Mosquito season in San Francisco County, CA: when it starts and ends

Mosquito months in San Francisco County

Mosquitoes in San Francisco County, CA can be active in any month. The worst stretch is August to August.

1. Jan
2. Feb
3. Mar
4. Apr
5. May
6. Jun
7. Jul
8. Aug
9. Sep
10. Oct
11. Nov
12. Dec

What to do in San Francisco County, and when

• No off-season here: tip out standing water every week, all year.
• A yard treatment matters most August.
• 2 West Nile cases since 2016 (last 2021): you should use repellent at dusk August, more so if over 60.

San Francisco County against the rest of CA

San Francisco County has the 3rd longest season of 58 CA counties, at 365 days against 365 in Los Angeles County and 110 in Mono County. Its mean temperature is 58°F.

• Los Angeles County: all year (365 days, 775 WNV)
• Orange County: all year (365 days, 117 WNV)
• Imperial County: January 2 to December 29 (361 days, 7 WNV)
• Santa Cruz County: January 3 to December 27 (358 days, 1 WNV)

West Nile in San Francisco County

From 1999 to 2025, CDC ArboNET counted 7 human West Nile cases in San Francisco County (6 neuroinvasive) across 6 years. The worst was 2005 with 2; the latest was 2021.

Background: where mosquitoes never stop; how the dates are worked out.