Mosquito season in Lassen County, CA: when it starts and ends
Mosquito months in Lassen County
In a normal year mosquito season in Lassen County, CA starts around May 30 and ends around October 1, about 18 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Lassen County, and when
- Start tipping out standing water weekly by May 16.
- Book any yard treatment for late May, ahead of the July peak.
- Repellent can go away after October 1.
- No West Nile cases since 2016, so bites in Lassen County are a nuisance; check window screens.
Lassen County against the rest of CA
Lassen County has the 57th longest season of 58 CA counties, at 124 days against 365 in Los Angeles County and 110 in Mono County. Its mean temperature is 48°F.
- Sierra County: May 24 to October 5 (133 days, 0 WNV)
- Alpine County: May 28 to October 3 (128 days, 0 WNV)
- Mono County: June 4 to September 22 (110 days, 0 WNV)
West Nile in Lassen County
From 1999 to 2025, CDC ArboNET counted 1 human West Nile case in Lassen County (1 neuroinvasive) across 1 year. The worst was 2004 with 1; the latest was 2004.
Background: when mosquitoes go away for the winter; how the dates are worked out.