Today's Mosquito Risk in California: Real-Time Forecast & Activity Map
Check your ZIP code for local mosquito activity, West Nile risk, and prevention insights.
Built from CDC and NOAA data
Mosquito Surveillance Insights
- Annual WNV Cases
- about 150 to 300 confirmed cases annually
- among the highest absolute counts in the country
- Based on CDC ArboNET surveillance data
- Risk Tier
- Tier 2: High Pressure
- Aedes aegypti invasion since 2013 added a daytime urban biter to Central Valley and LA Basin; Cx. tarsalis still drives WNV transmission.
- Per mosquito-risk-model regional tier
- Highest-Risk Areas
- Los Angeles, Riverside, San Bernardino (LA Basin and Inland Empire); Sacramento, Yolo, Stanislaus, Merced, Fresno, Tulare, Kern (Central Valley); Imperial (Imperial Valley); Santa Clara and Alameda (Bay Area)
Current Situation in California
California carries one of the heaviest WNV burdens in the country, with roughly 150 to 300 confirmed human cases annually and several years on record above 400. The state runs as a dense stack of mosquito climates: the irrigated Central Valley from Redding through Sacramento, Stockton, Fresno, and Bakersfield is the country's largest Cx. tarsalis engine, the LA Basin storm drain and dry-wash grid sustains year-round Cx. quinquefasciatus, the Salton Sea and Imperial Valley rice and alfalfa belt produces vast summer Aedes and Culex hatches, the San Francisco Bay marshes carry seasonal pressure, and the Sierra Nevada and Mojave run too cold or too dry for sustained transmission. The Aedes aegypti invasion that began in Madera County in 2013 has since expanded across the Central Valley, the LA Basin, and is moving north into Sacramento and the Bay Area, adding a daytime-biting urban container species to a state that already led on Culex burden. Aedes albopictus is established across southern California. Local dengue transmission has been documented sporadically. St. Louis Encephalitis remains a historic risk and Western Equine Encephalitis is documented but rare in modern surveillance. Biting season runs May through October across most of the state and stretches near year-round across the southern coast and Imperial Valley.
Snapshot of the current mosquito situation in California, drawn from CDC ArboNET surveillance and the regional risk model.
- Risk Level
- Very High
- Peak Activity
- May through October
- Dominant Vector
- Western Encephalitis Mosquito (Culex tarsalis) Central Valley and dominant WNV vector; Yellow Fever Mosquito (Aedes aegypti) invasion since 2013 across Central Valley and LA Basin
Where Mosquitoes Breed in California
California mosquito habitat is the most varied in the western United States. The Central Valley irrigation grid from the Sacramento Valley through the San Joaquin Valley holds enormous Cx. tarsalis populations off rice paddies, alfalfa, almond and stone-fruit orchards, and dairy lagoons, with Yolo, Sacramento, Stanislaus, Merced, Fresno, Tulare, and Kern counties producing the bulk of the state's WNV burden. The LA Basin from Long Beach and the Los Angeles River through the San Gabriel and San Fernando valleys carries year-round Cx. quinquefasciatus and Aedes aegypti through storm drains, abandoned pools, and ornamental container habitat. The Salton Sea and the Imperial and Coachella Valleys produce vast summer Cx. tarsalis and Psorophora hatches off rice, alfalfa, and date palm irrigation. The San Francisco Bay tidal marshes from San Pablo Bay through the Suisun Bay delta hold Aedes squamiger and seasonal Aedes washinoi. The Sacramento and San Joaquin River delta carries year-round Cx. tarsalis off slough and tule habitat. Coastal salt marsh from the Mendocino coast through Humboldt Bay and down the Big Sur and San Diego coast carries seasonal Aedes squamiger. The Sierra Nevada, Death Valley, and the high Mojave stay too cold or too dry for sustained populations.
Hotspot Counties & Areas
- Los Angeles County (LA Basin, San Fernando and San Gabriel valleys)
- Sacramento County (Sacramento metro, lower Sacramento Valley)
- Fresno County (Fresno, central San Joaquin Valley)
- Kern County (Bakersfield, southern San Joaquin Valley)
- San Bernardino and Riverside counties (Inland Empire, Coachella Valley)
- Imperial County (Imperial Valley rice and alfalfa, Salton Sea)
- Santa Clara and Alameda counties (San Jose, Oakland, Bay Area)
- San Diego County (San Diego, Aedes aegypti coastal cities)
Seasonal Activity Pattern
Biting starts in late April across the southern coast and Imperial Valley, ramps through May across the Central Valley, peaks in late July through September, and tapers through October. The southern coast and the Imperial and Coachella Valleys see near year-round Aedes aegypti biting in mild winters. WNV cases concentrate from July through October across the Central Valley and the LA Basin, with Yolo, Sacramento, Stanislaus, Merced, Fresno, Tulare, Kern, Los Angeles, and Riverside counties typically leading absolute case counts. The Aedes aegypti urban biting cycle holds steady from May through November in the Central Valley and runs longer in southern California. Sporadic local dengue transmission tends to flare in late summer in southern California neighborhoods with established Aedes aegypti. Atmospheric river flooding and tropical-storm remnants produce post-storm Psorophora and Aedes vexans hatches across the Central Valley and the LA Basin. The Sierra Nevada high country stays largely mosquito-free outside snowmelt Aedes pulses on the trail systems.
