FRESNO — Fruit flies that are highly destructive to crops are now permanently established in California and spreading, according to a new study published on Wednesday.

The study, published in the science journal Proceedings of the Royal Society B, found that despite decades of costly eradication efforts by the state, the Mediterranean fruit fly and the Oriental fruit fly have not been eliminated.

The flies’ populations are currently low, said study co-author and University of California, Davis entomologist James Carey.

But if the state does not change its long-term strategy to control the flies, the future could bring frequent, widespread outbreaks that would devastate California’s $43.5 billion agricultural industry, he said.

Outbreaks could lead to embargoes on California produce by foreign and domestic trading partners who want to protect their regions from fruit fly invasions.

At least five and as many as nine species of tropical fruit flies are now entrenched in California, the study determined. The Oriental fruit fly has been detected and captured in the state every year and the Medfly every other year for the past 44 years, Carey said.

State officials have said fruit flies are brought to California by visitors or cargo shipments from outside the country. But the study, which examined data on thousands of flies captured in California from 1950 to 2012, contradicts that claim.

“They’re here, they’re established and lurking,” Carey said. “It’s like an insidious cancer that’s just metastasizing and spreading, and it will eventually develop into a full-fledged cancer.”

Carey said that in addition to current strategies, the state should formulate long-term emergency plans, increase trapping and monitoring of the flies, and create a crop insurance program for farmers.

California farmers and packers should also develop production strategies based on knowledge of the fly’s presence, he added.

And the federal government should coordinate a national fruit fly program, because states such as Florida and Texas also have a fruit fly problem and the flies can spread, he said.

California agriculture officials said their methods are effective and environmentally friendly. And, they say, their approach would be the same even if the populations were established — though they welcome the input of scientists.

“We believe they come from out of the country. But it’s kind of a philosophical discussion with no practical significance,” said Dr. Robert Leavitt, director of plant health at the California Department of Food and Agriculture. “Even if they were here permanently, our response would be the same.”

Medfly outbreaks in the state led to aerial spraying in 1980s and 1990s, but the spraying was halted after it sparked a public outcry.

The state’s current strategies include releasing sterile male flies to break the breeding cycle of Medflies, squirting small amounts of pesticide in trees and bushes to attract and kill Oriental and guava male flies, and using pesticides approved for organic production for other fruit flies.

Growers within about 80 miles of a fly detection site are not able to ship their fruit unless they treat it with pesticides.

The state also asks homeowners and growers within a quarter of a mile of the detection area to strip and destroy fruit from trees.

Eradication costs are shared by the state and the U.S. Department of Agriculture. In 2012, the state spent $360,000 on Medfly eradication and a total of $437,500 on all fruit fly eradication. Officials said that prior to sterile fly releases and baiting, eradication efforts took more time and were much more costly.

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On the Web: Fruit fly study: http://bit.ly/13NzKs0