"We found one, and then we found two, and we found three," Santa Clara County's Agricultural Commissioner Joe Deviney told ABC7 News.
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On Wednesday, he said the count is now up to seven.
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Deviney said six flies usually signal a "breeding population," which is why the California Department of Food and Agriculture (CDFA) imposed a quarantine spanning about 96-square-miles.
"But that's just to ensure that we're focusing on the potential hosts in that area," Deviney said. "The businesses that might have those hosts, the farmers markets, the roadside vendors. We'll be making sure that they're all under compliance and not doing anything that would spread it to another county or further and further out."
This CDFA map shows the quarantine zone includes the fairgrounds, north to the Alum Rock neighborhood and south to Santa Teresa County Park.
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Residents in the quarantine zone are being asked to not share their backyard fruit to keep the flies from spreading.
"That would make farmers have a more difficult time growing the food, they'd probably have to do more pesticides- which no one wants to do," Deviney shared. "It would be more difficult for them to export things, because other countries don't want this, these flies."
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The OFF is a pest to more than 230 kinds of fruit and vegetables, and instead of attacking fallen, rotten fruit, Deviney said it infests the fruit on the tree.
"It likes to lay its eggs in those hosts. Those eggs turn into larva, maggots- and they ruin the fruit," he continued. "And they actually infest the fruit on the tree. We have some fruit flies that are native that after the fruit falls and starts to rot, then they come in."
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He said because the Bay Area doesn't have a real winter- which would otherwise kill the tropical flies- OFF could easily get established here.
Deviney said treatment kicked off last week, with more expected as needed.
"It's called 'male attractant treatment.' So we splat a little bit of a food attractant, mixed with a little organic pesticide on street trees, just about 10-feet up or eight feet out of the reach of anybody," he explained.
He said the fruit flies consume the mixture, then die.
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Deviney explained the county had a similar situation in 2017, but with fewer flies in Cupertino. He said earlier this year, the county found OFF in San Jose. Though he said it wasn't enough to trigger a quarantine.
He said OFF can have a life cycle of a month or two months, from egg to adult.
"Boom! It happens... full breeding population. But during the winter it does get cold, not enough to kill it. But the life cycle elongates because it slows the fly down. So it can be about three months for a lifecycle," Deviney elaborated. "So, if we can get through three lifecycles, let's say eight or nine months, and we don't get any more flies- that means that we have halted that population."
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"We've eliminated the males in the area, there's no breeding population, and we're not finding anything in all of our traps... That means the quarantine will be over," he added.
Deviney said the county is working closely with CDFA and the United States Department of Agriculture.