Mosquito season in King County, WA: when it starts and ends
Mosquito months in King County
In a normal year mosquito season in King County, WA starts around May 8 and ends around October 10, about 22 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in King County, and when
- Start tipping out standing water weekly by April 24.
- Book any yard treatment for early May, ahead of the August peak.
- Repellent can go away after October 10.
- 8 West Nile cases since 2016 (last 2025): you should use repellent at dusk August, more so if over 60.
King County against the rest of WA
King County has the 33rd longest season of 39 WA counties, at 155 days against 205 in Walla Walla County and 132 in Asotin County. Its mean temperature is 47.9°F.
- Skamania County: May 3 to October 9 (159 days, 0 WNV)
- Klickitat County: May 2 to October 7 (158 days, 0 WNV)
- Stevens County: April 30 to September 28 (151 days, 0 WNV)
- Pend Oreille County: May 4 to September 28 (147 days, 0 WNV)
West Nile in King County
From 1999 to 2025, CDC ArboNET counted 12 human West Nile cases in King County (6 neuroinvasive) across 10 years. The worst was 2025 with 2; the latest was 2025.
See how the dates are worked out, and reading the West Nile counts.