Mosquito season in St. Croix County, WI: when it starts and ends

Mosquito months in St. Croix County

In a normal year mosquito season in St. Croix County, WI starts around May 2 and ends around October 7, about 23 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 St. Croix County, and when

• Start tipping out standing water weekly by April 18.
• Book any yard treatment for early May, ahead of the July peak.
• Repellent can go away after October 7.
• 4 West Nile cases since 2016 (last 2025): you should use repellent at dusk July, more so if over 60.

St. Croix County against the rest of WI

St. Croix County has the 45th longest season of 72 WI counties, at 158 days against 180 in La Crosse County and 139 in Rusk County. Its mean temperature is 42.8°F.

• Ozaukee County: May 6 to October 12 (160 days, 2 WNV)
• Burnett County: May 1 to October 6 (158 days, 0 WNV)
• Barron County: May 2 to October 6 (157 days, 2 WNV)
• Marathon County: May 2 to October 6 (157 days, 3 WNV)

West Nile in St. Croix County

From 1999 to 2025, CDC ArboNET counted 5 human West Nile cases in St. Croix County (4 neuroinvasive) across 3 years. The worst was 2025 with 3; the latest was 2025.

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