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This summer, scientists in California are releasing 20 million mosquitoes in an effort to shrink the population of mosquitoes that can carry diseases.

It sounds counterintuitive. But the plan is to release millions of sterile male mosquitoes, which will then mate with wild female mosquitoes. The eggs the females lay won't hatch, researchers say.

 and it's being undertaken by Verily, a subsidiary of Alphabet, Google's holding company. It's the company's first field study involving sterile mosquitoes in the U.S.
Scientists say the goal is to cut the population of 

 mosquitoes — the species responsible for spreading Zika, dengue and chikungunya. 

 have been present in California's Central Valley since 2013 and have been a problem in Fresno County.

"It's a terrible nuisance, a terrible biting nuisance. It's changed the way people can enjoy their back yard and it's a threat for disease transmission," Steve Mulligan of Fresno County's Consolidated Mosquito Abatement District told 

.
Each week for 20 weeks, the company plans to release 1 million of the sterile, non-biting male mosquitoes in two neighborhoods in Fresno County. The male mosquitoes are bred and infected with 

, a bacterium that is "naturally found in at least 40 percent of all insect species," according to 

.
"Over time, we hope to see a steep decline in the presence of 

 in these communities," Verily says in a statement.
In a phenomenon called cytoplasmic incompatibility, "matings between 

-infected males and uninfected females results in embryo lethality or low hatch rates," William Sullivan and Scott L. O'Neill 

 also helps mosquitoes to avoid carrying diseases like Zika in the first place. 

 bacteria were highly resistant to Zika virus infection, and were unable to transmit the virus via their saliva," 

The magazine says the bacterium "is not known" to infect humans.


Verily is partnering in the project with the Kentucky-based company MosquitoMate and the Consolidated Mosquito Abatement District of Fresno County. They plan to study the 

 population density and number of eggs hatching in the experimental areas and compare to control neighborhoods to measure the experiment's effectiveness.


It's not the first time the bacteria-carrying mosquitoes have been released into the wild in the U.S.


MosquitoMate and the Florida Keys Mosquito Control District 

-carrying mosquitoes each week for several weeks this spring in the Florida Keys.
Last year in Fresno, MosquitoMate and county officials released 

 of the mosquitoes. This experiment is a 25-fold increase on that.


Verily says it developed "automated mass rearing and sex-sorting processes," which enable it to breed and release so many of the mosquitoes in such a short time.

 is an invasive species to Fresno County, and officials told the 

 they don't predict an impact on the local ecosystem from reducing or eliminating it.

The method Verily is using to cut the mosquito population differs from the experiment the British company Oxitec proposed last year in the Florida Keys. In contrast to the mosquitoes being infected by bacteria, Oxitec has released males that are genetically modified with "a gene that causes the mosquito's offspring to die in the first two to three days of life," according to 

 the proposed experiment last year; some Texas residents are reacting similarly to a proposed trial there, the 

 reports.
From the spring to the fall of 2016, 

 mosquito, according to the Centers for Disease Control and Prevention. Since the beginning of 2015, 

 symptomatic cases of Zika were reported to the CDC in the U.S., though the vast majority stemmed from people traveling to affected areas. As of Wednesday, the agency said there were 

A previous version of this story implied that Verily is releasing 

-infected males and females as part of its strategy to control mosquito populations. Only infected males are released.

Science

Health

Scientists plan to release millions of sterile, male bacteria-infected mosquitoes in California, to breed with wild females. They're hoping for a "steep decline" in the species that carries Zika.

To shrink the mosquito population, scientists are releasing 20 million of them

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