12/07/2018

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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Kerry Klein

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

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