Mosquito season in Washington County, OR: when it starts and ends
Mosquito months in Washington County
In a normal year mosquito season in Washington County, OR starts around April 16 and ends around October 24, about 27 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Washington County, and when
- Start tipping out standing water weekly by April 2.
- Book any yard treatment for mid April, ahead of the August peak.
- Repellent can go away after October 24.
- No West Nile cases since 2016, so bites in Washington County are a nuisance; check window screens.
Washington County against the rest of OR
Washington County has the 10th longest season of 36 OR counties, at 191 days against 225 in Curry County and 95 in Klamath County. Its mean temperature is 52.2°F.
- Lincoln County: April 27 to November 8 (195 days, 0 WNV)
- Clatsop County: April 22 to November 1 (193 days, 0 WNV)
- Umatilla County: April 14 to October 19 (188 days, 0 WNV)
- Polk County: April 22 to October 26 (186 days, 0 WNV)
West Nile in Washington County
CDC ArboNET lists no human West Nile cases for Washington County from 1999 to 2025, though virus turned up in birds, mosquitoes or horses in 2 of those years.
Background: when the season starts across the US; how the dates are worked out.