Mosquito season in Orange County, CA: when it starts and ends
Mosquito months in Orange County
Mosquitoes in Orange County, CA can be active in any month. The worst stretch is July to September.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Orange County, and when
- No off-season here: tip out standing water every week, all year.
- A yard treatment matters most July to September.
- 117 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to September, and check CA spray notices.
Orange County against the rest of CA
Orange County has the 2nd longest season of 58 CA counties, at 365 days against 365 in Los Angeles County and 110 in Mono County. Its mean temperature is 65.1°F.
- Los Angeles County: all year (365 days, 775 WNV)
- San Francisco County: all year (365 days, 2 WNV)
- Imperial County: January 2 to December 29 (361 days, 7 WNV)
West Nile in Orange County
From 1999 to 2025, CDC ArboNET counted 705 human West Nile cases in Orange County (512 neuroinvasive) across 22 years. The worst was 2014 with 263; the latest was 2025.
Background: where mosquitoes never stop; how the dates are worked out.