Controlling Mosquitoes, By Releasing Mosquitoes

5:17 minutes

A mosquito resting on a leaf
A mosquito on a leaf. Credit: yod67, via Shutterstock.

state of science icon

This segment is part of The State of Science, a series featuring science stories from public radio stations across the United States. A version of this story, by Kerry Klein, originally appeared on Valley Public Radio in Fresno, California.

Summer is approaching in the San Joaquin Valley, and that means it’s not only the season for sunscreen and paletas, but also mosquitoes—something local authorities are working on. For the last two years, the Fresno area has been the site of an experimental mosquito control program. And it’s back again. Here we examine the project’s latest, scaled-up season, and why it appears to be working.

If you’re expecting the ice cream truck on these hot May afternoons, you may be disappointed if you’re one of the few neighborhoods in Fresno County where a white van rolls up each day with a cartoon insect painted on the side.

Steve Mulligan is director of the Fresno Consolidated Mosquito Abatement District, and he says this white, high-top Mercedes van is part of a project called Debug Fresno. What’s inside? Lots and lots of mosquitoes. “We are releasing male mosquitoes in 3 neighborhoods in Fresno County to implement and evaluate a new, innovative mosquito control strategy against an invasive mosquito species,” Mulligan says.

Throughout 2018, vans like these will be releasing millions upon millions of male mosquitoes in three neighborhoods in Fresno and Clovis. It’s a kind of pest-control paradox. “We’re hoping by sustaining these releases over a period of time, we can reduce the local population of biting females,” Mulligan says.

And Mulligan and his partners believe it’s working. After a small test in 2016, and Debug’s debut in 2017, preliminary results suggest the biting female population dropped by two-thirds. So it’s back for another year, in the Fancher Creek area of Fresno and the Harlan Ranch and Loma Vista neighborhoods of Clovis—an expansion over last year’s test area, which included Fancher Creek and limited releases at Harlan Ranch. After running from July to November last year, this year’s releases began last month and will continue through the end of 2018.

But Mulligan says: Don’t worry. Debug’s all-male mosquitoes shouldn’t bother you. “It’s only female mosquitoes that bite,” he says. “Male mosquitoes cannot bite.”

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The Fresno area is home to close to 30 mosquito species. Most don’t transmit disease, but that’s recently changed—first, with a mosquito that began carrying West Nile virus in the early 2000s, then even more recently with a different species. “We have this new invasive mosquito now since 2013 in CA,” Mulligan says: “The Aedes aegypti mosquito.”

A. aegypti mosquitoes are more aggressive than native ones, biting throughout the day rather than only at night. But there’s a much bigger reason this species is concerning. “This is the primary vector of some very serious diseases,” Mulligan says, “including dengue, chikungunya, zika virus, and, historically, yellow fever, which decimated populations in the United States centuries ago.”

These diseases affect hundreds of Californians each year. And although most cases are probably picked up internationally, Mulligan wants to prevent local transmission. So he found some project partners to help: A Kentucky company called MosquitoMate, and Verily, a research and development company owned by Google parent company Alphabet.

The partners provide Mulligan’s district with sterile mosquitoes. They’ve been injected with wolbachia, a bacteria that occurs naturally in most insect species—but not A. aegypti. The mosquitoes are grown in Verily’s San Francisco lab. Senior scientist Jacob Crawford says the genders are then meticulously sorted so that only males are released. “Those males go out into the environment and mate with females, and so any females that mate with our males cannot have viable offspring,” he says.

This is not genetic modification. In fact, this technology in a different mosquito species has officially been recognized as a biopesticide by the EPA.

Fresno may have been the Debug program’s first test site, but Debug itself is far from the first to test the so-called sterile insect technique. One of the technique’s earliest demonstrations took place just after World War II. And not on mosquitoes but on an insect that harms livestock.

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Fresno State entomologist Jorge Gonzalez says today, the technique is commonplace in other insects. It’s used in California to control fruitflies and a cotton pest called the pink bollworm. “So the technique is simple, it is really beautiful, and it really works,” he says.

What is new is the use of wolbachia. Earlier programs sterilized insects using different technology like radiation.

Gonzalez says wolbachia can be used to control mosquitoes in other ways, too. One is to infect both males and females with the bacteria. Then, the population wouldn’t diminish nearly as much—it would adapt. But the technique would still get at the root problem: Disease transmission. “When they are highly infected with wolbachia, the female becomes a very lousy transmitter of dengue,” Gonzalez says.

This technique is already being used to control dengue fever in parts of Africa. For now, Debug Fresno is only operating in three test neighborhoods. But if it’s successful, partners hope it could someday help eliminate A. aegypti here. But one thing it should never eliminate is the ice cream truck.

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Segment Guests

Kerry Klein

Kerry Klein is a reporter at Valley Public Radio in Fresno, California.

Segment Transcript

IRA FLATOW: And now it’s time to check in on the state of science. 

