it.

According to an estimate by the Stanford Research Institute, American farmers, faced with the spread of damaging insects, will spend $6 billion in 1982 on pesticides, a 67 percent increase over 1980. Even after taking inflation into account, U.S. farmers are spending twice as much on pesticides now as they did a decade ago.

"It's about impossible to get people not to transport things around," said Edward Sylvester, chairman of the department of entomological sciences at the University of California at Berkeley. Sylvester numbers himself among many other entomologists who see mankind continuing to lose battles against its old adversary, the insect.

"One never knows what the insect world will do until it does it," Sylvester said, pointing out that the most recent California medfly infestation spread partly because experts thought the pest could not survive a cold northern California winter.

The threat of the medfly to California's rich central valley, source of nearly half of the nation's fruits and vegetables, has focused attention on what has been a growing problem throughout the country. While much of the rest of the nation has grown accustomed to regular pest infestations, California has been protected by mountains on the east and the ocean on the west.

The speed of modern travel has softened up the country to the spread of not only the medfly, but the Oriental fruit fly, the gypsy moth, the Japanese beetle, the European corn borer, the apple maggot and the European elm bark beetle.

Robert Nave, acting director of the U.S. Plant Protection and Quarantine Service in California, Arizona and Nevada, said his inspectors have tried to keep pace with the problem, but the changes they face have been rapid.

"In 1967 we were dealing with a lot of slow, bulk type vessels. In 1981 we are no longer unloading five or six cases at a time, taking days to get off the ship . . . . The speed by which cargo moves internationally has let more hitchhiking pests survive the trip because cargo moves so much quicker," he said.

Thomas Wallenmaier, an entomolgist with the U.S. Agriculture Department, provided statistics showing the rapid increase in the insect threat. In 1970, 12,518 finds of what the department calls "important" pests were made in baggage and cargo coming into the United States. By 1978 that figure was 15,880 and in 1980 increased to 24,189.

Wallenmaier said the total number of insect finds in 1980 was 18,978 compared with 14,002 in fiscal 1979 and 8,058 in 1970.

Of the total number of insect finds by inspectors, 55 percent appeared in the baggage of passengers coming into the country. Entomolgists and government officials acknowledge that, despite their efforts, inspectors cannot catch all of the increased volume of infested material entering the United States. Passenger travel has mushroomed from 2.3 million overseas travelers entering the United States in 1970 to 7.2 million entering in 1979.

Nave,who supervises inspections at the big West Coast ports of San Francisco, Los Angeles and Long Beach, said much of the cargo arrives in the huge new containers "stuffed from front to back," and ready to be placed directly on railroad cars or trucks and trailers. He said it is extremely difficult to unpack such cargo for inspection, and so "We do not inspect 100 cent of containerized cargo."

California officials, who turned over responsibility for inspecting incoming cargo to federal officials in 1967, have complained that federal budget restraints put further limits on the amount of work Nave and his men can do.

Nave contradicted that, saying that his inspectors were available on a 24-hour basis to look at any new cargoes of fruits and that 17 more inspectors had been added in Los Angeles because of increasing air traffic there. He added, however, that budget pressures change from year to year. "Sometimes we have enough money and people and sometimes we don't," he said.

The medfly has created so much attention, and forced the state to budget more than $60 million for its eradication because it is difficult to trap and reproduces rapidly, each female laying as many as 200 eggs.

The Oriental fruit fly, which feeds on many of the same crops, has been turning up in California almost annually for some time, but has not gotten as much attention because it is easier to stop. Its males are easily drawn to a chemical that can be used to trap and kill the insect.

Other insects now threatening California include the Japanese beetle, which covers much of the eastern United States, and the gypsy moth, also on the East Coast and recently found in three California counties.