Mosquito season in Chouteau County, MT: when it starts and ends

Mosquito months in Chouteau County

In a normal year mosquito season in Chouteau County, MT starts around May 9 and ends around September 29, about 20 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 Chouteau County, and when

• Start tipping out standing water weekly by April 25.
• Book any yard treatment for early May, ahead of the July peak.
• Repellent can go away after September 29.
• 1 West Nile case since 2016 (last 2018): you should use repellent at dusk July to July, more so if over 60.

Chouteau County against the rest of MT

Chouteau County has the 24th longest season of 56 MT counties, at 143 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 45°F.

• Garfield County: May 8 to September 29 (144 days, 1 WNV)
• Flathead County: May 8 to September 28 (143 days, 1 WNV)
• Big Horn County: May 10 to September 28 (141 days, 4 WNV)
• Mineral County: May 12 to September 30 (141 days, 0 WNV)

West Nile in Chouteau County

From 1999 to 2025, CDC ArboNET counted 12 human West Nile cases in Chouteau County (3 neuroinvasive) across 4 years. The worst was 2007 with 8; the latest was 2018.

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