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

Mosquito months in Butte County

In a normal year mosquito season in Butte County, CA starts around February 4 and ends around December 3, about 43 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 Butte County, and when

• Start tipping out standing water weekly by January 21.
• Book any yard treatment for early February, ahead of the July peak.
• Repellent can go away after December 3.
• 95 West Nile cases since 2016: you should wear EPA-registered repellent at dusk June to September, and check CA spray notices.

Butte County against the rest of CA

Butte County has the 20th longest season of 58 CA counties, at 303 days against 365 in Los Angeles County and 110 in Mono County. Its mean temperature is 62.8°F.

• Contra Costa County: February 8 to December 10 (305 days, 35 WNV)
• Sutter County: February 2 to December 3 (304 days, 29 WNV)
• Sonoma County: February 4 to December 4 (302 days, 1 WNV)
• Yolo County: February 6 to December 2 (300 days, 93 WNV)

West Nile in Butte County

From 1999 to 2025, CDC ArboNET counted 296 human West Nile cases in Butte County (122 neuroinvasive) across 22 years. The worst was 2015 with 53; the latest was 2025.

See how the dates are worked out, and reading the West Nile counts.