For decades Los Angeles was ground zero in an escalating pesticide war against Medflies. Then the evolution of an invaluable tool for knocking down populations radically changed the battlefield in the 1990s. But a sudden infestation of fruit flies across California this year has officials falling back on a reliable insecticide to prevent the spread.

As the Newsom administration begins to phase out the use of hazardous pesticides in the state, the Department of Pesticide Regulation is probing researchers for the latest alternatives to scale up for broader deployment in California. One such tool, sterile insect programs, offer a strategic weapon against agricultural pests as well as mosquitoes that threaten people with infectious diseases like West Nile virus and Zika, which are gaining ground with a warming climate.

The Medfly war began with the advent of steamships, according to Jason Leathers, an environmental program manager at CDFA, who spoke at a DPR workshop last week on nonchemical technologies for pest management. As more travelers took to the sea, so did the Mediterranean fruit fly, migrating from sub-Saharan Africa to colonize Hawaii in 1910. Officials, suddenly aware of the gravity of an infestation, scrambled to prevent the Medfly from establishing a foothold in California. Inspectors boarded ships off the coast and if they found mangos with the fly, arrests were made.

The Medfly is considered one of the world’s worst invasive pests, according to USDA. Unlike typical fruit flies found in kitchen sinks, Medflies are the size of a house fly and more colorful. They feed on more than 250 fruits and vegetables, devouring the flesh from inside out, leaving farmers unaware until an infestation is well underway. The fly’s low profile works to its favor in the era of jet travel, according to Leathers, who estimated in California alone about $20 billion in crops are vulnerable to the pest.

Beginning in the 1930s, the state found success in rudimentary glass traps, he explained. Yet in 1975 the infestations grew at a rapid clip. By the time specialists found one it was already a large population and consumed a massive area, requiring aerial insecticide applications to knock it down.

“The projects were so big we had to have the National Guard help with fruit removal,” said Leathers. “Caltrans loaded up all the infested fruit to take it away for deep burial.”

A two-year infestation in 1980 cost about $100 million to eradicate. Officials stopped 5.2 million cars and trucks to inspect for host material and monitored more than 100,000 traps.

Peter AtkinsonUC Riverside Prof. Peter Atkinson

“We kept having multiple projects like this every year and in increasing frequency,” he said.

Scientists began experimenting with sterilizing male flies by exposing them to radiation and releasing them to trick females into mating. With no viable offspring, populations would dramatically drop—the fly precipitates its own collapse. In the 1980s trucks drove through Southern California neighborhoods with a state worker in the back tapping sterile flies out of a bucket. By the early 1990s, CDFA was running its first aerial operations.

“Every year we'd find Medfly infestations, knock them down with insecticides, then release sterile flies to mop up the population,” said Leathers. “Then the project would be over, we'd lay off all the seasonal employees and shortly later find more Medflies and do the whole thing over again.”

To break the cycle, the state made the release program permanent in 1996. Now twice weekly planes release about 62,000 flies per square mile over portions of Los Angeles, Orange, Riverside and San Bernardino counties, bumping that up to as many as 125,000 flies for high-risk areas. A remote laboratory in Waimānalo, Hawaii, radiates the flies before loading them onto commercial aircraft to ship overnight to LAX for subsequent deployment. The average number of Medfly investigations have dropped from about seven per year to just one or two every two years or so, according to Leathers. The low number of infestations means growers use less pesticide to control the flies.

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“A lot of people think that this is just a program to protect big agriculture,” he said. “But really the opposite is true.”

About half of all Californians own fruit trees or vegetable gardens and most residents are not going to take the time to develop their own integrated pest management programs and treatment schedules, he reasoned. But with food insecurity “a real issue” and higher prices for fruits and vegetables, the sustainable release program “is really valuable to all of our residents.”

Emerging technologies are taking the approach to a new level.

Scientists are on the verge of replacing radiation with gene editing to combat the glassy-winged sharpshooter. The pest spreads Pierce’s disease in grapes and in the 1980s threatened to wipe out California’s wine industry before UC Davis plant biologists developed disease-resistant rootstocks. According to UC Riverside Entomology Professor Peter Atkinson, gene editing was in its infancy at the time and not readily available as a pest management tool. In the 2010s, however, the advent of CRISPR-Cas9 changed the game. It has enabled scientists to precisely snip sections out of DNA sequences.

“There's never been a better time to be a geneticist,” he said.

Atkinson and his team are hoping to modify the mouths of sharpshooters to prevent them from picking up the bacteria that causes Pierce’s disease. The genetic mutations would spread to the insect’s offspring, leading to entire populations that are unable to transmit the disease.

Missouri-based Agragene is using the Cas9 technology to sterilize spotted wing drosophila, a small fly that can decimate blueberry and strawberry crops. Stephanie Gamez, who directs research and development at Agragene, said that radiating males is a difficult strategy to scale up, but the company’s gene editing technology “works very effectively,” with the potential to quickly collapse populations. Gamez and her team will begin trials in 2024 in partnership with USDA.

While such technologies may be effective at established pests, they are slow to pivot to new targets.

CDFA is currently battling oriental fruit fly activity across 15 counties, a sixfold increase in activity over a typical year. The flies prey on more than 230 fruits and vegetables, destroying crops as they mass produce.

The exotic fruit fly has evaded tools like sterile insect breeding—because it is not a single species. CDFA entomologist Andrew Cline explained to the State Board of Food and Agriculture last week that the name covers a complex of cryptic but distinct species.

“We're just starting to disentangle with genomics what that actually looks like from a molecular level,” said Cline. “Until we can figure out what truly is an oriental fruit fly, trying to develop a sterile insect program for that would fail.”

Instead, CDFA relies on STATIC Spinosad ME, an insecticidal bait developed by Dow AgroSciences. Specialists apply a small dose of the organic pesticide to trees and other surfaces off the ground in a 1.5-mile area surrounding each detection. The approach has successfully eliminated dozens of infestations, according to the department.

But each incident involves a team of specialists and the extensive outbreak has stretched staff and resources thin across the state, according to Cline.

Any silver bullet solution to change the battlefield dynamic —like sterile flies and gene editing—remains several years out of reach.

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