Mosquito season in Portage County, WI: when it starts and ends
Mosquito months in Portage County
In a normal year mosquito season in Portage County, WI starts around May 1 and ends around October 8, about 23 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Portage 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 8.
- 1 West Nile case since 2016 (last 2025): you should use repellent at dusk July, more so if over 60.
Portage County against the rest of WI
Portage County has the 42nd longest season of 72 WI counties, at 160 days against 180 in La Crosse County and 139 in Rusk County. Its mean temperature is 44.1°F.
- Jackson County: April 30 to October 7 (160 days, 0 WNV)
- Vernon County: April 30 to October 8 (160 days, 0 WNV)
- Ozaukee County: May 6 to October 12 (160 days, 2 WNV)
- Burnett County: May 1 to October 6 (158 days, 0 WNV)
West Nile in Portage County
From 1999 to 2025, CDC ArboNET counted 4 human West Nile cases in Portage County (3 neuroinvasive) across 4 years. The worst was 2025 with 1; the latest was 2025.
Background: when mosquitoes go away for the winter; how the dates are worked out.