Mosquito season in Clark County, WA: when it starts and ends
Mosquito months in Clark County
In a normal year mosquito season in Clark County, WA starts around April 21 and ends around October 20, about 26 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Clark County, and when
- Start tipping out standing water weekly by April 7.
- Book any yard treatment for late April, ahead of the August peak.
- Repellent can go away after October 20.
- 2 West Nile cases since 2016 (last 2023): you should use repellent at dusk August, more so if over 60.
Clark County against the rest of WA
Clark County has the 13th longest season of 39 WA counties, at 182 days against 205 in Walla Walla County and 132 in Asotin County. Its mean temperature is 50.9°F.
- Mason County: April 20 to October 20 (183 days, 0 WNV)
- Columbia County: April 18 to October 16 (182 days, 0 WNV)
- Lewis County: April 21 to October 18 (181 days, 0 WNV)
- Pacific County: April 24 to October 20 (179 days, 0 WNV)
West Nile in Clark County
From 1999 to 2025, CDC ArboNET counted 4 human West Nile cases in Clark County (3 neuroinvasive) across 4 years. The worst was 2023 with 1; the latest was 2023.
Background: when the season starts across the US; how the dates are worked out.