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

Mosquito months in Jefferson County

In a normal year mosquito season in Jefferson County, WI starts around April 25 and ends around October 16, 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 Jefferson County, and when

• Start tipping out standing water weekly by April 11.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 16.
• 7 West Nile cases since 2016 (last 2024): you should use repellent at dusk July to August, more so if over 60.

Jefferson County against the rest of WI

Jefferson County has the 9th longest season of 72 WI counties, at 174 days against 180 in La Crosse County and 139 in Rusk County. Its mean temperature is 47.2°F.

• Green County: April 23 to October 15 (175 days, 1 WNV)
• Dane County: April 24 to October 15 (174 days, 36 WNV)
• Buffalo County: April 23 to October 13 (173 days, 1 WNV)
• Grant County: April 24 to October 14 (173 days, 0 WNV)

West Nile in Jefferson County

From 1999 to 2025, CDC ArboNET counted 16 human West Nile cases in Jefferson County (9 neuroinvasive) across 10 years. The worst was 2018 with 3; the latest was 2024.

Background: reading the West Nile counts; how the dates are worked out.