Mosquito season in Snohomish County, WA: when it starts and ends
Mosquito months in Snohomish County
In a normal year mosquito season in Snohomish County, WA starts around April 29 and ends around October 12, about 24 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Snohomish County, and when
- Start tipping out standing water weekly by April 15.
- Book any yard treatment for late April, ahead of the August peak.
- Repellent can go away after October 12.
- No West Nile cases since 2016, so bites in Snohomish County are a nuisance; check window screens.
Snohomish County against the rest of WA
Snohomish County has the 27th longest season of 39 WA counties, at 166 days against 205 in Walla Walla County and 132 in Asotin County. Its mean temperature is 49°F.
- Kittitas County: April 25 to October 9 (167 days, 0 WNV)
- Spokane County: April 26 to October 10 (167 days, 12 WNV)
- Clallam County: April 30 to October 13 (166 days, 1 WNV)
- Jefferson County: May 1 to October 13 (166 days, 0 WNV)
West Nile in Snohomish County
From 1999 to 2025, CDC ArboNET counted 1 human West Nile case in Snohomish County (1 neuroinvasive) across 1 year. The worst was 2015 with 1; the latest was 2015.
See how the dates are worked out, and when mosquitoes go away for the winter.