Mosquito season in St. Louis County, MN: when it starts and ends

Mosquito months in St. Louis County

In a normal year mosquito season in St. Louis County, MN starts around May 13 and ends around September 26, about 19 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. Louis County, and when

• Start tipping out standing water weekly by April 29.
• Book any yard treatment for mid May, ahead of the July peak.
• Repellent can go away after September 26.
• 7 West Nile cases since 2016 (last 2025): you should use repellent at dusk July, more so if over 60.

St. Louis County against the rest of MN

St. Louis County has the 85th longest season of 87 MN counties, at 136 days against 176 in Winona County and 118 in Cook County. Its mean temperature is 38°F.

• Carlton County: May 12 to September 30 (141 days, 0 WNV)
• Koochiching County: May 12 to September 25 (137 days, 1 WNV)
• Lake County: May 16 to September 25 (132 days, 1 WNV)
• Cook County: June 2 to September 28 (118 days, 0 WNV)

West Nile in St. Louis County

From 1999 to 2025, CDC ArboNET counted 14 human West Nile cases in St. Louis County (8 neuroinvasive) across 7 years. The worst was 2025 with 5; the latest was 2025.

More: how the dates are worked out · reading the West Nile counts.