Mosquito season in Grant County, WA: when it starts and ends
Mosquito months in Grant County
In a normal year mosquito season in Grant County, WA starts around April 12 and ends around October 16, about 27 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Grant County, and when
- Start tipping out standing water weekly by March 29.
- Book any yard treatment for mid April, ahead of the July peak.
- Repellent can go away after October 16.
- No West Nile cases since 2016, so bites in Grant County are a nuisance; check window screens.
Grant County against the rest of WA
Grant County has the 7th longest season of 39 WA counties, at 187 days against 205 in Walla Walla County and 132 in Asotin County. Its mean temperature is 51.2°F.
- Franklin County: April 10 to October 18 (191 days, 1 WNV)
- Yakima County: April 12 to October 17 (188 days, 4 WNV)
- Wahkiakum County: April 20 to October 24 (187 days, 0 WNV)
- Island County: April 17 to October 20 (186 days, 0 WNV)
West Nile in Grant County
From 1999 to 2025, CDC ArboNET counted 5 human West Nile cases in Grant County (1 neuroinvasive) across 3 years. The worst was 2015 with 3; the latest was 2015.
Background: when mosquitoes go away for the winter; how the dates are worked out.