Mosquito season in Jefferson County, WA: when it starts and ends
Mosquito months in Jefferson County
In a normal year mosquito season in Jefferson County, WA starts around May 1 and ends around October 13, about 24 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Jefferson County, and when
- Start tipping out standing water weekly by April 17.
- Book any yard treatment for early May, ahead of the August peak.
- Repellent can go away after October 13.
- No West Nile cases since 2016, so bites in Jefferson County are a nuisance; check window screens.
Jefferson County against the rest of WA
Jefferson County has the 29th 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.
- Snohomish County: April 29 to October 12 (166 days, 0 WNV)
- Clallam County: April 30 to October 13 (166 days, 1 WNV)
- Douglas County: April 29 to October 9 (163 days, 0 WNV)
- Skamania County: May 3 to October 9 (159 days, 0 WNV)
West Nile in Jefferson County
CDC ArboNET lists no human West Nile cases for Jefferson County from 1999 to 2025.
More: how the dates are worked out · when mosquitoes go away for the winter.