Summer is mosquito season in California. Not only are the biting, bloodsucking insects nuisances that disturb quiet evenings, but they can also spread infections such as West Nile virus, St. Louis Encephalitis and other diseases to humans. To combat the spread of mosquito-borne diseases, the San Mateo County Mosquito and Vector Control District employs hundreds of tiny secret weapons: mosquitofish.
SMCMVCD is a local agency tasked with helping the people of San Mateo County prevent mosquito bites, among other things. Mosquitoes have been a problem for residents of the Bay Area as long as people have lived here, so the district has had to come up with many creative solutions to keep mosquitoes under control.
“The first mosquito control work in San Mateo County was as early as 1904 [or] 1905 when some residents of Burlingame wrote to an entomologist to ask for assistance with an overwhelming mosquito population,” said Rachel Curtis-Robles, the public health education and outreach officer at the Vector Control District. “He and his students collected samples that revealed the main source of the mosquitoes was the marshes along the Bay. Today, those areas are Foster City, Redwood Shores, and Bair Island. The very first mosquito control work focused on controlling mosquitoes in those marshes.”
Though mosquitoes can be found year round throughout the county, people tend to have itchy encounters with mosquitoes more frequently as the weather warms up.
Public Health Education and Outreach Officer Rachel Curtis-Robles checks on a cage of mosquitoes that the San Mateo County Mosquito and Vector Control District uses for testing, educational, and observational purposes on June 20, 2024. Photo by Anna Hoch-Kenney. 
A tray full of mosquito larvae at the San Mateo County Mosquito and Vector Control District offices, where they are bred for testing, educational, and observational purposes, on June 20, 2024. Photo by Anna Hoch-Kenney. 
A cage full of mosquitoes at the San Mateo County Mosquito and Vector Control District offices, where they are bred for testing, educational, and observational purposes, on June 20, 2024. Photo by Anna Hoch-Kenney.
“Mosquitoes can grow through their lifecycle a little more quickly in warmer weather,” said Curtis-Robles. “We say it’s always mosquito season year round, because there’s different species of mosquito that can grow even in the cool weather. But particularly in the summer, we’re very worried about the West Nile virus transmitting mosquito, which here is primarily Culex pipiens, the house mosquito.”
To combat the issue of mosquitoes breeding and biting in backyards, San Mateo County Mosquito and Vector Control District first advises residents to dump or drain all sources of standing water. However, some areas such as backyard ponds, decorative fountains, livestock watering troughs or abandoned pools can not be drained or dumped.
Though chemical interventions such as mosquito-specific pesticides can be used in these undrainable water sources, the chemicals are not permanent solutions and have to be frequently reapplied. Curtis-Robles said that mosquitoes can also sometimes develop resistance if particular chemicals are used too frequently.
So instead, the district also employs hundreds of tiny fish to help them stop mosquitoes in their tracks.
What are mosquitofish?
Mosquitofish (Gambusia affinis), are small fish that have a voracious appetite for mosquito larvae. An adult mosquitofish can eat up to three times its body weight in larvae each day, which means that just one fish could wipe out several hundred larvae daily.
“Mosquitofish are a great type of what we call biological control, using one organism to control another organism. An example of this not in the mosquito world would be ladybugs eating aphids,” said Curtis-Robles. “This allows us to use fewer chemical pesticides in our work, because mosquito fish are able to eat the mosquitoes in their larval and pupal stages so that we don’t have to put chemicals in that water to control the mosquitoes from that water source.”
Mosquitofish can be delivered to San Mateo County residents free of charge, or can be picked up from the district’s office by anyone regardless of where they live. Curtis-Robles said that people who live outside of San Mateo County can also get in on the mosquitofish action, they just have to pick up the fish at the district headquarters in Burlingame.
“We sometimes get people from San Francisco who come down just to pick up fish,” she said.
The district’s mosquitofish program has existed for at least a couple decades, and they give out about 1,500 fish yearly to interested residents.
The fish are bred and stored in large tanks at SMCMVCD’s headquarters, ready to be deployed. Sometimes, the demand for the mosquitofish is high enough that the district has to source fish from elsewhere.
“Our system at this time is not optimized to maximize their breeding potential,” said Curtis-Robles. “So sometimes we’ll go to other mosquito control districts that do have a really robust mosquitofish breeding program. We have a special enclosed tank that we will take, filled with water. One of our staff will go really early in the morning to beat the rush hour traffic and that district will sell us these live fish by the pound. … They’ll come back to our headquarters and then they’ll be quarantined for a few weeks before they join the rest of our fish to make sure we’re not introducing any pathogens to our fish.”
These fish, which are related to guppies, mollies and other popular aquarium fish, give birth to live young rather than laying eggs, and breed quite prolifically. Often just a small amount of fish is enough to establish the population that is required to control mosquitoes.
“For typical backyard fishponds, decorative fountains, and green swimming pools, no more than about 10-15 mosquitofish are needed to start a robust mosquitofish population,” said Curtis-Robles. “In some smaller situations, as few as four to six mosquitofish may be enough for that particular water source. We consider the volume of the water when distributing fish so that we are providing enough fish to start the population.”
The fish are an especially useful tool, because the district focuses most of its mosquito-prevention efforts on the aquatic larvae and pupae, rather than the flying adults.
“It’s much easier to control mosquitoes when they’re larvae and pupae in the water than when they’re adults flying all over the place, sometimes many miles from the source,” said Curtis-Robles. “The vast majority of what we do at the District focuses really on what we call larviciding and controlling mosquito larvae in the water sources before they can even grow and emerge into adult mosquitoes that are capable of flying.”
Not native, but necessary?
Though the fish are helpful in controlling mosquitoes, they are not native to California and can pose a threat to native wildlife if released into a natural water source such as a stream, lake or natural pond. Some researchers estimate that the mosquitofish has the widest distribution across the globe of any freshwater fish.
Since the fish can survive in water with a wide range of temperatures, acidity and oxygenation, and have large appetites for anything they can fit in their mouths, they can often outcompete native fish and amphibians when introduced. The fact that mosquito fish give birth to live young also means that they don’t need specialized habitats to rear their babies, unlike many native fish.
Since the fish was first introduced to California in 1922, the species has become well established throughout the state. Several endangered species are threatened by the introduction of this hungry fish to their native ranges.
However, the qualities that make these fish an invasive nuisance are the same traits that make them uniquely capable of controlling mosquito populations. Their adaptability means that they can thrive in backyard ponds of all sizes, with widely different water quality.
SMCMVCD must therefore be cautious about where the fish are deployed in order to make sure they stay out of natural waterways, and away from sensitive native species.
“Mosquitofish are not native to California, they’re actually native to the southeastern United States,” said Curtis-Robles. “We’re really very particular about where we can put the fish, they can’t go into any water sources that can drain to natural waterways. … So they’re in really restricted and very contained situations.”
Curtis-Robles says that public education also plays a role in preventing fish from being released into the wild.
“We aim to educate every person who picks up or receives a delivery of mosquitofish, letting them know that the fish are not to be placed in natural or open waterways and that they can request we retrieve the fish if they are done with the fish at some point in the future,” she said.
It is illegal to release mosquitofish into any water that drains to a natural source, such as ponds, creeks, rivers, marshes and storm drains.
Ultimately, the fish are another tool in the Vector Control District’s toolbox to prevent the spread of mosquito-borne diseases to humans. The recent wet winters in California have caused high levels of West Nile virus to circulate in the state, according to the district’s website. California recorded 433 cases of the virus and 19 related fatalities in 2023. San Mateo County had only five recorded West Nile virus cases and no fatalities in the same year.
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