A mob of mosquitoes is a “scourge.” And the cardboard tube Jodi Holeman holds in her hands buzzes with more than enough mosquitoes to fill a scourge’s ranks---a thousand, to be exact. She uncaps the cylinder and taps gently, encouraging the stragglers out. “Get to work, boys,” she says.

On this Tuesday morning in July, Holeman and her colleagues at the Consolidated Mosquito Abatement District are releasing 20 tubes full of mosquitoes in Clovis, California, at the northern tip of Fresno County in the Central Valley. They do this twice weekly, bringing a scourge upon the same 20 spots in the same subdivision, where the two-car-garage houses range in color from beige to brown. By summer's end, they will have blanketed the subdivision with 400,000 mosquitoes.

The point of this? To rid the neighborhood of mosquitoes.

Counterintuitive, sure. But these are no ordinary mosquitoes. They’re not locals, for one: They were flown in that night from Kentucky, where the biotech startup MosquitoMate breeds mosquitoes carrying a bacterium called Wolbachia within their cells. And the mosquitoes in the tube are all male. Once Holeman sets them free, the idea is that they’ll mate with local females that don’t carry Wolbachia. And the offspring won’t be able to hatch.

This is all a big experiment. The district hopes the Wolbachia-infected male mosquitoes will become a vital tool against Aedes aegypti, a mosquito species invading the US that can carry diseases like dengue, yellow fever, chikungunya, and Zika. As temperatures rise, Aedes aegypti could push further north, bringing these diseases to communities that have never experienced them. Like in Florida, where some travelers to Latin America have returned with Zika and mosquitoes are now---officials said this week---transmitting Zika to locals. In July, Fresno County saw its first case of travel-related Zika. The disease is generally mild, but can cause birth defects. Nobody wants Zika to keep spreading.

To prevent an incursion, scientists have spent years---decades, even---exploring ways to kill mosquitoes and the diseases they carry. In Brazil, where the risk of Zika and other mosquito-borne diseases is high, scientists have tested genetically modified mosquitoes that are sterile. But in the US, a proposal to test the insects in Florida remains stuck within the gears of the Food and Drug Administration, which regulates genetically modified animals. Last year, the FDA approved its first genetically modified animal, a salmon. It took 20 years.

The route MosquitoMate is following in Clovis is less fraught. And it involves a different agency. Earlier this summer, the company submitted data to the Environmental Protection Agency to get Wolbachia in a related mosquito, Aedes albopictus, approved as a pesticide---an easier process than the FDA's. Three states had already tested Aedes albopictus. If the Wolbachia-infected Aedes aegypti work in Clovis, approval for that species could follow. Together, they could herald a new wave of mosquito-control strategies.

But will it work with this new mosquito? To get a brood of doomed eggs, you must get these males to mate. These bugs have spent their lives in a lab and the last 12 hours crowded in cardboard tubes---so they’re not exactly studs. “It’s a numbers game,” says Holeman. To increase the odds, the district will release orders of magnitude more males than there are females in the district.

So: 400,000 male mosquitoes, released over 10 weeks and 120 acres of suburban landscape. That’s the plan. Hopefully, it will be enough.

Stephen Dobson has spent most of his life studying an obscure microbe that lives inside insects. His PhD: Wolbachia. Twenty years of lab work: Wolbachia.

It’s not as unimaginative as it sounds. For evolutionary biologists, Wolbachia is the best kind of puzzle. It not only lives in insects; it lives inside their cells, passed down from mother to offspring. Humans have such bacteria, or remnants of them, inside their cells, too. Mitochondria were once free-living bacteria that found refuge inside animal cells, and they remain a key part of cells' energy-processing machinery. Studying Wolbachia offers a window into the symbiosesis between bacteria and insects---a niche topic, sure, but exactly the kind of basic research question a scientist like Dobson can happily burrow into for decades.

Scientists still aren’t exactly sure what Wolbachia does inside insect cells. Years into his research, Dobson noticed that when mosquitoes infected by different strains of Wolbachia---or one with Wolbachia and one without---mated, it interfered with how DNA duplicates. The eggs never hatched. This wasn’t just a niche research question anymore; this could be a new way to control mosquitoes.

He began mixing and matching different strains of Wolbachia in different mosquito species. Aedes aegypti does not naturally carry the bacteria, so in 2005, his lab created the first line of Wolbachia-infected Aedes aegypti by injecting the bacteria into mosquito eggs. Once the bacteria is within the cells, it passes from mother to offspring. The mosquitoes Holeman is releasing descend from these original Wolbachia-infected ones.

By 2013, Dobson had founded MosquitoMate. That year, at an Aedes aegypti conference in Panama City, he ran into Steve Mulligan, the longtime manager of the Consolidated Mosquito Abatement District. They’d long known each other from meetings like this. But this year, for the first time, Mulligan had an Aedes aegypti problem.

