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Mosquitoes Are Abuzz in San Francisco. You Can Thank Climate Change

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Black and white spotted mosquito on the surface of liquid.
An Aedes japonicus mosquito rests on the water surface from which it just emerged. (doug4537/Getty Images)

The Bay Area’s epic winter rainfall means that a certain pesky, blood-sucking summertime pest is having the time of its short life. (For males, that’s about a week — and that’s if they aren’t swatted sooner!)

“This year you’re going to see some pretty bad mosquito conditions — good conditions if you’re a mosquito, bad conditions if you’re a human being,” said Kaitlyn Trudeau, senior research associate at Climate Central. “Mosquitos are awful. I’m not a fan.”

In the short term, lots of rain and snow means plentiful puddles, marshes, ponds and other opportunities for mosquitoes to lay their eggs and reproduce rapidly.

But there’s a long-term trend playing out, and it has to do with warming temperatures — and it’s bad news for any San Franciscan with bare ankles and plans for an outdoor picnic.

Trudeau and her research colleagues looked closely at mosquito activity trends between 1979 and 2022 at 242 locations across the U.S. They found that rising summertime temperatures are affecting mosquitoes all over the place.


Across the country, 173 places showed an annual increase in “mosquito days” by an average of 16 days; these are days when conditions are optimal for mosquitoes, with an average relative humidity of 42% or higher, and daily temperatures ranging from 50 to 95 degrees.

San Francisco saw one of the sharpest increase by a whopping 42 days.

“What’s really causing this is the rise in minimum temperatures,” Trudeau said. “There are many more days where the minimum temperature in San Francisco is 50 degrees or above.”

The number of mosquito-friendly days around the coastal Bay Area has increased dramatically as the cooler days warm up, but San Francisco’s warmer days on average are still well below 95 degrees, making it a sweet spot for mosquitos.

This is also the case in other humid coastal areas like Monterey and Salinas, which share these increasingly optimal conditions for mosquitoes to survive, according to Trudeau.

“It’s the coastal curse,” she said.

A graph showing an increase in mosquito days in San Francisco.

But that trend is not true everywhere: Already hot places are getting even hotter, too warm for mosquitoes to thrive.

Other inland locations throughout the state like Stockton, Sacramento and Bakersfield are much hotter and regularly roast with temperatures above 95 degrees.

The rising temperatures in these places are causing mosquito activity to plummet each year.

“The Central Valley isn’t humid, and it’s likely getting too hot for mosquitoes,” Trudeau said.

A graph showing the decrease in the number of annual mosquito days in Sacramento.

Places like San José, which gets warmer temperatures than the coastal areas, experienced a lower annual increase of mosquito days, too.

Fifty-five U.S. locations saw a significant increase of 21 days or more, primarily in the Ohio Valley and Northeast regions. The majority of the 61 locations with a decrease in mosquito days were in the Southern areas, where temperatures were too high for mosquitos to thrive.

A sign of climate change

Spring and fall temperatures are rising, and that means mosquitoes will come out earlier and survive longer, increasing the opportunities for mosquito bites and disease transmission.

There are over 200 mosquito species in the U.S., with around a dozen species that can transmit viruses and parasites to humans. West Nile virus is the primary mosquito-borne disease in the U.S. and the Bay Area.

Compared to tropical regions, the U.S. has lower infection rates and milder health effects from mosquito-borne diseases. Globally, malaria and dengue pose more significant risks, particularly in Africa and Asia. Tick-borne diseases are more prevalent than mosquito-borne diseases in the U.S., although West Nile virus cases are widespread, especially in the Plains and Central regions.

A map of the Bay Area with shades of blue, yellow, orange, and red dots.
A dot map of the annual change in mosquito days in the Bay Area between 1979 and 2022. (Kaitlyn Trudeau/Climate Central)

Climate change affects mosquito populations and disease transmission, with increasing mosquito days and potential health risks. While mosquito-borne diseases are relatively less common in the U.S., officials say it remains crucial to address their impact through public health measures and understanding the varying risks posed by different mosquito species.

“The things that we do on a daily basis are impacting the environment. An increase in mosquito days is just one of the many, many impacts that we are seeing around the U.S., around the world, or in California because of climate change,” Trudeau said.

Innovative efforts to reduce mosquito population

Last April, the Santa Clara County Board of Supervisors approved an innovative initiative to reduce the mosquito population.

They launched a six-propeller drone to drop larvicide on the county’s remote marshlands, replacing work that was typically conducted by helicopter.

Edgar Nolasco, who directs the agency spearheading the program, said using drones instead of helicopters reduces the county’s carbon footprint and is more sustainable and efficient. It decreases larvicide waste and saves on costs.

“A drone is able to get closer to areas that helicopters can’t get to because of the drift caused by their propellers,” said Nolasco, who works for the Consumer and Environmental Protection Agency.

Adult mosquitoes can travel up to a 25-mile radius. “Treating adult mosquitos becomes very difficult,” Nolasco said, adding that using a strong larvicide program is the most effective way for the county to combat mosquitos.

Nolasco emphasizes that drones will only be used in uninhabitable and remote areas not accessible by the Vector Control team.

And, he said, the county is making every effort to eliminate mosquito sources, but is asking the community to help reduce the mosquito population.

“We need everybody in the community to do their part with standing water,” Nolaso said. “With the amount of rain that we got this year, there are many areas of standing water that can hold water that can reproduce mosquitoes.”

What to do if you get a mosquito bite, and how to protect yourself

If you get bitten by a mosquito, the California Department of Public Health recommends using a topical lotion to reduce itching. In California, most mosquito bites do not result in any infection. If you develop a fever two to 14 days after getting bitten by a mosquito and are concerned about West Nile virus disease, you should see a doctor. Most people with West Nile recover completely, according to CDPH.

The San Francisco Department of Public Health recommends using mosquito repellant such as DEET, installing window screens and wearing long sleeves when outdoors at night if possible.

A young Black woman with locs twirled on the top of her head sprays insect repellent on her skin while in the outdoors. Background shows lush green trees.
Insect repellent can deter mosquitoes and ticks during hikes in nature. (stefanamer/Getty Images)

Residents can also protect themselves by recognizing and reducing the source where mosquito larvae are commonly found. Mosquitos lay eggs in standing water, such as water in outdoor containers, so it’s important for residents to clear this water and clean out clogged roof gutters. Large drains that hold water are also a possible source of mosquito activity. Placing screens and under-drain covers could prevent mosquitoes from breeding.

San Francisco health officials frequently survey areas where mosquito complaints are received, make monthly checks on monitoring devices for invasive Aedes mosquitoes in select fire stations and inspect apartment buildings regularly for mosquito sources. To report complaints, call 311. For more information, see San Francisco’s Mosquito Prevention 101 public service announcement.


The West Nile virus website, handled by CDPH’s Vector-Borne Disease Section, is updated weekly on Fridays with the latest findings to ensure public health partners and the public have current information on the risk of transmission in the state.

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