Mosquito season in Washington County, WI: when it starts and ends

Mosquito months in Washington County

In a normal year mosquito season in Washington County, WI starts around May 1 and ends around October 12, about 24 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 Washington County, and when

• Start tipping out standing water weekly by April 17.
• Book any yard treatment for early May, ahead of the July peak.
• Repellent can go away after October 12.
• 2 West Nile cases since 2016 (last 2024): you should use repellent at dusk July to July, more so if over 60.

Washington County against the rest of WI

Washington County has the 26th longest season of 72 WI counties, at 165 days against 180 in La Crosse County and 139 in Rusk County. Its mean temperature is 45.6°F.

• Green Lake County: April 29 to October 11 (165 days, 0 WNV)
• Calumet County: April 30 to October 12 (165 days, 3 WNV)
• Adams County: April 29 to October 10 (164 days, 1 WNV)
• Outagamie County: April 29 to October 11 (164 days, 5 WNV)

West Nile in Washington County

From 1999 to 2025, CDC ArboNET counted 5 human West Nile cases in Washington County (2 neuroinvasive) across 4 years. The worst was 2024 with 2; the latest was 2024.

More: how the dates are worked out · when mosquitoes go away for the winter.