Mosquitos are on the minds of Southern Californians a lot these days. Also their ankles.
Although the insects are now a year-round problem in the region, mosquito season kicks into high gear at this time of year, adding a layer of nuisance to barbecues and other outdoor activities.
But the insects are also a more serious concern for public health.
West Nile virus is spreading in the United States at rates not seen in two decades, according to federal public health officials. Scientists are also logging a rise in infected insects in Southern California, where the virus is carried by a native species of mosquito.
Meanwhile, a different, invasive species of the insect is jumping in population. Aedes aegypti can transmit the diseases dengue fever, Zika and yellow fever.
In response, vector control officials around the region are releasing millions of modified mosquitoes to try to reduce overall populations.
West Nile virus
The virus is the most common and serious mosquito-borne disease in the state, according to the California Department of Public Health.
And there are more mosquitos testing positive for West Nile in Southern California this summer than normal. Data from the Greater Los Angeles County Vector Control District show the number of West Nile-infected mosquitos hit levels in June that weren’t hit until August of 2025.
But it's not yet clear if the higher rate of infected insects will translate to more human cases. Southern California has seen two human cases of the virus this summer — one reported in the Antelope Valley and one in Long Beach.
“Having two cases reported in our area is not unusual. It's already July,” said Dr. Aiman Halai with the Los Angeles County Department of Public Health. “Our mosquito activity picks up around May and goes on till November, December, and it's typically around this time that we start to get human cases.”
L.A. County has seen an average of 41 cases of West Nile virus per year over the past five years. The actual number of cases is likely much higher, however, as the majority of infected people do not show symptoms.
In cases where symptoms do develop, they can include fever, body aches, rash, nausea, vomiting and headache. Approximately 1 in 150 people also develop serious symptoms, such as brain inflammation and paralysis. West Nile is a neurological virus that attacks a person’s central nervous system.
Public health officials say people over 55, people with chronic health conditions or those who are at high risk of illness are most susceptible to West Nile. There is no vaccine or treatment for the virus.
Aedes aegypti
West Nile virus is transmitted by a mosquito species native to Southern California called Culex quinquefasciatus, a.k.a. the southern house mosquito, which feeds on birds as well as humans.
In fact, West Nile virus can’t be transmitted directly from human to human. It has to be carried by a mosquito from a bird to a human, because birds carry a higher concentration of the virus in their blood.
But that’s not the case for other diseases transmitted by a different mosquito species, Aedes aegypti. The invasive mosquito has made itself more than comfortable in Southern California since it established itself here over a decade ago.
The species was responsible for an outbreak of dengue in L.A. County in 2024.
“We saw local transmission of dengue happening, and that is obviously a great concern to us, and that's something that we're trying to avoid,” said Steve Vetrone with the Greater Los Angeles County Vector Control District.
Symptoms of dengue fever include fever, body aches, nausea, vomiting and rashes. About one in four people infected with dengue will get sick. One in 20 will contract a more severe version of the disease, which can result in shock and internal bleeding.
Aedes aegypti can also transmit Zika and yellow fever.
Tristan Hallum releases mosquitoes in Bassett Park. The San Gabriel Valley Mosquito and Vector Control District is releasing about 30,000 in the park and the surrounding neighborhood each week.
Don't worry. The males released by San Gabriel Valley vector control officials don't bite.
Sterilized Insect Technique
Public health officials use an array of strategies to manage mosquito populations.
Some of the most effective identify standing water where mosquitos can reproduce, whether that’s a natural pond or water collected in garden pots. Officials will sometimes use satellite imagery to identify unmaintained backyard pools that have turned into mosquito breeding grounds.
Severe situations can be treated with pesticides. Crews will treat gutters and underground storm drains where water can collect, for example.
They are also rolling out a new strategy called SIT, or Sterilized Insect Technique, that specifically targets Aedes aegypti.
The technique involves releasing hundreds of thousands of male Aedes mosquitos that have been sterilized, meaning they can no longer reproduce, using a specialized virus or radiation.
Once released, the male mosquitos will mate with females in the natural population, but since they will not produce viable offspring, the overall Aedes aegypti population decreases over time.
The San Gabriel Valley Vector Control District launched its SIT program this summer. Earlier this month, district scientists began releasing 30,000 sterilized mosquitos each week in Bassett Park and the surrounding neighborhood. They plan to release almost 500,000 in the area by the end of October.
“We are talking about established populations of these types of invasive species in essentially every city across our district,” said Tristan Hallum, a scientist with the district. “So we're seeing them fairly consistently across our area, and we're trying to identify the best possible way to alleviate some of this bite pressure for our residents.”
L.A. County launched a similar program in Sunland-Tujunga in 2024. Orange County’s program has been focused in Mission Viejo.
“People are seeing a good reduction in the number of mosquitoes that are out there, and they're pretty happy with it,” said Miquel Jacobs with the Orange County Vector Control District.
However, as promising as these Southern California SIT programs are, they are only test cases. It remains to be seen if they can be scaled up.
The best thing to do in the short term, officials say, is to get rid of the standing water that mosquitos use to reproduce.
