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Climate and Environment

The Invasive Mosquito Species Vexing Californians And Driving A 500% Jump In San Gabriel Valley Mosquitoes

An Aedes aegypti mosquito is seen through a microscope.
An Aedes aegypti mosquito captured in Brazil in 2019. The invasive species is now menacing Californians across the state.
(Mauro Pimentel
AFP via Getty Images)
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Not so long ago one of the great pleasures of life in Southern California was little or no pesky mosquitoes to bite us up.

Those days are long gone. Still, this latest massive jump in mosquito residents of the San Gabriel Valley is notable: 500% over the same time last year. No, that's not a typo.

“When our track results came in, I was definitely a little dumbfounded at how much of an increase we saw," said Levy Sun, director of communications for the San Gabriel Valley Mosquito Vector and Control District. "It wasn't a surprise though, considering how warm of a weather we've had the past couple of weeks."

Mosquitoes aren't just an irritation. They can cause a range of diseases — including West Nile Virus. Sun is urging residents to take precautions:

  • dump stagnant water
  • throw out unused containers that can collect water
  • use mosquito repellent (he says look for one of these four ingredients: oil of lemon eucalyptus, picaridin, IR3535 or DEET)
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One more startling thing Sun mentioned: Like fire season, mosquito season in Southern California is now basically year round.

So What Happened?

Back in 2010 a new invasive mosquito series made its way to Southern California. Called the Aedes mosquito, it first showed up in El Monte in 2010. By 2018, it was prevalent enough that my colleague Emily Guerin investigated. She reported:

... it's been spreading all throughout Southern California. And it's way more vicious than our native mosquito, the Culex.

Unlike the Culex, Aedes mosquitoes bite during the day. And they go for the legs and ankles, instead of buzzing around your ears, so you can't hear them coming.

And they need much less water to breed.

That's one key difference between the invasive Aedes and native Culex mosquitoes. Culex prefer to lay their eggs in big sources of water, like swimming pools, but the Aedes mosquitoes hardly need any water at all. Both mosquitoes, however, can carry diseases: Culex has successfully transmitted West Nile to six people in L.A. County already this year, while Aedes can transmit Zika virus and dengue fever, although that hasn't happened yet.

In the four years since it's gotten considerably worse. Here's where the Aedes mosquito was in 2018:

A simple line map of California counties shows the location of Aedes aegypti in red and Aedes albopictus in blue. There are about 30 dots, most in the southern part of the state.
The distribution of invasive Aedes mosquitoes in Southern California, as of Aug. 3, 2018.
(Courtesy California Department of Public Health)

And this month:

A simple line map of California counties shows the location of Aedes aegypti in red and Aedes albopictus in blue. The dots are now spread throughout the state.
The distribution of invasive Aedes mosquitoes in California, as of Feb. 4, 2022.
((Courtesy California Department of Public Health))
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If you want details about which species have been spotted in your area, the state has a comprehensive updating list.

"I think the ultimate thing that people are most concerned with is the amount of biting that can occur," Sun explained.

Bottom line, he said, Aedes mosquitoes "don't belong in our ecosystem."

What questions do you have about Southern California?