For most of California, mosquitoes fade in late fall, when cooling nights push their activity down. Going away here usually means going quiet rather than gone, because the mosquitoes are waiting out winter in forms people rarely notice. And where you live matters more than the calendar, since the state’s mild coast and hot inland valleys keep very different schedules.
The Months Mosquito Season Ends Across California
In most inland areas of California, mosquito season typically ends as temperatures drop in October and November, because cold nights push mosquito activity down sharply.1Corky’s Pest Control. Mosquito Season in California: When It Starts and How to Stop It The first stretch of chilly nights, not a set date, is the best sign the season is winding down.
Mild areas keep mosquitoes active far longer. In Southern California coastal neighborhoods and warmer inland valleys, biting mosquitoes can stay active well into November.2Corky’s Pest Control. Mosquito Season in California: When It Starts and How to Stop It Some species stay active year-round where temperatures stay mild.3Corky’s Pest Control. Mosquito Season in California: When It Starts and How to Stop It Invasive Aedes mosquitoes in Southern California can be active nearly all year, with far less of an off-season than in colder regions.4Mosquito Squad. Are invasive mosquitoes active all year?
The same week can feel completely different across the state. A Sacramento Valley yard may be nearly bite-free in mid-November while an Orange County patio is not. Elevation and distance from the ocean change how cold nights get, and irrigated farmland and lawns can keep water around after the rains have stopped. Local mosquito and vector control district websites often post seasonal activity for your area, which beats guessing.
A single chilly night slows mosquitoes but rarely ends the season, because the survivors speed back up as soon as evenings warm. The season is really over when cool nights come in a run and the bites stop even on mild afternoons. Even then, mosquitoes sheltered in garages or crawl spaces may bite if you disturb them on a warm day.
Your own yard may not match the regional pattern either. Mosquitoes breed in whatever water is near you, so a neighbor’s clogged gutter or a nearby creek can keep your block biting after the surrounding area has quieted. A sunny, sheltered patio against a warm wall stays friendlier to mosquitoes than an open, shaded one. And different species peak at different times, so a lull in one kind of bite can be followed by another kind showing up.
In the high Sierra Nevada, snowpack and water temperature shape when mosquito larvae develop and when their pools dry up.5PubMed Central. The ecology of mosquito larvae (Diptera: Culicidae) in the subalpine zone of the eastern Sierra Nevada Mountains, California, U.S.A. Mountain season follows snowmelt rather than the coastal calendar. It often starts later than in the lowlands, because pools stay frozen or cold into late spring or summer, and once they dry up and nights turn freezing, mountain mosquitoes fade quickly.
Why Mosquitoes Linger Longer Than They Used To
In recent years, climate change has softened winter temperatures, lengthening the season mosquitoes can thrive.6NPR. How climate change plays a role in bringing dengue fever to L.A. Mosquitoes are cold-blooded, so their growth speeds up and slows down with the temperature around them. A mild winter means fewer nights cold enough to halt development, so larvae keep maturing when they once would have stalled. Adults that would have died in a hard cold spell are more likely to live through it, leaving more of them to start the next season.
In much of California, hotter temperatures and late summer and fall rains are projected to extend the mosquito season.7PubMed Central. Regional and seasonal response of a West Nile virus vector to climate change Rain in late summer or fall refills pools, containers, and ditches just when mosquitoes would normally be tapering off, and each fresh pool can produce a new batch of adults. A very dry, hot spring or summer, on the other hand, may delay the season’s start in the southwest.8PubMed Central. Regional and seasonal response of a West Nile virus vector to climate change
Irrigation is linked to more mosquitoes and steadier numbers through the year for California’s two main West Nile carriers.9PubMed Central. Irrigation increases and stabilizes mosquito populations and increases West Nile virus incidence It keeps their numbers high for longer, partly freeing them from natural rainfall patterns.10PubMed Central. Irrigation increases and stabilizes mosquito populations and increases West Nile virus incidence Irrigated fields, flooded refuges, and watered landscapes keep water around through California’s dry months. Peaks in one major species follow crop irrigation and the flooding of wildlife refuges in late summer.11PubMed Central. Seasonal Abundance of Culex tarsalis and Culex pipiens Complex Mosquitoes (Diptera: Culicidae) in California
At home, bites in late October or November are becoming less unusual. Garden and lawn watering acts like small-scale irrigation and can keep breeding spots wet.
