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California · Mosquito Season Guide

Is it mosquito season in California?

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II. California Mosquito Season Month by month

California spans more climate zones than almost any other US state, and its mosquito season reflects that range — from near-year-round activity in coastal San Diego to a compressed June-to-August window in the Sierra Nevada foothills.

Central Valley
Sacramento · Fresno · Bakersfield
Southern California
Los Angeles · San Diego · Riverside
Bay Area / NorCal
San Francisco · San Jose · Sacramento foothills
Very Low Low Moderate High Very High taller = more active · color = severity
53+
mosquito species recorded in California
#1
state for West Nile Virus cases most years
20+
counties with invasive Ae. aegypti established
~270
days of possible activity in coastal Southern California

The Central Valley: California's Mosquito Hotspot

Sacramento, Fresno, Bakersfield, and the agricultural corridor between them form the most active West Nile Virus zone in the state — and among the most active in the entire continental US. Irrigated fields, rice paddies, and drainage canals provide relentless breeding habitat, while the Central Valley's extreme summer heat (routinely above 100°F) accelerates larval development. Culex tarsalis, the primary WNV vector west of the Rocky Mountains, thrives here from May through October. Sacramento and Fresno counties have ranked among the top West Nile burden counties nationally in multiple recent years.

Southern California and the Invasive Species Problem

Los Angeles and San Diego have a milder coastal season, but inland valleys — San Bernardino, Riverside, and the San Fernando Valley — amplify summer activity significantly. The more critical development: invasive Aedes aegypti has been established in Los Angeles, San Diego, Orange, Fresno, and surrounding counties since first detected in 2013, and continues to spread. Unlike native Culex species, Ae. aegypti bites aggressively during daylight hours and is capable of transmitting dengue, Zika, and chikungunya. Standard dusk-only protection is insufficient anywhere this species is established.

Bay Area and Northern California

San Francisco's coastal fog keeps temperatures moderate, making it one of the lower-risk major metros in the state. The South Bay (San Jose, Fremont) and East Bay (Oakland, Concord) see more meaningful activity — particularly in marshy areas along the Bay shoreline and creek corridors. Season typically runs May through September. Northern California and the Sierra Nevada foothills follow a compressed June-to-August pattern driven by snowmelt and warming temperatures at elevation.

III. California's Three Species to Know
Culex tarsalis mosquito macro photograph — Davefoc, CC BY-SA 4.0, Wikimedia Commons
Culex tarsalis
Western Encephalitis Mosquito

The dominant West Nile Virus vector west of the Rocky Mountains. Bridges between bird and human hosts most aggressively at dusk and dawn. Thrives in California's irrigated Central Valley — Sacramento, Fresno, and Kings counties are historically among the highest WNV-burden counties in the US. Active May through October; population crashes below 50°F.

Aedes aegypti macro photograph — CDC / James Gathany, public domain
Aedes aegypti
Yellow Fever Mosquito

First detected in California in 2013, now established in over 20 counties. A daytime biter capable of transmitting dengue, Zika, and chikungunya. Concentrated in Los Angeles, San Diego, Orange, Fresno, and Riverside counties. Unlike native Culex, it bites throughout the day — standard dusk-only precautions are not sufficient.

Culex quinquefasciatus macro photograph
Culex quinquefasciatus
Southern House Mosquito

A secondary West Nile Virus vector established in urban Southern California. Bites at dusk and dawn, breeding in stagnant water — neglected pools, birdbaths, and storm drains. Most prevalent in the LA basin and Inland Empire, where mild winters allow populations to persist at low levels year-round.

Invasive Aedes aegypti bites during the day. If you're in Los Angeles, San Diego, or Fresno, applying repellent only at dusk leaves a significant exposure window open during warm months. Check your local CDPH Mosquito-Borne Disease page for current invasive species distribution in your county.

