Mosquito season in Big Horn County, MT: when it starts and ends
Mosquito months in Big Horn County
In a normal year mosquito season in Big Horn County, MT starts around May 10 and ends around September 28, about 20 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Big Horn County, and when
- Start tipping out standing water weekly by April 26.
- Book any yard treatment for early May, ahead of the July peak.
- Repellent can go away after September 28.
- 4 West Nile cases since 2016 (last 2025): you should use repellent at dusk July to July, more so if over 60.
Big Horn County against the rest of MT
Big Horn County has the 25th longest season of 56 MT counties, at 141 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 46.8°F.
- Flathead County: May 8 to September 28 (143 days, 1 WNV)
- Chouteau County: May 9 to September 29 (143 days, 1 WNV)
- Mineral County: May 12 to September 30 (141 days, 0 WNV)
- Park County: May 14 to October 1 (140 days, 1 WNV)
West Nile in Big Horn County
From 1999 to 2025, CDC ArboNET counted 24 human West Nile cases in Big Horn County (11 neuroinvasive) across 9 years. The worst was 2003 with 12; the latest was 2025.
More: how the dates are worked out · reading the West Nile counts.