Months before, Holeman received a call from a neighboring mosquito-control district office. Employees there had picked up an unusual bug in a trap, and they wanted her to confirm the species. Sure enough, she saw *Aedes aegypti'*s distinct black-and-white-striped legs.

Soon after Holeman identified that mosquito, her district started finding Aedes aegypti in its traps, too.

Mulligan knew he had to act quickly, before Aedes aegypti made a home in Clovis. This species isn’t just a disease carrier: It’s also hardier and more aggressive than most other mosquitoes. Aedes aegypti bite during the day, not just at dusk and dawn. They love biting humans especially. And they need only the smallest amounts of water to lay their eggs---the water in the dish under a potted plant is sufficient.

District employees went door to door in the neighborhood looking for water sources, emptying small ones and spraying bigger ones---abandoned swimming pools were common after foreclosures hit Clovis---with pesticides. “We basically threw everything we had at it, and it didn’t do anything,” says Holeman. Even the cold winter didn’t kill them. Aedes aegypti was in Clovis for good.

When Mulligan and Dobson ran into each other in Panama City, each found the partner they were looking for. MosquitoMate needed someone to test the sterile Aedes aegypti; Clovis needed some way of eliminating Aedes aegypti.

Before Mulligan started trying to eradicate mosquitos in Clovis a quarter-century ago, he studied pesticides at the University of California. The pesticides he tested are still being used today. “Sterile insects, I think they’re the way of the future,” says Mulligan. The district received an experimental use permit from the EPA and California’s department of pesticide regulation, and their study started in June.

When Holeman and Mulligan go around releasing mosquitoes, they’re quick to greet passersby and explain what they’re doing. A person standing in the street letting out swarms of insects is a weird sight.

A few residents have complained about the buzzing insects. Although the males don’t bite humans---only females need human blood to nourish their developing eggs---they like to follow people and buzz in their ears. Others complain about government meddling with the environment. Yet by and large, says Holeman, most people are happy somebody is doing something about the mosquito problem.

Like one man who was out on a walk, pulling his toddler in a toy car. “Oh good, the mosquitoes are killing us,” he said, before moving on. Before the district started releasing mosquitoes, the team went door to door three times leaving flyers on doors explaining the project and announcing a community meeting to answer questions. One person showed up. Two others came only after Mulligan called them. He and Holeman are trying to get people on board with the Wolbachia mosquitoes. “Even if it’s very effective,” says Holeman, “if residents aren’t supportive, it’s going to be very difficult to implement.”

Community support will be key as the EPA considers MosquitoMate’s applications for the use of Wolbachia-infected mosquitoes. The application has received 10 comments, at least three of them from people who confused the Wolbachia mosquitoes with genetically modified insects. For example: “I'm 100 percent against this plan to release GMO mosquitos in FL.”

If Clovis were experimenting with genetically modified mosquitoes, the scene might look different. The plan that comment referred was in the Florida Keys, where the biotech company Oxitec is attempting to release its GM mosquitoes in a field trial---it would be the first in the US. An online petition against the release garnered over 150,000 signatures, and opponents swarmed community meetings carrying “No Consent” signs. Oxitec has received more than 1,200 public comments on the FDA’s assessment about mosquitoes’ environmental impact. (The FDA found no significant impact, but that hasn’t cleared the roadblocks.)

“I’m an entomologist. I understand insects a lot better than I understand people,” says Dobson. “But it appears people are less resistant to the Wolbachia approach because it’s non-GMO, because it’s a naturally occurring bacterium.”

It’s not just people that he needs to convince, though. The EPA has never dealt with a “pesticide” like this before. “We kind of blew their minds,” says Dobson, who has been working with the agency on the application since 2008. “This has been a long process, a lot of emails, a lot of flights, a lot of stacks of paper going back and forth.”

Dobson says he expects approval in a few months, which would let mosquito-control districts in the US add the method to their arsenal against Aedes albopictus. As for the Wolbachia-infected Aedes aegypti in Clovis, MosquitoMate has sought permits for additional tests in California and Florida. If those go well, MosquitoMate will ask the EPA to approve its mosquitoes as a pesticide as well.

Elsewhere in the world, China is releasing millions of Wolbachia-infected Aedes albopictus mosquitoes every week. And entomologists with the project Eliminate Dengue are taking advantage of another quirk of Wolbachia: its ability to prevent mosquitoes from passing the Zika, dengue, and yellow fever viruses onto humans. The downside is the number of mosquitoes remains unchanged, so they keep biting and biting. The upside is that this method is cheaper; a one-time release of female mosquitoes can infect an entire population with Wolbachia, whereas males must be continuously released.

These projects all come back to the same goal: creating a new tool to control mosquitoes carrying diseases. So on a 105-degree day, Holeman’s team is out in the sun, releasing scourges and scourges of mosquitoes.