Mosquito season in Sweet Grass County, MT: when it starts and ends
Mosquito months in Sweet Grass County
In a normal year mosquito season in Sweet Grass County, MT starts around May 3 and ends around October 5, about 22 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Sweet Grass County, and when
- Start tipping out standing water weekly by April 19.
- Book any yard treatment for early May, ahead of the July peak.
- Repellent can go away after October 5.
- 1 West Nile case since 2016 (last 2025): you should use repellent at dusk July, more so if over 60.
Sweet Grass County against the rest of MT
Sweet Grass County has the 9th longest season of 56 MT counties, at 155 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 47°F.
- Rosebud County: May 3 to October 6 (156 days, 3 WNV)
- Prairie County: May 2 to October 5 (155 days, 1 WNV)
- Lake County: May 4 to October 4 (153 days, 2 WNV)
- Fallon County: May 5 to October 3 (151 days, 0 WNV)
West Nile in Sweet Grass County
From 1999 to 2025, CDC ArboNET counted 2 human West Nile cases in Sweet Grass County (1 neuroinvasive) across 2 years. The worst was 2025 with 1; the latest was 2025.
More: how the dates are worked out · when mosquitoes go away for the winter.