Fruit Flies Are Invading Los Angeles. The Solution? More Fruit Flies.

The winged creatures were squat, striped and just a quarter-inch in size. But the three Mediterranean fruit flies prompted officials to quarantine the produce of an entire Los Angeles neighborhood. They had a plan to stop the spread: more fruit flies — millions of them.

The flies, considered the most notable agricultural pest in the world, lay eggs inside fruit and vegetables that then hatch into maggots, rendering the produce inedible. The three so-called wild flies were discovered in early October in traps hung on a persimmon tree and a pomegranate tree in the South Los Angeles neighborhood of Leimert Park, according to the California Department of Food and Agriculture.

“In the early stages, the fruit looks just fine, and so people can spread it to a new area without realizing they’re moving the maggots,” said Jason Leathers, an entomologist who oversees the department’s fly release program.

He added: “It’s really important to get on top of this fast,” noting that while the department had found just three flies, “there’s probably more.”

To combat what the department called an “infestation,” officials have quarantined a 69-square-mile area surrounding the neighborhood. They’ve also urged gardeners to consume the produce they grow only at home, and told people to double bag any fruit scraps before putting them into the trash. Officials said they planned to drop around 250,000 sterile male flies per square mile each week into the area near where the wild flies were found.

The aim? Fly birth control. When sterile males mate with females, the eggs are infertile and do not hatch to produce offspring. The infertile eggs are so tiny that they most likely will not affect fruit, Dr. Leathers said, noting that residents were also “unlikely to notice” the influx of sterile flies. The department is also removing fruit and laying organic pesticide near where the wild flies were found.

Jamiah Hargins, the director of Crop Swap LA, which grows and collects surplus fruit and vegetables from residents in Leimert Park to deliver them to residents experiencing food insecurity, said the quarantine meant that the group would no longer be able to harvest much of its produce. Deliveries have been halved, Mr. Hargins said, noting that he had been left with a surplus of peppers, tomatoes and eggplants that he was planning to turn into sauce and salsa.

The Mediterranean fruit fly is not the only recent invader. Last week, officials found two Queensland fruit flies in Ventura County, Calif., northwest of Los Angeles. In July, more than 20 invasive Tau fruit flies were detected near the city of Santa Clarita, in Los Angeles County. Last year, workers confiscated a crop of oranges in northern San Diego County in response to the discovery of five Mexican fruit flies.

Humans may unwittingly introduce the pests through fruits and vegetables they bring home from traveling, experts say. Although the effects of climate change on fruit flies are not well documented, warmer temperatures are likely to reduce winter mortality rates and speed up the flies’ life cycle, Dr. Leathers said.

In response to the Queensland fruit fly outbreak, the Ventura County agricultural commissioner, Korrine Bell, told reporters that California was “facing an agricultural crisis.” She urged residents: “Please, don’t pack a pest.”

The Mediterranean fruit fly is among the most undiscerning, and therefore, destructive — it has made its home inside more than 250 kinds of produce, including avocados, tomatoes, mangoes and walnuts, and has spread across parts of Europe, the Middle East, Australia and the Americas. California — which grows a majority of the nation’s fruit and nuts — is among the states most at risk, according to the United States Department of Agriculture.

Although the Mediterranean fly does not pose a current risk to California’s crops, the potential repercussions, if the situation is not managed properly, could be devastating, agricultural experts say. “Growers would go out of business; packing houses would go bankrupt; the damage would be in the billions,” said James Cranney, the president of the California Citrus Quality Council. It would be, he added, an “untold level of catastrophe.”

The eradication plan for the flies in Leimert Park is part of a broader preventive release program that is run by the state food and agriculture department and the U.S.D.A., and airdrops millions of sterile male Mediterranean fruit flies across counties in Southern California throughout the year. In the nearly three decades since its inception, the program has helped reduce the number of infestations by more than 90 percent, according to the California agriculture department.

The barren males are bred at a labs in Hawaii and Guatemala (where the pest is already present), and as pupae are dyed pink or orange to distinguish them from fertile flies. Each week, more than 200 million of the fluorescent insects are shipped to Los Alamitos, Calif., for distribution. After being carefully incubated, and later, fed a diet of sugar, water and an algae derivative, the adult flies are then loaded into planes with “release chutes” that evenly distribute them.

On average, it takes 28 flights per week to cover a 1,750-square-mile region, according to the state agriculture department. To combat the Leimert Park outbreak, officials said they would divert two flights per week to target the affected neighborhoods.

The program began in 1996, after concerns over the use of the pesticide malathion, which was used to control the invasive flies. Having been found in Hawaii in 1910, the flies had made their way to California by 1975. In response, officials dropped 2.5 million sterile male Mediterranean fruit flies in western Los Angeles County. But in 1981, the U.S.D.A. accidentally released hundreds of thousands of fertile males, posing a threat to California’s agriculture industry, and reviving support for the pesticide. For years, the department used both approaches, but ultimately ceased the use of the pesticide.

Strict procedures are now in place to prevent another accidental release of fertile flies, Dr. Leathers said. “We really need to make sure there’s a lot more sterile flies out there, than the wild flies,” he said, “to make it more likely to work.”