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

Mosquito months in San Juan County

In a normal year mosquito season in San Juan County, WA starts around April 20 and ends around October 22, about 26 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 San Juan County, and when

• Start tipping out standing water weekly by April 6.
• Book any yard treatment for mid April, ahead of the August peak.
• Repellent can go away after October 22.
• No West Nile cases since 2016, so bites in San Juan County are a nuisance; check window screens.

San Juan County against the rest of WA

San Juan County has the 10th longest season of 39 WA counties, at 185 days against 205 in Walla Walla County and 132 in Asotin County. Its mean temperature is 50.9°F.

• Wahkiakum County: April 20 to October 24 (187 days, 0 WNV)
• Island County: April 17 to October 20 (186 days, 0 WNV)
• Mason County: April 20 to October 20 (183 days, 0 WNV)
• Columbia County: April 18 to October 16 (182 days, 0 WNV)

West Nile in San Juan County

CDC ArboNET lists no human West Nile cases for San Juan County from 1999 to 2025.

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