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What's Behind San Francisco's Plague of Flies?

A brief history of the California city’s hopeless battle against invasive insects.

Spotted-wing drosophila, aka vinegar flies, like the ones infesting cities in California. (Blair Sampson/USDA)

To walk while yawning in San Francisco is to risk getting a fly stuck in your gullet. The city is lousy with clouds of tiny, drifting insects, so much so you might think the roads are paved with rotten garbage.

“I feel like they come in evil waves in the city,” writes one local in a thread titled Screw Fruit Flies. “I got an infestation REAL bad,” complains another. “Was out of the apartment for a week, and my roommate had left some bananas out. There were tens of thousands, everywhere, in swarms.”

One woman has gone so far as to petition her landlord, who had pest-bombed the retail building twice already, to find a deadlier way of dealing with the horde. “It is a constant fight against the flies,” she writes, in prose reminiscent of a post-apocalyptic war journal. “We are now unable to eat lunch in our showroom, since the flies find a way to crawl on our food or into the refrigerator even when covered or closed.”

OK State

Fruit flies love San Francisco perhaps more than any other U.S. burg, and can be found plopped on walls and doing lazy doughnuts in homes, streets, bars, dirty restaurants, fancy restaurants, public bathrooms, the transit system, and probably in your beer if you’re not vigilant. So what gives—is there a terrestrial explanation for the plague, or is it that God really does hate this heathen city?

Probably the former. Fruit flies have happily buzzed around in California since arriving from abroad in the 1950s, probably in cargo shipments. They are now found almost everywhere, including at least 330 cities. They represent a serious problem for the state’s legion of growers, who must deal with fly maggots burrowing into and destroying produce. A study from the 1990s estimated that a wide infestation of just one species, the famously voracious Medfly, could cost California $1.2 billion in gross revenue and roughly 14,000 jobs.

California’s Medfly wars were the stuff of killer-bee-grade 1980s headlines.  Fears of the “Satan bug” were so intense that, in the late ‘80s, there were rumors they were weaponized near L.A. in a biological-terror attack. Municipalities around the San Francisco Bay have historically responded to the invasion by using helicopters to shower hundreds of square miles with pesticides, flooding the region with more than a billion sterile male flies, and sending workers to strip the fruit from trees in peoples’ yards.

Victory is occasionally declared in certain areas, but the insects seem to find a way to counterattack. A few years ago, scientists affiliated with the University of California, Davis, announced that since arriving the “trickle” of flies had become a “flood.” Their dire warning: “At least five and as many as nine species of tropical fruit flies, including the infamous Medfly, are permanently established in California and inexorably spreading, despite more than 30 years of intervention and nearly 300 state-sponsored eradication programs aimed at the flies.”

And in San Francisco, the winged menaces may have found their El Dorado: A city paved in rotting fruit. One of the lovelier aspects of Bay Area is its abundance of lemon, orange, plum, and other fruit trees, which throughout the year litter the sidewalks and dirt with a pungent, decaying ground-harvest. These squishy shoe-mines attract (among other creatures) small fruit flies called vinegar flies, which are probably what many folks see hovering around town. “They like rotting fruit, close to fermenting,” says George Roderick, curator of invasive species at Berkeley’s Essig Museum of Entomology. “And they likely do well here because of our relatively mild climate and abundance of fruit in markets, stores, restaurants, kitchens, but also outside in yards and gardens.”

Peter Oboyski, a collections manager at the Essig Museum, observes in an email that the city is particularly “conducive to continuous populations of flies.” He continues:

It is amazing how fruit flies appear, as if by magic, if you ignore a piece of fruit even one day too long. In many cases the eggs may already be on the fruit when you buy it at the market, but if eaten soon after would not be noticed. But as George points out, with all the fresh fruit markets around there are plenty of fruit flies lurking nearby to colonize your fruits and soft vegetables. But there are a few other flies one might see around the city as well.

For those who keep house plants, dark-winged fungus gnats (Sciaridae) will inhabit the soil in potted plants. The larvae feed on fungal hyphae but are not destructive to the plants, and they can be abundant. I once had a carnivorous sundew plant next to some potted plants on my kitchen window sill. My sundew was never hungry—it had a constant supply of fungus gnats.

The sort-of-good news for San Franciscans is that the thriving fly populations support a robust food chain of somewhat less-annoying creatures, from wasps to backyard chickens. Many serious fruit-fly predators are themselves considered pests, unfortunately. (Frogs are effective, but a urban plague of them would cause its own problems.) Inside the home, this natural gardener recommends sticking cloves into a lemon, which is said to have some fly-repellent powers. But ultimately, co-habitation may be the only real option: “I’ve come to see them as a symbol of abundance.” Along with sky-high housing prices and un-bikeably steep hills, consider the flies part of the karmic toll one pays to live in this part of the world.

About the Author

  • John Metcalfe
    John Metcalfe is CityLab’s Bay Area bureau chief, based in Oakland. His coverage focuses on climate change and the science of cities.