Engineered mosquitoes the newest weapon to combat infestations
Bryan Ruiz moved his family into a newly built home in this Central Valley farming center seven months ago and almost immediately found they were under assault.
Mosquitoes bit and harassed them in broad daylight. He looked around, trying to find a water source where they were breeding, and noticed a freshly dug pipe, meant to drain water from the backyard to the front. He lifted its cap and inside found a small puddle in the drainage line, which didn’t have enough slope to fully empty.
He grabbed a turkey baster and drew water, already knowing what he would find: the larvae of Aedes aegypti, one of the greatest threats to humans on the planet.
Ruiz knew what he was looking for because he is in charge of a newly formed team that spends the summer months traveling around northern Tulare County to combat the invasion of Aedes aegypti, a mosquito capable of infecting humans with the deadly diseases of dengue, chikungunya, yellow fever, and Zika.
Since gaining a foothold in California less than a decade ago, Aedes aegypti has spread quickly across the state, its territory now ranging from the desert terrain of Imperial County at the U.S.-Mexico border to the city of Redding in woody Shasta County, about 750 miles north.
To combat aegypti, mosquito control districts rely on the same tools they were using decades ago — pesticides and rooting out water sources — even as climate change and agricultural practices are allowing the mosquitoes to thrive in places previously uninhabitable.
But Tulare County officials hope the region will soon be a testing ground for a new generation of technology, including a genetically modified mosquito, as they try to prevent the kind of disease outbreaks now common in regions of the Caribbean and Central and South America where Aedes aegypti is widespread.
The most immediate mosquito danger in Tulare County comes from a different genus, Culex, a type that typically bites at dawn and dusk and can carry West Nile virus, St. Louis encephalitis, and western equine encephalomyelitis virus, all of which can be fatal. Over the past decade, California has registered more than 4,000 cases of West Nile and at least 220 deaths. Tulare’s mosquito control districts have poured extensive resources into that battle, including releasing a range of chemicals, maintaining a hatchery for larvae-eating fish, and, most recently, buying a drone to ferry pesticides deep into cornfields.
Aedes aegypti, however, is a growing concern, and much harder to combat.
To understand this fight, you first have to understand just how stealthy Aedes aegypti
is. The mosquitoes can lay eggs in spaces as small as a bottle cap, and females spread their eggs across multiple locations — scientists often refer to their “cryptic habitats.” Most other types of mosquito eggs need water to survive, but Aedes aegypti’s can lie dormant for months, springing to life when water eventually does come. And one mosquito can bite many times over, snacking repeatedly on the same human or moving from one to the next. They become a lethal threat if one of those people happens to be carrying dengue or some other virus.
In an increasingly global world, people regularly travel to places where dengue is endemic and bring it back to the U.S. If a mosquito bites them, the illness can spread locally. That’s what happened in the Florida Keys in 2020, and more than 70 people were infected before the outbreak was stopped.
California so far has avoided local outbreaks of the diseases carried by Aedes aegypti. But it does import cases — California has registered seven cases of imported dengue this year — and with the mosquito population growing, experts say it’s likely only a matter of time.
Aedes aegypti is a frequent flyer, traveling the world in cargo. It no doubt has been introduced into California millions of times. But it wasn’t until recently that it took hold, said Chris Barker, an associate professor of pathology, microbiology, and immunology at University of California-Davis. It was first detected in 2013 in three California cities: Menlo Park, Clovis, and Madera.
Today, Aedes aegypti has spread across more than 200 California cities and 22 counties. And it has strained mosquito control districts. “It’s been a lot of extra work, extra staffing, extra financial demand,” said Barker. “And regardless of disease risk, a huge issue is the nuisance biting.”
Because of the threat the mosquitoes pose, when aegypti was detected in 2014 by the Delta Mosquito and Vector Control District, where Ruiz works, the district rushed to stamp out the menace. Its eradication method required searching every nook and cranny of the area where Aedes aegypti had taken up residence and cleaning out water sources multiple times a week. People were so annoyed by the full-court press, said the district’s assistant manager, Mir BearJohnson, that nearly five years passed before that community reached out again. Which was a problem, because the district partly relies on reports from residents to know where mosquitoes are.
The eradication was also short-lived. In 2015, Aedes aegypti was reintroduced, and this time the ferocious biters spread out across Visalia, the area’s largest city. Because Aedes aegypti can now be found all around the Central Valley, eradication no longer feels like an option, said Mustapha Debboun, an entomologist who moved from Harris County, Texas, in March 2020 to lead the Delta district.
Aedes aegypti mosquitoes in the area are also broadly resistant to pyrethroids, the family of chemicals most frequently used to kill adult mosquitoes. Pyrethroids are in heavy use among the area’s agricultural companies, likely contributing to what Barker described as nearly 100% resistance.
Which is why Debboun and colleagues are interested in the genetically modified mosquito. Their hope is that the engineered mosquitoes reduce the number of wild Aedes aegypti as they interbreed and produce short-lived offspring.
The U.S. Environmental Protection Agency recently approved a trial in Tulare of the engineered mosquito, which is made by a company called Oxitec. The company says its