Mosquito season in Kittitas County, WA: when it starts and ends

Mosquito months in Kittitas County

In a normal year mosquito season in Kittitas County, WA starts around April 25 and ends around October 9, about 24 weeks.

1. Jan
2. Feb
3. Mar
4. Apr
5. May
6. Jun
7. Jul
8. Aug
9. Sep
10. Oct
11. Nov
12. Dec

What to do in Kittitas County, and when

• Start tipping out standing water weekly by April 11.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 9.
• No West Nile cases since 2016, so bites in Kittitas County are a nuisance; check window screens.

Kittitas County against the rest of WA

Kittitas County has the 25th longest season of 39 WA counties, at 167 days against 205 in Walla Walla County and 132 in Asotin County. Its mean temperature is 48.2°F.

• Whitman County: April 26 to October 11 (168 days, 1 WNV)
• Okanogan County: April 24 to October 8 (167 days, 1 WNV)
• Spokane County: April 26 to October 10 (167 days, 12 WNV)
• Snohomish County: April 29 to October 12 (166 days, 0 WNV)

West Nile in Kittitas County

CDC ArboNET lists no human West Nile cases for Kittitas County from 1999 to 2025, though virus turned up in birds, mosquitoes or horses in 7 of those years.

Background: when the season starts across the US; how the dates are worked out.