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

Mosquito months in Olmsted County

In a normal year mosquito season in Olmsted County, MN starts around April 28 and ends around October 9, 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 Olmsted County, and when

• Start tipping out standing water weekly by April 14.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 9.
• 2 West Nile cases since 2016 (last 2023): you should use repellent at dusk July to July, more so if over 60.

Olmsted County against the rest of MN

Olmsted County has the 31st longest season of 87 MN counties, at 164 days against 176 in Winona County and 118 in Cook County. Its mean temperature is 44.3°F.

• Big Stone County: April 28 to October 9 (164 days, 5 WNV)
• Le Sueur County: April 28 to October 9 (164 days, 4 WNV)
• Pipestone County: April 28 to October 9 (164 days, 4 WNV)
• Swift County: April 28 to October 9 (164 days, 2 WNV)

West Nile in Olmsted County

From 1999 to 2025, CDC ArboNET counted 6 human West Nile cases in Olmsted County (2 neuroinvasive) across 5 years. The worst was 2003 with 2; the latest was 2023.

See how the dates are worked out, and when the season starts across the US.