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

Mosquito months in Jefferson County

In a normal year mosquito season in Jefferson County, OR starts around May 2 and ends around October 12, about 23 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 Jefferson County, and when

• Start tipping out standing water weekly by April 18.
• Book any yard treatment for early May, ahead of the July peak.
• Repellent can go away after October 12.
• 1 West Nile case since 2016 (last 2016): you should use repellent at dusk July, more so if over 60.

Jefferson County against the rest of OR

Jefferson County has the 24th longest season of 36 OR counties, at 163 days against 225 in Curry County and 95 in Klamath County. Its mean temperature is 49°F.

• Union County: April 26 to October 12 (169 days, 0 WNV)
• Sherman County: April 28 to October 14 (169 days, 0 WNV)
• Wheeler County: May 6 to October 11 (158 days, 0 WNV)
• Marion County: May 10 to October 12 (155 days, 0 WNV)

West Nile in Jefferson County

From 1999 to 2025, CDC ArboNET counted 1 human West Nile case in Jefferson County (1 neuroinvasive) across 1 year. The worst was 2016 with 1; the latest was 2016.

Background: when the season starts across the US; how the dates are worked out.