LA County’s latest weapon against biting mosquitoes is more mosquitoes

BASSETT — Twice a week this summer, crews from the local mosquito agency walk a shaded path through Bassett Park and a neighboring housing tract and let tens of thousands of mosquitoes loose into the air.

The mosquitoes are exactly the ones the neighborhood wants gone. One caveat—they are all male, they do not bite and they carry a bacterium meant to make sure the next generation never hatches.

That is the idea behind SGV Mosquito Match, a program the San Gabriel Valley Mosquito and Vector Control District launched in Bassett on July 7. Over 16 weeks, ending Oct. 30, the district will release about 480,000 male mosquitoes into a 25 acre stretch of the unincorporated community, roughly 15,000 at a time, twice a week, to shrink the population of an invasive mosquito that does bite.

The target is Aedes aegypti, a mosquito that bites during the day, reached Southern California in 2011 and can spread dengue, Zika and chikungunya.

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The males the district puts out carry Wolbachia, a bacterium found naturally in about 70% of insects. The strain lives in the males‘ sperm. When one of these males mates with a wild female, the eggs she lays never develop, a process scientists call cytoplasmic incompatibility.

“When they match, they don’t hatch,” said Tristan Hallum, the district’s  Director Of Scientific Programs.

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Because a female Aedes aegypti mates only once, a single pairing with one of these males ends her whole line. Hallum said the technique does not sterilize the males. It leaves them fertile but unable to father surviving offspring, which is why programs like this one are known as the incompatible insect technique rather than the sterile insect technique.

The catch, Hallum said, is timing and numbers. The special strain only works while the released males are alive, and fresh females and males keep flying into the release zone. So the district has to keep putting out males, week after week, hoping they find enough wild females to crash the population over the season.

To learn how the males behave in the field, the district ran a tracking study late last year, known as Mark-Release-Recapture. Early results, still being finalized, showed most released males lived only about a week, with 80 to 90% gone after that. The district recovered about 90% of them, most within roughly 200 meters of where they started, in line with the short distances Aedes aegypti usually travel.

Why Bassett, and why now?

The district picked Bassett after five years of trap data showed it running higher than the district‘s average, and because the neighborhood’s biting mosquitoes are almost all Aedes aegypti, which makes the results easier to read.

Aedes aegypti can carry dengue, Zika, chikungunya and yellow fever, and California has started seeing dengue spread locally rather than only through travel.

The state recorded its first two locally acquired dengue cases in 2023, then 18 in 2024 and six in 2025, according to the California Department of Public Health. Communications director Anais Medina Diaz said 11 of those 2024 cases were in this district, and the county’s  first locally transmitted case, in 2023, was here as well.

These mosquitoes are hard to fight with the usual tools. The district’s traditional methods work best on the native Culex mosquito, which breeds in large pools of standing water. Aedes aegypti breed in a spoonful, laying eggs on the sides of containers that can stay viable for a year and hatch in as little as five days once wet. The species can also build resistance to store bought pesticides, Medina Diaz said.

For residents, the change has been hard to miss. Medina Diaz said the invasive mosquito, first found in the district in South El Monte around 2015 after arriving in bamboo shipments from Southeast Asia, has spread across Los Angeles County over the past decade. It bites aggressively during the day and prefers human blood, she said, and residents often tell the district they cannot sit outside or grill dinner without getting bitten.

“Mosquitoes are the deadliest creature on Earth,” Medina Diaz said, pointing to the diseases they carry.

What residents will see

The district started in July on purpose, ahead of the usual Aedes surge in late summer and early fall, hoping the males mate with wild females before the population peaks. In the meantime, residents will see more mosquitoes, not fewer, right after each release. Those are the males, and Hallum said biting pressure should ease over the season.

The district will judge the program three ways by the end of October. It is running surveillance traps inside the release zone and as far as 300 meters out, collecting them every other week. It is comparing Bassett against an untouched control site in El Monte. And its communications team, which knocked on doors before the releases to ask residents about their mosquito troubles, will go back at the end to ask again.

So far the response has been mostly positive, Medina Diaz said, with many residents curious to see whether it works. She said attitudes toward mosquito control have shifted as the biting has gotten worse.

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However, programs like this have limits. Because the district is suppressing the population rather than trying to establish Wolbachia permanently, the effect fades once releases stop, which is part of why it runs for months.

The program costs the district about $30,000 this year, roughly half of it for overnight shipping from MosquitoMate, a company out of the University of Kentucky that rears the males. The money comes from a district reserve set aside for the technique, not a grant, Medina Diaz said.

The district is quick to add that the releases do not replace the basics. Wolbachia targets only this one species, so West Nile virus, carried by a different mosquito, remains a threat, and there is still no vaccine or cure for it. Officials are asking residents to keep tipping out standing water, tossing unused containers and using repellent with DEET, picaridin, oil of lemon eucalyptus or IR3535.

For now, Bassett is the only neighborhood getting the treatment. The district says it will use this first run to work out its process and its costs, and to decide whether releasing mosquitoes to get rid of mosquitoes is worth bringing to the rest of the San Gabriel Valley.

Aani Nagaiah is a correspondent with the Southern California News Group.