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

Mosquito months in Waukesha County

In a normal year mosquito season in Waukesha County, WI starts around April 28 and ends around October 15, 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 Waukesha County, and when

• Start tipping out standing water weekly by April 14.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 15.
• 15 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to July, and check WI spray notices.

Waukesha County against the rest of WI

Waukesha County has the 18th longest season of 72 WI counties, at 170 days against 180 in La Crosse County and 139 in Rusk County. Its mean temperature is 46.5°F.

• Fond du Lac County: April 27 to October 14 (171 days, 4 WNV)
• Winnebago County: April 27 to October 14 (170 days, 2 WNV)
• Marquette County: April 27 to October 12 (168 days, 0 WNV)
• Richland County: April 27 to October 12 (168 days, 0 WNV)

West Nile in Waukesha County

From 1999 to 2025, CDC ArboNET counted 29 human West Nile cases in Waukesha County (20 neuroinvasive) across 9 years. The worst was 2012 with 7; the latest was 2025.

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