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

Mosquito months in Walla Walla County

In a normal year mosquito season in Walla Walla County, WA starts around April 2 and ends around October 24, about 29 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 Walla Walla County, and when

• Start tipping out standing water weekly by March 19.
• Book any yard treatment for early April, ahead of the July peak.
• Repellent can go away after October 24.
• 4 West Nile cases since 2016 (last 2023): you should use repellent at dusk July to August, more so if over 60.

Walla Walla County against the rest of WA

Walla Walla County has the 1st longest season of 39 WA counties, at 205 days against 205 in Walla Walla County and 132 in Asotin County. Its mean temperature is 54°F.

• Benton County: April 5 to October 22 (200 days, 10 WNV)
• Cowlitz County: April 11 to October 28 (200 days, 0 WNV)

West Nile in Walla Walla County

From 1999 to 2025, CDC ArboNET counted 5 human West Nile cases in Walla Walla County (5 neuroinvasive) across 4 years. The worst was 2023 with 2; the latest was 2023.

More: how the dates are worked out · reading the West Nile counts.