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

Mosquito months in San Mateo County

In a normal year mosquito season in San Mateo County, CA starts around December 31 and ends around December 12, about 50 weeks.

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 Mateo County, and when

• Start tipping out standing water weekly by December 17.
• Book any yard treatment for late December, ahead of the August peak.
• Repellent can go away after December 12.
• 5 West Nile cases since 2016 (last 2023): you should use repellent at dusk August, more so if over 60.

San Mateo County against the rest of CA

San Mateo County has the 7th longest season of 58 CA counties, at 347 days against 365 in Los Angeles County and 110 in Mono County. Its mean temperature is 57.9°F.

• Santa Cruz County: January 3 to December 27 (358 days, 1 WNV)
• Riverside County: January 10 to December 31 (355 days, 110 WNV)
• Santa Clara County: January 18 to December 11 (327 days, 16 WNV)
• Fresno County: January 22 to December 10 (321 days, 201 WNV)

West Nile in San Mateo County

From 1999 to 2025, CDC ArboNET counted 6 human West Nile cases in San Mateo County (6 neuroinvasive) across 4 years. The worst was 2023 with 3; the latest was 2023.

More: how the dates are worked out · where mosquitoes never stop.