Why California's 2026 Mosquito Season Is the Worst in Years

Here is what is driving the California mosquito season in 2026, where activity is concentrated, why it is running late, and a short list for this week. Outside California, the same routine applies; the last section points you to your local program.

What the state is actually saying

The California Department of Public Health describes this year as the "highest levels of West Nile virus activity in five years."

The numbers behind it, as of October 2, 2026, per CDPH: 252 human West Nile cases in 23 counties, 19 deaths, 702 dead birds, 3,839 mosquito samples, 149 sentinel chickens, and 14 horses. For comparison, the state's history table shows 117 cases in all of 2025. This year has more than doubled that, and the season is still open.

Figures change through the season. Check the live CDPH and county dashboards linked below for current counts.

For the full county-by-county breakdown, see West Nile virus in California: the 2026 numbers.

Three drivers, one season

Heat and humidity

The Los Angeles Times reported on September 22, 2026 that El Niño-linked humidity, on top of heat, has been boosting mosquitoes this year.

The general picture: mosquitoes tend to develop faster in warm water, and humid air tends to favor adults. A hot, sticky stretch can give them both.

Two kinds of mosquito, not one

• Native Culex mosquitoes are, per the CDC, "most active from dusk to dawn," and in US states they can spread West Nile virus.
• Invasive Aedes aegypti bite in daylight. Ventura County's invasive Aedes page states that Aedes aegypti and Aedes albopictus "commonly bite during the day," unlike most native species. They lay eggs in a container holding "as little as a teaspoon of water."

The Desert Sun reported on September 25, 2026 that Aedes aegypti has now been found in at least 28 California counties. It is not the main West Nile mosquito. Ventura County frames its disease risk carefully: it has the potential to transmit dengue, chikungunya, Zika, and yellow fever if an infected traveler returns to the area. The everyday problem is the bites, often from containers close to home.

More on the day-biter in Aedes aegypti: the daytime-biting mosquito.

A season that runs late

The season now runs later than most people plan for. This year, the state was still reporting West Nile activity into October.

Warmth is a large part of it. The invasive Aedes add a second layer: the CDC says Aedes females lay eggs on the inner walls of containers, above the waterline, and the eggs "can survive drying out for up to 8 months." A refilled saucer can bring them back. Plan for mosquitoes until the weather actually turns cold where you live, not until the calendar says fall.

Where activity is concentrated

West Nile activity is not spread evenly. Per the CDPH county table dated October 6, 2026:

• Los Angeles County: 111 human cases, the most in the state. Los Angeles County Public Health's report of October 2, 2026 counts 180 infections and 5 deaths, excluding Long Beach and Pasadena.
• San Joaquin Valley: Fresno 19, Kings 11, Kern 10, and Stanislaus 7 human cases.
• Orange County: 23 human cases in the state table. The Daily Pilot reported on September 18, 2026 that the Orange County Mosquito and Vector Control District had 747 positive mosquito samples by that date, against 186 in all of 2025. District spokesman Brian Brannon called it "one of the worst years we've seen."
• Sacramento Valley: Butte 13, Sacramento 11, Yolo 5, and Glenn 5 human cases.
• Ventura County: 0 human cases in the same table, with 7 birds and 1 mosquito sample listed.

A quiet county is not a reason to skip the routine. The daytime biter is about bites, not case counts, and it can breed in a backyard container.

What the county does vs what you do

Your local vector control program is already working. Many districts inspect private property on request, give away mosquitofish, and treat for disease risk. The Ventura County Vector Control Program, for example, runs a mosquito complaint hotline, free mosquitofish, and dead-bird reporting.

The gap is narrow and clear. No public program keeps your patio on a weekly calendar. Container breeding on private property stays your job.

What to do this week

1. Walk the yard with a flashlight. The CDC and Ventura County both advise that once a week you "empty and scrub, turn over, cover, or throw out" items that hold water. Lift every pot. Check the saucers, the tarp fold, and the dog bowl. Our ten-minute weekly yard check walks the order.
2. Call your district. Ask what free services it offers, such as inspections or mosquitofish. In Ventura County, the hotline is (805) 658-4310. In the Greater Los Angeles County district, it is (562) 944-9656. In Orange County, it is (714) 971-2421.
3. Match repellent to the hour. Dusk and dawn for Culex, daytime for Aedes. The CDC lists six EPA-registered repellent active ingredients, including DEET, picaridin, and IR3535; the EPA's search tool helps you compare. Follow the label, including its age cautions.
4. Leave the yard-wide spraying to a licensed applicator. If you hire one, follow their guidance and the product label.

None of this acts on the virus. The goal of a routine is fewer bites, and fewer bites means fewer chances of exposure.

Outside California

California is the epicenter this year, but mosquito control is local everywhere. The CDC notes there is no national operational mosquito-control agency; local governments and districts do the work, and their services vary. Find yours on the NPIC list of mosquito control agencies by state, or call your local health department. The weekly container check works the same in any state.