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

Mosquito months in Stillwater County

In a normal year mosquito season in Stillwater County, MT 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 Stillwater 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.
• 1 West Nile case since 2016 (last 2017): you should use repellent at dusk July, more so if over 60.

Stillwater County against the rest of MT

Stillwater County has the 34th longest season of 56 MT counties, at 136 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 44.3°F.

• Custer County: May 11 to September 25 (137 days, 6 WNV)
• Hill County: May 12 to September 26 (137 days, 2 WNV)
• Wheatland County: May 16 to September 27 (134 days, 0 WNV)
• Wibaux County: May 17 to September 26 (132 days, 0 WNV)

West Nile in Stillwater County

From 1999 to 2025, CDC ArboNET counted 14 human West Nile cases in Stillwater County (6 neuroinvasive) across 5 years. The worst was 2007 with 5; the latest was 2017.

See how the dates are worked out, and when mosquitoes go away for the winter.