Millions Of Fruit Flies Are Being Dropped On LA. Here's How They're Raised

The small, two-engine Beechcraft King Air is getting jostled by light winds over the L.A. basin. Having taken off from Los Alamitos Army Airfield Base, it’s flying toward one of the 1,750 square miles across Southern California it regularly covers.

A photo of a mediterranean fruit fly against maps of Southern California that line the walls of Mediterranean Fruit Fly Preventive Release Program offices (Medfly PRP) on the Los Alamitos Joint Forces Training Base in Los Alamitos.

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Brian Feinzimer

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LAist

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The pilot is guiding the plane while hundreds of thousands of his passengers are crammed inside metal chambers right behind his seat. They're kept in comas by refrigeration, meant to keep them from hurting themselves while in the tight space.

A Medfly PRP representative shows the area of Leimert Park on a map where there is an infestation of non-native fruit flies.

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Brian Feinzimer

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LAist

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They’ll wake up only after the pilot opens the trap door, and they start to warm as they fall through the sky, cascading over the city of Los Angeles some 2,000 feet below.

Every week, 3 to 9 million sterile male Mediterranean fruit flies are dropped over Los Angeles, Orange, Riverside and San Bernardino counties as part of the California Medfly Preventative Release Program — a critical tool in stopping the spread of invasive fruit flies, which can threaten the state's agricultural industry.

The inside of a Medfly PRP airplane that releases Mediterranean fruit flies over Southern California.

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Brian Feinzimer

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LAist

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Once they land on yards and in fruit trees, they're hardwired to seek out female flies to copulate with. The idea being that if the state drops enough sterile flies, they’ll preoccupy fertile females that get brought in on produce, and stop them from reproducing. The flies can lay their eggs beneath the skins of various crops like pomegranates and figs. When the larvae hatch, they eat the produce, drop to the ground and pupate in the soil, before turning into adult flies. Leaving behind damaged goods.

Just last month, five invasive Mediterranean fruit flies were discovered in the Leimert Park area. Since then, the program has increased its drops over the area to 6 million flies per week, and it will continue with the schedule until at least next June to make sure the problem is solved.

A fruit fly’s trans continental journey

While the program’s home is at the Los Alamitos Joint Forces Training base near Long Beach, a sterile fly’s journey begins in either Hawaii or Guatemala.

“We want to keep our breeding colonies offshore in an area where the fly is already established in the event any flies escape the facility, they don’t start an infestation,” said Ian Walters, director of the medfly preventative release program at Los Alamitos.

A Mediterranean fruit fly at the Medfly PRP facility.

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Brian Feinzimer

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LAist

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Adult female flies have their eggs collected from large mating cages, and placed on an edible bedding material, so the larvae have something to munch. Once they’re ready to pupate, they’ll pop themselves out of the trays and into water baths, where they’re once again gathered. This time though, they’re tossed into a giant mixer and covered in pink dye that’ll help scientists identify them with a UV light if they’re caught in traps throughout L.A.

Along the way they’re irradiated with Cesium-137 to make sure that they’re sterile, and then shipped via commercial airlines to California, where about 225 million pupae arrive per week.

Turned on by ginger and dropped from a plane

Once at Los Alamitos, they go into a hot and humid bungalow, where they become adult flies, munching on a mixture of seaweed extract, sugar and water.

Strangely enough, when you walk into check them out in their giant metal towers, the entire room smells like fresh baked gingerbread.

Mediterranean fruit fly incubation crates at the Medfly PRP facility.

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Brian Feinzimer

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LAist

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“Ginger root oil has a natural organic compound in it, which increases the mating competitiveness of the sterile males. They also tested orange oil, which has that same compound. But we found that ginger root oil was the most effective,” Walters said.

From there, they get put into refrigerated trucks and are chilled to 38 degrees so that they fall into comas. They're then placed into special hoppers ready to be dropped from an airplane.

An open Mediterranean fruit fly incubation crate at the Medfly PRP facility.

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Brian Feinzimer

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LAist

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If they wake up when they’re crammed into the tight space, they could injure themselves, so they sleep until they warm up as they fall through the sky.

The length of an individual fly’s life can vary, but typically lasts around 30 days.

How effective is the program?

There doesn’t seem to be much debate that the program has helped mitigate the number of medfly outbreaks since it began.

“They haven’t had as many outbreaks as prior to that. So in that sense it’s effective in suppressing them,” said James Carey, an entomologist at UC Davis, who’s an expert in invasion biology.

However, there is disagreement about where the invasive flies the state keeps finding in traps are coming from.

Carey argues that it’s likely the medfly has made itself at home here, in part, because they keep showing up in the same relative areas.

But when I asked Jason Leather, the entomologist who oversees the fruit fly program, how likely it is that they’re established, he said, “I think it’s actually zero.”

“Airline passengers are the biggest pathway for fruit flies into California," he added. "During the pandemic, air travel stopped and our traps were really empty."

Again, Carey disagrees.

“The only way you can monitor the wild population is by trapping. Well, guess what? The sterile flies also go to these traps,” Carey said. “It’s not entirely clear if it’s suppressing the populations or suppressing the detection. I would say it’s a little bit of both. But that’s a huge challenge for monitoring.”

A Medfly PRP airplane that releases Mediterranean fruit flies over Southern California prepares to take off from the Los Alamitos Joint Forces Training Base.

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Brian Feinzimer

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LAist

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What is certain is that the program will continue for the foreseeable future. And that threats from other exotic pests like the oriental fruit fly need to be dealt with.

If you'd like to help monitor for medflys, you can contact the CDFA and ask if they can put traps in your yard.