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

Mosquito months in Benton County

In a normal year mosquito season in Benton County, WA starts around April 5 and ends around October 22, 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 Benton County, and when

• Start tipping out standing water weekly by March 22.
• Book any yard treatment for early April, ahead of the July peak.
• Repellent can go away after October 22.
• 10 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check WA spray notices.

Benton County against the rest of WA

Benton County has the 2nd longest season of 39 WA counties, at 200 days against 205 in Walla Walla County and 132 in Asotin County. Its mean temperature is 52.9°F.

• Walla Walla County: April 2 to October 24 (205 days, 4 WNV)
• Cowlitz County: April 11 to October 28 (200 days, 0 WNV)
• Kitsap County: April 15 to October 25 (193 days, 0 WNV)

West Nile in Benton County

From 1999 to 2025, CDC ArboNET counted 40 human West Nile cases in Benton County (20 neuroinvasive) across 8 years. The worst was 2015 with 13; the latest was 2021.

See how the dates are worked out, and reading the West Nile counts.