Where Mosquitoes Go in Winter
Many of the common Culex mosquitoes that grow up in fall’s cool, short days become females that pause egg-making for winter. In response to short autumn days, they stop developing eggs, store energy instead, and stop seeking blood meals.
The effect is strong: most females of one key West Nile carrier exposed as larvae to autumn conditions entered this winter rest.12PubMed Central. Overwintering Biology of Culex (Diptera: Culicidae) Mosquitoes in the Sacramento Valley of California In California these resting females don’t feed on blood, but they stay active and sip sugars.13PubMed Central. Effects of Warm Winter Temperature on the Abundance and Gonotrophic Activity of Culex (Diptera: Culicidae) in California They appear to stay in winter rest until after the winter solstice, after which warming temperatures seem to end it.14PubMed Central. Effects of Warm Winter Temperature on the Abundance and Gonotrophic Activity of Culex (Diptera: Culicidae) in California
A winter-resting Culex female lives on stored energy and sugars, so a mosquito in your garage in December often isn’t hunting you, though disturbed mosquitoes on a warm day may still bite. In spring the survivors become the first egg layers of the new year.
Some mosquitoes overwinter as adults in sheltered places like basements, storm drains, hollow logs, or under houses.15Thermacell. Winter Mosquito Myths—Busted Underground storm drain systems in urban Southern California serve as major breeding and resting sites for mosquitoes.16PubMed. Spatial and temporal distribution of mosquitoes in underground storm drain systems in Orange County, California
Invasive Aedes take a different route. They stick single eggs to the inside of water-holding containers, just above the waterline.17California Mosquitoes. Winter Mosquitoes in California and Your Home Checklist These eggs can survive drying for six months or longer, and when warm weather comes and water covers them, they hatch into larvae.18Contra Costa Mosquito and Vector Control District. Invasive Mosquito Species A saucer that’s dry today may still carry eggs from when it held water.19California Mosquitoes. Winter Mosquitoes in California and Your Home Checklist
The eggs are tiny and look like a faint stain or dirt line, which is hard to spot, and the first warm rain or watering can hatch a whole batch. That’s why the problem returns in the same spots each year.
The Mosquito That Comes Out in Winter
The winter salt marsh mosquito is one of 53 kinds of mosquitoes in California and is distinctive for its black and white markings.20Napa County Mosquito Abatement District. Supplemental Information: Mosquitoes It breeds in coastal pickleweed tidal and diked marshes, favoring pools diluted by winter and early spring rains.21Napa County Mosquito Abatement District. Supplemental Information: Mosquitoes High tides and rains, along with winter’s short days and cool temperatures, can hatch its eggs as early as November.22Santa Clara County Vector Control District. Helicopter Treatments to Reduce Emergence of Adult Winter Salt Marsh Mosquitoes Postponed For this species, the cool, rainy season is the trigger, so the season you think of as mosquito-free is its peak.
The species lays eggs in moist soil in late spring and early summer.23Santa Clara County Vector Control District. Helicopter Treatments to Reduce Emergence of Adult Winter Salt Marsh Mosquitoes Postponed Those eggs can stay dormant for many years, with only part of a batch hatching in any one flood.24Napa County Mosquito Abatement District. Winter Salt Marsh Mosquito (Aedes squamiger) One flood doesn’t use up the supply, so marsh areas can produce winter mosquitoes year after year even after a dry season.
If you live near the San Francisco Bay or other coastal marshes, winter bites may come from this species rather than a late-summer one. These mosquitoes can fly into nearby neighborhoods from the marsh. Local districts sometimes treat marshes, including by helicopter, to cut adult emergence. Yard cleanup alone won’t stop a marsh-bred mosquito.
How Drought and Heavy Rain Change Mosquito Numbers
Big storms can actually clear urban breeding sites. Heavy rainfall in Southern California scoured and unclogged storm drains and sewer lines, removing standing water where house mosquitoes breed.25Journal of Vector Ecology. Effects of the El Niño – Southern Oscillation (ENSO) cycle on mosquito populations in southern California Without rain, water stagnates in stormwater pipes and drains, giving mosquitoes good breeding conditions.26Beat the Bites: Mosquito Research and Management. Why would a Californian drought trigger an outbreak of mosquito-borne disease? The cleaning effect is temporary, though, since heavy rain also leaves new puddles and fills containers.
