Mosquito season in Columbia County, WA: when it starts and ends
Mosquito months in Columbia County
In a normal year mosquito season in Columbia County, WA starts around April 18 and ends around October 16, about 26 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Columbia County, and when
- Start tipping out standing water weekly by April 4.
- 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 Columbia County are a nuisance; check window screens.
Columbia County against the rest of WA
Columbia County has the 12th longest season of 39 WA counties, at 182 days against 205 in Walla Walla County and 132 in Asotin County. Its mean temperature is 51.1°F.
- San Juan County: April 20 to October 22 (185 days, 0 WNV)
- Mason County: April 20 to October 20 (183 days, 0 WNV)
- Clark County: April 21 to October 20 (182 days, 2 WNV)
- Lewis County: April 21 to October 18 (181 days, 0 WNV)
West Nile in Columbia County
CDC ArboNET lists no human West Nile cases for Columbia County from 1999 to 2025, though virus turned up in birds, mosquitoes or horses in 1 of those years.
More: how the dates are worked out · when mosquitoes go away for the winter.