ESCONDIDO - A destructive fruit fly that has prompted multimillion-dollar quarantines since 1999 in Fallbrook and Valley Center has been discovered in Escondido, state and county agriculture officials said Wednesday.

Officials said they trapped five Mexican fruit flies in two locations near Oak Hill Drive and Bear Valley Parkway in Escondido, a discovery that will trigger spraying and a new quarantine that could last well into next summer.

California Department of Food and Agriculture spokesman Steve Lyle said the discovery will automatically trigger a quarantine, inside which growers and nurseries will not be allowed to sell or ship fruit and agricultural products without being treated.

Lyle said the state would not know how large the quarantine area would be for several days because inspectors need to set and monitor more traps to find out how spread out the fly infestation is.

County Agriculture Commissioner Bob Atkins said that any quarantine will have to be at least 81 square miles, a little more than five miles in every direction from the initial find.

A county worker discovered the female fruit flies in two weekly monitored traps Tuesday, Atkins said. He said state Department of Food and Agriculture officials confirmed Wednesday that they were Mexican fruit flies.

Lyle said state crews working with county workers would begin spraying the areas by hand with backpacks carrying a pesticide today, and hoped to spray areas within 200 yards by Friday.

Mexican fruit flies are extremely destructive pests that have vexed local growers in the last decade.

The discovery of two fruit flies in 1999 in Fallbrook led to an eight-month quarantine that cost an estimated $2.83 million in crop losses, according to county officials. A 2002 Valley Center infestation prompted several months of quarantine over 130 square miles that cost about $2 million.

The female Mexican fruit fly destroys more than 50 kinds of fruit by injecting its eggs beneath a fruit's skin. The resulting maggots dine on the fruit's flesh until it rots and falls to the ground, where the pupae crawl out of the host fruit and into the ground. Adult flies emerge from the ground in 12 to 25 days.

Lyle said that the state would eventually begin bombing the quarantine area with sterile Mexican fruit flies in the hope of eradicating the pest.

State officials in past interviews have said that fruit flies typically mate just once, and if you pair a fertile female with a sterilized male, it does an incredible job of stopping future generations.

Federal agriculture officials have routinely bombarded the U.S. Mexico border with sterile fruit flies for decades.

Flies are grown by the millions in laboratories in Mission, Texas, and in Mexico, and eventually zapped with radiation to make them sterile and dyed to make them recognizable to inspectors.

Giant balls of flies are chilled to put them to sleep, then dropped out of airplanes over infested areas. The flies warm up, and wake up, as they drop to the ground.

- Contact staff writer Gig Conaughton at (760) 739-6696 or gconaughton@nctimes.com.