Drought lowers mosquito numbers overall.27PubMed Central. The paradoxical impact of drought on West Nile virus risk: insights from long-term ecological data Natural pools, creeks, and ponds shrink or vanish, so fewer breeding places exist. But drought concentrates mosquitoes and birds around the few remaining water sources.28PubMed Central. The paradoxical impact of drought on West Nile virus risk: insights from long-term ecological data In dry years, birds and mosquitoes gather in urban areas where people provide water.29Beat the Bites: Mosquito Research and Management. Why would a Californian drought trigger an outbreak of mosquito-borne disease?
In a dry year, pet water bowls, birdbaths, and dripping hoses become the main water for miles around. Drains and gutters that never flush can hold water all season. Birds matter here because they are the main hosts of West Nile virus and need water too. When birds and mosquitoes share a few water sources, the virus has many chances to pass between them, so a year with fewer mosquitoes can still bring more infected ones to your yard.
When West Nile Virus Risk Is Highest
August and September are the most active months for West Nile virus in Southern California.30Compton Creek Mosquito Abatement District. Why West Nile Virus Risk Is Highest in Late Summer Mosquitoes there can stay active into fall while temperatures remain warm.31Compton Creek Mosquito Abatement District. Why West Nile Virus Risk Is Highest in Late Summer The risk doesn’t end on Labor Day: warm weather keeps the virus moving among mosquitoes, so risk follows temperature rather than the calendar.
West Nile virus passes between mosquitoes and birds, and people can be infected by the bite of an infected mosquito.32KFF Health News. Heavy Rains, End Of Drought Could Help Keep West Nile Virus Subdued — For Now It is rarely passed from one person to another.33KFF Health News. Heavy Rains, End Of Drought Could Help Keep West Nile Virus Subdued — For Now Drought increases the share of infected mosquitoes, even though there are fewer mosquitoes overall,34PubMed Central. The paradoxical impact of drought on West Nile virus risk: insights from long-term ecological data because crowding mosquitoes and birds around limited water raises the chance of infection.35PubMed Central. The paradoxical impact of drought on West Nile virus risk: insights from long-term ecological data
Most people who are infected have no symptoms or only mild ones like fever and aches. A small share become seriously ill, and older adults and people with weakened immune systems are at higher risk.
During peak months, use an EPA-registered repellent when outside at dawn and dusk, and wear long sleeves and pants in the evening near water. Keep window and door screens repaired. Reporting dead birds to your county or state West Nile hotline helps track the virus.
What to Do Now So Fewer Come Back Next Year
Because Aedes eggs cling above the waterline, scrub saucers, buckets, and pots rather than only emptying them. A quick rinse or dump leaves many eggs behind. Use a stiff brush or scrub pad on the inside surface at the waterline, then let the container dry or turn it over. Doing this in fall removes eggs before winter rain can wet them and start the next generation. Store containers upside down or under cover so winter rain can’t fill them.
Tall grass and dense plants give adult mosquitoes shaded resting places near where you spend time outdoors.36Corky’s Pest Control. Mosquito Season in California: When It Starts and How to Stop It Mowing and trimming shrubs along fences and patios removes the cover that keeps mosquitoes on your property.37Corky’s Pest Control. Mosquito Season in California: When It Starts and How to Stop It
A fall and winter yard walk helps. Check gutters, rain barrels without screens, and tire swings for trapped water. Look under houses and in crawl spaces for standing water where adults can rest. Fix leaky irrigation that keeps soil wet through winter, and repeat after big storms, when new water appears.
In one experiment, a predatory water insect cut the number of adult house mosquitoes emerging by about a third to a half.38PubMed Central. Eutrophication and predator presence overrule the effects of temperature on mosquito survival and development Natural predators can help in ponds and water features, and your local mosquito and vector control district can inspect problem spots and often provides mosquito fish for free.
Some problems are beyond one yard. Neglected swimming pools nearby can breed huge numbers, and districts can treat them. Small black-and-white mosquitoes biting in the daytime may be invasive Aedes, which districts want reported, though near coastal marshes in winter they may be the black-and-white winter salt marsh mosquito. Persistent bites near drains or marshes are also worth a call.