Mosquito season in Contra Costa County, CA: when it starts and ends

Mosquito months in Contra Costa County

In a normal year mosquito season in Contra Costa County, CA starts around February 8 and ends around December 10, about 44 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 Contra Costa County, and when

• Start tipping out standing water weekly by January 25.
• Book any yard treatment for early February, ahead of the August peak.
• Repellent can go away after December 10.
• 35 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to September, and check CA spray notices.

Contra Costa County against the rest of CA

Contra Costa County has the 18th longest season of 58 CA counties, at 305 days against 365 in Los Angeles County and 110 in Mono County. Its mean temperature is 60.8°F.

• Stanislaus County: January 30 to December 5 (310 days, 180 WNV)
• Alameda County: February 3 to December 6 (305 days, 4 WNV)
• Sutter County: February 2 to December 3 (304 days, 29 WNV)
• Butte County: February 4 to December 3 (303 days, 95 WNV)

West Nile in Contra Costa County

From 1999 to 2025, CDC ArboNET counted 87 human West Nile cases in Contra Costa County (60 neuroinvasive) across 21 years. The worst was 2005 with 11; the latest was 2025.

More: how the dates are worked out · reading the West Nile counts.