Mosquito season in Santa Clara County, CA: when it starts and ends

Mosquito months in Santa Clara County

In a normal year mosquito season in Santa Clara County, CA starts around January 18 and ends around December 11, about 47 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 Santa Clara County, and when

• Start tipping out standing water weekly by January 4.
• Book any yard treatment for mid January, ahead of the August peak.
• Repellent can go away after December 11.
• 16 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check CA spray notices.

Santa Clara County against the rest of CA

Santa Clara County has the 8th longest season of 58 CA counties, at 327 days against 365 in Los Angeles County and 110 in Mono County. Its mean temperature is 60.4°F.

• Riverside County: January 10 to December 31 (355 days, 110 WNV)
• San Mateo County: December 31 to December 12 (347 days, 5 WNV)
• Fresno County: January 22 to December 10 (321 days, 201 WNV)
• Marin County: January 24 to December 10 (321 days, 0 WNV)

West Nile in Santa Clara County

From 1999 to 2025, CDC ArboNET counted 53 human West Nile cases in Santa Clara County (33 neuroinvasive) across 17 years. The worst was 2014 with 10; the latest was 2025.

Background: reading the West Nile counts; how the dates are worked out.