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

Mosquito months in Walworth County

In a normal year mosquito season in Walworth County, WI starts around April 23 and ends around October 17, about 25 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 Walworth County, and when

• Start tipping out standing water weekly by April 9.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 17.
• 4 West Nile cases since 2016 (last 2025): you should use repellent at dusk July to August, more so if over 60.

Walworth County against the rest of WI

Walworth County has the 3rd longest season of 72 WI counties, at 177 days against 180 in La Crosse County and 139 in Rusk County. Its mean temperature is 48.2°F.

• La Crosse County: April 20 to October 17 (180 days, 3 WNV)
• Kenosha County: April 25 to October 21 (179 days, 3 WNV)
• Rock County: April 23 to October 16 (176 days, 6 WNV)
• Milwaukee County: April 28 to October 21 (176 days, 29 WNV)

West Nile in Walworth County

From 1999 to 2025, CDC ArboNET counted 13 human West Nile cases in Walworth County (10 neuroinvasive) across 9 years. The worst was 2013 with 4; the latest was 2025.

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