Monthly Biting Intensity (West Coast)
California Mosquito Risk Profile
- WNV burden level
- Very High
- about 150 to 300 confirmed cases annually; among the highest absolute counts in the country
- EEE risk
- Rare
- Peak biting season
- May through October; near year-round in Imperial and southern coast
- Top urban risk areas
- Los Angeles (Los Angeles)
- San Diego (San Diego)
- San Jose (Santa Clara)
- Fresno (Fresno)
Signature habitat: Central Valley irrigated agriculture, LA Basin storm drains, Sacramento Delta, coastal salt marsh.
Climate driver: Aedes aegypti invasion since 2013 added a daytime urban biter to Central Valley and LA Basin; Cx. tarsalis still drives WNV transmission.
Check Your Local Mosquito Risk
Ready to see current mosquito activity in your area? Get today's forecast and personalized risk assessment for your ZIP code.
Common Mosquitoes in California
Understanding which mosquitoes are active in California helps you identify threats and take appropriate precautions. Click any card for details.
Southern House Mosquito
Culex quinquefasciatus
Risk: West Nile virus, St. Louis encephalitis, dog heartworm.
Key feature: Medium brown body with banded abdominal segments and no leg stripes.
Active: Year-round in the deep South; summer peak elsewhere. Most active from dusk through the first few hours after sunset.
Yellow Fever Mosquito
Aedes aegypti
Risk: Zika, dengue, yellow fever, chikungunya.
Key feature: Black body with white lyre-shaped markings on the thorax.
Active: Year-round in southern Florida and the Gulf; summer-only further north.
Asian Tiger Mosquito
Aedes albopictus
Risk: Eastern Equine Encephalitis, dengue, chikungunya, Zika, dog heartworm.
Key feature: A single bright white stripe down the center of the back.
Active: Spring through first hard frost. Eggs overwinter in cold climates and hatch when flooded by spring rain.
West Nile Virus
Vector: Southern House Mosquito
West Nile Virus
The most common mosquito-borne illness in the US. About 80% of infections cause no symptoms; less than 1% develop neuroinvasive disease.
What to watch for
- Onset typically 2 to 15 days after a bite
- Most cases mild or asymptomatic, small fraction severe
- Neurological signs (severe headache, neck stiffness, confusion) need ER care
Breaking the Cycle in California
The mosquito lifecycle takes about a week. These habits cut local biting pressure faster than anything else, building multiple layers of protection.
Drain the Yard Weekly
Empty buckets, planter saucers, kiddie pools, tarps, and clogged gutters. A week of standing water is one full mosquito generation.
Repel Before Going Out
EPA-registered DEET (20-30%), picaridin (20%), IR3535, or oil of lemon eucalyptus. Apply before going outside, especially at dusk and dawn.
Seal the House
Window and door screens with no tears. AC and porch fans both work. Mosquitoes struggle to fly upwind even at 1-2 mph air speed.
Treat Outdoor Clothing
Permethrin-treated long sleeves and pants for evening yard work, hiking, or fishing. Remains effective through 70+ wash cycles.
If You Develop Symptoms
Most mosquito bites are harmless. A small fraction can transmit illness, and knowing the timeline matters more than panicking at every itch.Five things to know.
Watch for fever
Fever, body aches, headache, or fatigue 2 to 15 days after a bite. West Nile often feels like flu, and most infections never get diagnosed.
Note where and when
A rough record of where and when you got bitten helps your doctor decide what to test for. Date, location, mosquito-heavy areas you visited.
Get help fast for neuro signs
Severe headache, neck stiffness, confusion, vision changes, or sudden muscle weakness can mean neuroinvasive WNV or EEE. Both are medical emergencies.
Pregnant exposure?
If pregnant and bitten in an area with documented Zika or local dengue transmission, mention to your OB. Same goes for travel exposure.
Children and seniors
Higher risk of severe disease from WNV, EEE, and dengue in adults over 60 and children under 5. Early call to the pediatrician or PCP if symptoms appear.
Heading to a lake in California?
Live water temperatures and swim conditions for Lake Tahoe, Big Bear Lake, Shasta Lake.
See California lake temperaturesWhen does mosquito season end in California?
Mosquito activity winds down after the first hard freeze, which in California typically arrives around Dec 1. See the local first-frost window and city-by-city freeze dates.
California first frost datesAlso worried about ticks in California?
Lyme disease risk and tick activity track different seasonal patterns than mosquitoes. See your California tick forecast.
View California tick forecastStay Safe in California
Get personalized mosquito forecasts and prevention tips for your area