IV. California Cities at a Glance
City Peak Season Off-Season Notes
Los Angeles Mar – Nov Low (rarely zero) Coastal urban; Ae. aegypti established in multiple neighborhoods — daytime bites possible spring through fall Check live
San Diego Feb – Nov Very low Longest season in the state; mild winters allow year-round low-level activity; invasive Ae. aegypti well established Check live
Sacramento May – Oct Minimal Highest West Nile Virus burden of any CA metro; rice fields and river delta sustain Culex tarsalis through peak summer Check live
Fresno May – Oct Minimal Central Valley heat drives intense July–August peak; both Culex tarsalis (WNV) and invasive Ae. aegypti present Check live
San Francisco May – Sep Very low Coastal fog suppresses activity significantly; South Bay and East Bay see noticeably more pressure than the city itself Check live
San Jose May – Sep Low Bay shoreline marshes and creek corridors sustain local populations; generally milder than Central Valley Check live
Bakersfield May – Oct Minimal Kern County agriculture and drainage canals; Culex tarsalis dominant; extreme summer heat pushes peak activity to dawn and dusk Check live
More State Guides
Florida Year-round activity in the south — 80+ species including invasive Ae. aegypti and the Asian tiger mosquito across all 67 counties Texas ~85 species across four climate zones — Gulf Coast active March through November, with year-round pressure in the Rio Grande Valley Georgia Atlanta metro leads the Southeast in Asian tiger mosquito density — active statewide March through November New York Ground zero for West Nile in the US (1999 Queens outbreak) — NYC metro and Long Island see the highest Northeast seasonal pressure Illinois Home of the 2002 Cook County West Nile outbreak — the deadliest urban WNV event in US history — peak season June through September Ohio Lake Erie to the Ohio River — WNV active in Franklin and Cuyahoga counties, with floodwater species surging after summer storms Pennsylvania Philadelphia WNV corridor meets rural EEE risk in western PA — the densest human population in the mid-Atlantic mosquito belt Michigan Site of the 2019 EEE outbreak — the deadliest in decades — 10 human cases and 6 deaths concentrated in SW Michigan Great Lakes wetlands New Jersey Oldest organized county mosquito control in the US (1914) — Pine Barrens EEE corridor and fierce salt marsh Shore species Virginia Tidewater season runs April–October — the longest mid-Atlantic window — with the Great Dismal Swamp as an active EEE hotspot North Carolina The coastal plain runs one of the longest seasons in the Southeast — NC leads the nation in La Crosse encephalitis cases from the Appalachian foothills Maryland Chesapeake Bay watershed creates extraordinary mosquito habitat — tidal marshes, Eastern Shore wetlands, and the DC-Baltimore corridor combine for dense seasonal pressure Tennessee Memphis and the Mississippi River corridor historically record the highest WNV case counts in the mid-South — Ae. albopictus has colonized all 95 counties Indiana Indiana sits at the crossroads of the Great Lakes EEE belt — Lake Michigan wetlands in the north and the Wabash River floodplain drive intense seasonal pressure Louisiana The Gulf Coast's most intense pressure — coastal marshes and summer rains keep Cx. quinquefasciatus and Ae. aegypti active from March through November Alabama Mobile Bay to the Tennessee Valley — Ae. albopictus has reached all 67 counties and peak WNV season runs June through September South Carolina Low Country marshes produce some of the Southeast's densest Anopheles and Culex populations — season runs March through November Minnesota 10,000 lakes means 10,000 breeding sites — the Northwoods sees intense but short July–August peaks, while the Twin Cities face a longer June–September window Wisconsin Great Lakes wetlands and the Mississippi River backwaters drive strong WNV pressure in the Milwaukee corridor — peak season July through August Missouri At the confluence of the Missouri and Mississippi rivers — St. Louis historically ranks among the highest WNV cities in the US — season May through October Arkansas Delta rice-field flooding and the Arkansas River bottomlands create some of the highest rural mosquito pressure in the mid-South — active April through October Washington Western WA stays mild and damp year-round; the Yakima Valley and Columbia Basin face the highest WNV risk in the Pacific Northwest — peak July through September Arizona Monsoon season (July–September) drives sudden mosquito explosions across the Valley of the Sun — WNV detected annually Colorado Front Range WNV corridor stretches Denver to Fort Collins — eastern plains carry the state's highest risk Connecticut Connecticut River valley WNV pressure meets EEE risk in inland freshwater wetlands — active May through October Delaware Chesapeake and Delaware Bay marshes create outsized pressure for the nation's second-smallest state Hawaii Year-round activity at sea level — no native species, but Ae. aegypti and tiger mosquitoes established statewide Idaho Snake River Plain irrigation districts fuel summer WNV pressure — season June through September Iowa Mississippi River corridor and prairie wetlands drive intense June–September pressure — WNV active statewide Kansas Great Plains WNV corridor — Wichita and Kansas City see peak pressure July–August; floodwater species surge after storms Kentucky Ohio River corridor and Kentucky Lake wetlands sustain a long April–October season across all 120 counties Maine Short but intense July–August season — EEE detected in some years in the southern coastal plain Massachusetts EEE fatality rate exceeds 30% — the Plymouth County swamp complex is the active epicenter — season May through October Mississippi Gulf Coast near-year-round pressure meets Delta rice-field flooding for one of the South's most intense mosquito environments Montana Yellowstone and Missouri River corridors drive a compressed June–August season — WNV detected most years in eastern MT Nebraska Platte River valley and eastern Nebraska are among the Great Plains' most active WNV zones — peak July–August Nevada Las Vegas monsoon season drives July–September activity; Reno irrigation districts fuel a parallel northern peak New Hampshire EEE detected most years — the southern lakes and coastal plain are in the active Northeast EEE corridor New Mexico Monsoon season (July–September) creates sudden breeding habitat; WNV circulates in the Rio Grande valley annually North Dakota Red River Valley flooding produces some of the Great Plains' most extreme floodwater mosquito events — peak June–August Oklahoma One of the South-Central's most intense seasons — the Arkansas and Red River corridors sustain populations April through October Oregon Eastern Oregon Columbia Basin is the state's WNV hotspot; western Oregon runs a mild coastal season June through September Rhode Island New England's densest coastal pressure — Narragansett Bay salt marshes and WNV active in all five counties South Dakota Missouri River and James River valleys drive summer WNV activity — eastern SD sees the most intense pressure Utah Salt Lake Valley WNV corridor and irrigation districts sustain a July–September peak — drier areas see limited activity Vermont Champlain Valley runs Vermont's longest season — EEE detected in the Lake Champlain lowlands in some years West Virginia Kanawha and Ohio River valleys sustain a May–October season — Ae. albopictus now established statewide Wyoming The shortest meaningful season of any contiguous US state — North Platte valley WNV window is just 6–8 weeks
West Nile, dengue, and EEE: US disease risk by region → Asian tiger mosquitoes: why you're getting bitten during the day → Eliminating standing water: the highest-leverage yard move →