Mosquito season in Sierra County, CA: when it starts and ends
Mosquito months in Sierra County
In a normal year mosquito season in Sierra County, CA starts around May 24 and ends around October 5, about 19 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Sierra County, and when
- Start tipping out standing water weekly by May 10.
- Book any yard treatment for late May, ahead of the July peak.
- Repellent can go away after October 5.
- No West Nile cases since 2016, so bites in Sierra County are a nuisance; check window screens.
Sierra County against the rest of CA
Sierra County has the 55th longest season of 58 CA counties, at 133 days against 365 in Los Angeles County and 110 in Mono County. Its mean temperature is 44.5°F.
- Modoc County: May 11 to September 25 (137 days, 0 WNV)
- Plumas County: May 17 to October 1 (137 days, 0 WNV)
- Alpine County: May 28 to October 3 (128 days, 0 WNV)
- Lassen County: May 30 to October 1 (124 days, 0 WNV)
West Nile in Sierra County
CDC ArboNET lists no human West Nile cases for Sierra County from 1999 to 2025, though virus turned up in birds, mosquitoes or horses in 1 of those years.
See how the dates are worked out, and when mosquitoes go away for the winter.