Mosquito season in Placer County, CA: when it starts and ends
Mosquito months in Placer County
In a normal year mosquito season in Placer County, CA starts around April 3 and ends around November 10, about 32 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Placer County, and when
- Start tipping out standing water weekly by March 20.
- Book any yard treatment for early April, ahead of the July peak.
- Repellent can go away after November 10.
- 35 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check CA spray notices.
Placer County against the rest of CA
Placer County has the 44th longest season of 58 CA counties, at 221 days against 365 in Los Angeles County and 110 in Mono County. Its mean temperature is 56.8°F.
- Inyo County: March 31 to November 12 (226 days, 4 WNV)
- Humboldt County: March 27 to November 7 (225 days, 1 WNV)
- Mariposa County: April 4 to November 8 (217 days, 0 WNV)
- Mendocino County: March 31 to October 30 (213 days, 0 WNV)
West Nile in Placer County
From 1999 to 2025, CDC ArboNET counted 117 human West Nile cases in Placer County (65 neuroinvasive) across 18 years. The worst was 2005 with 35; the latest was 2025.
Background: reading the West Nile counts; how the dates are worked out.