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

Mosquito months in Cowlitz County

In a normal year mosquito season in Cowlitz County, WA starts around April 11 and ends around October 28, about 29 weeks.

What to do in Cowlitz County, and when

Start tipping out standing water weekly by March 28.
Book any yard treatment for mid April, ahead of the August peak.
Repellent can go away after October 28.
No West Nile cases since 2016, so bites in Cowlitz County are a nuisance; check window screens.

Cowlitz County against the rest of WA

Cowlitz County has the 3rd longest season of 39 WA counties, at 200 days against 205 in Walla Walla County and 132 in Asotin County. Its mean temperature is 52.7°F.

Walla Walla County: April 2 to October 24 (205 days, 4 WNV)
Benton County: April 5 to October 22 (200 days, 10 WNV)
Kitsap County: April 15 to October 25 (193 days, 0 WNV)
Franklin County: April 10 to October 18 (191 days, 1 WNV)

West Nile in Cowlitz County

CDC ArboNET lists no human West Nile cases for Cowlitz County from 1999 to 2025, though virus turned up in birds, mosquitoes or horses in 1 of those years.

See how the dates are worked out, and when mosquitoes go away for the winter.