Mosquito season in Chelan County, WA: when it starts and ends
Mosquito months in Chelan County
In a normal year mosquito season in Chelan County, WA starts around May 11 and ends around September 30, about 20 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Chelan County, and when
- Start tipping out standing water weekly by April 27.
- Book any yard treatment for mid May, ahead of the July peak.
- Repellent can go away after September 30.
- No West Nile cases since 2016, so bites in Chelan County are a nuisance; check window screens.
Chelan County against the rest of WA
Chelan County has the 37th longest season of 39 WA counties, at 142 days against 205 in Walla Walla County and 132 in Asotin County. Its mean temperature is 43.6°F.
- Pend Oreille County: May 4 to September 28 (147 days, 0 WNV)
- Lincoln County: May 9 to October 1 (145 days, 0 WNV)
- Ferry County: May 12 to September 24 (135 days, 0 WNV)
- Asotin County: May 13 to September 22 (132 days, 0 WNV)
West Nile in Chelan County
From 1999 to 2025, CDC ArboNET counted 1 human West Nile case in Chelan County (0 neuroinvasive) across 1 year. The worst was 2010 with 1; the latest was 2010.
Background: when the season starts across the US; how the dates are worked out.