Mosquito season in Washington County, MN: when it starts and ends

Mosquito months in Washington County

In a normal year mosquito season in Washington County, MN starts around April 27 and ends around October 11, 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 Washington County, and when

• Start tipping out standing water weekly by April 13.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 11.
• 5 West Nile cases since 2016 (last 2025): you should use repellent at dusk July to July, more so if over 60.

Washington County against the rest of MN

Washington County has the 18th longest season of 87 MN counties, at 167 days against 176 in Winona County and 118 in Cook County. Its mean temperature is 44.7°F.

• Waseca County: April 26 to October 10 (167 days, 2 WNV)
• Morrison County: April 27 to October 10 (167 days, 2 WNV)
• Steele County: April 28 to October 11 (167 days, 3 WNV)
• Lyon County: April 27 to October 11 (166 days, 18 WNV)

West Nile in Washington County

From 1999 to 2025, CDC ArboNET counted 13 human West Nile cases in Washington County (5 neuroinvasive) across 11 years. The worst was 2014 with 2; the latest was 2025.

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