Mosquito season in Multnomah County, OR: when it starts and ends

Mosquito months in Multnomah County

In a normal year mosquito season in Multnomah County, OR starts around March 30 and ends around November 3, about 31 weeks.

1. Jan
2. Feb
3. Mar
4. Apr
5. May
6. Jun
7. Jul
8. Aug
9. Sep
10. Oct
11. Nov
12. Dec

What to do in Multnomah County, and when

• Start tipping out standing water weekly by March 16.
• Book any yard treatment for late March, ahead of the August peak.
• Repellent can go away after November 3.
• 1 West Nile case since 2016 (last 2023): you should use repellent at dusk August, more so if over 60.

Multnomah County against the rest of OR

Multnomah County has the 2nd longest season of 36 OR counties, at 218 days against 225 in Curry County and 95 in Klamath County. Its mean temperature is 54.4°F.

• Curry County: March 27 to November 8 (225 days, 0 WNV)
• Coos County: April 12 to November 10 (212 days, 0 WNV)
• Josephine County: April 4 to November 1 (211 days, 0 WNV)

West Nile in Multnomah County

From 1999 to 2025, CDC ArboNET counted 2 human West Nile cases in Multnomah County (0 neuroinvasive) across 2 years. The worst was 2023 with 1; the latest was 2023.

Background: when mosquitoes go away for the winter; how the dates are worked out.