SPEAKER 1: This is KER– 


SPEAKER 3: St. Louis Public Radio News– 

SPEAKER 4: Public Radio News– 

IRA FLATOW: Local science stories of national significance– and now that it’s December, peak mosquito season is finally over in the San Joaquin Valley of California. And so public health officials around Fresno have a chance to take stock of their battle against mosquitoes. 

And they’ve teamed up with a Kentucky company called MosquitoMate and with Verily– that’s a company owned by Google’s parent company, Alphabet– to test out an innovative mosquito control program that relies on producing and releasing millions of male mosquitoes. 

Joining me now to talk about the project is Kerry Klein, reporter at Valley Public Radio in Fresno. Welcome to the program. 


IRA FLATOW: Hi, how are you? 

KERRY KLEIN: I’m great. Thank you. 

IRA FLATOW: So how does releasing millions of mosquitoes help the mosquito population? 

KERRY KLEIN: Yeah. That sounds like quite a paradox, doesn’t it? 


KERRY KLEIN: But the way this project works is that, as you said, it’s releasing male mosquitoes, millions upon millions of male mosquitoes, in fact, and they’ve been infected with a bacteria called wolbachia, and it essentially interferes with their ability to reproduce, that when these wolbachia-infected male mosquitoes try to reproduce with females in the wild who don’t have wolbachia, they can lay the eggs and the eggs will never hatch. So it’s an attempt to really bring down this population by basically preventing them from reproducing. 

IRA FLATOW: Pretty tricky, if you say so. OK. Give me an idea of what it looks like. How does it work in practicality? 

KERRY KLEIN: Right. Well, so as you said, there are these three different companies or organizations involved, Verily and MosquitoMate being two and then this Consolidated Mosquito Abatement District here locally in Fresno. The mosquitoes are all grown in a lab in San Francisco owned by Verily. They’re brought down to Fresno, and they’re released in these vans. They’re kind of unassuming-looking, white, high-top vans. But they have a “Debug Fresno” logo on the side, a big cartoon mosquito. Yeah. You see those driving around and you know exactly what it is. And they have these little vents out the side of the van that just kind of pump out, a couple of times a day, lots of mosquitoes into a handful of neighborhoods in the Fresno area. 

IRA FLATOW: So you don’t see, like, a swarm coming out of the van, right? 

KERRY KLEIN: You don’t. You don’t, at least I didn’t when I was there watching these. Yeah. It’s over the course of a day, and so you don’t see that many. 

IRA FLATOW: Let’s be clear about this. These are not genetically-modified or some kind of mutant mosquito, right? 

KERRY KLEIN: That’s absolutely right. And that’s one of the biggest concerns that people raise about this is that they think that they’re genetically modified. But no, there’s absolutely no changes to the genetic code being done to these mosquitoes. It’s a naturally occurring bacterium, this wolbachia that they’re infected with. And in fact, estimates are that as much as 2/3 of all insect species have wolbachia, carry wolbachia in them. And so they’d have this bacteria carried in them, but there’s absolutely no genetic changes going on. 

IRA FLATOW: You know, the cynics among us are going to say, hey, you’re releasing all these millions of mosquitoes. They’re all going to be biting us. 

KERRY KLEIN: Right. It’s also a major concern. Fun fact, it’s only female mosquitoes that bite of any species. And so they’re only releasing these male ones. And so you’ll see more of them, but they shouldn’t bite. 

IRA FLATOW: Have they tested this out already? 

KERRY KLEIN: Yeah, actually. They just wrapped up their third year testing this program here in Fresno. It was the biggest one. They released– estimates are around 15 million mosquitoes over the course of the mosquito season. And the companies involved have said that it’s been a tremendous success, and they estimate that, in these test areas at least, it’s brought down the biting female population by 95%, 95% reduction. 

IRA FLATOW: And which kind of mosquito are they trying to get rid of? 

KERRY KLEIN: So this is a species called Aedes aegypti, and it’s a particularly nasty species of mosquito that is not native in the US, but it’s begun showing up here in the last couple of decades. In California, it showed up in 2013, at least officially. 

And it’s really nasty because– you know, most of us may think of mosquitoes as biting around dusk, but these guys will bite all throughout the day. They’re real pernicious. And they also are known to carry a bunch of really nasty diseases. So things like Zika, Chikungunya, Dengue are all carried by this species. 

IRA FLATOW: And so why is Alphabet. Google’s parent, involved in this, Verily? 

KERRY KLEIN: Yeah. So Verily, it’s a life sciences company, and they’ve given themselves this broad mission statement of trying to solve a number of major health problems around the world. And they’re getting involved with this one because they can bring tremendous, or at least they’re attempting to bring tremendous automation technologies into it. 

And so as you can imagine, if you’re trying to release millions of mosquitoes and they’re only male ones– 

IRA FLATOW: You got to make them and sort them and get them out there, and that’s what they’re good at, right? 

KERRY KLEIN: That’s it that’s exactly. 

IRA FLATOW: All right. Thank you very much, Kerry Klein, reporter at Valley Public Radio in Fresno.

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