Mosquito season in Jefferson County, MT: when it starts and ends
Mosquito months in Jefferson County
In a normal year mosquito season in Jefferson County, MT starts around May 22 and ends around September 21, about 17 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Jefferson County, and when
- Start tipping out standing water weekly by May 8.
- Book any yard treatment for late May, ahead of the July peak.
- Repellent can go away after September 21.
- 1 West Nile case since 2016 (last 2025): you should use repellent at dusk July, more so if over 60.
Jefferson County against the rest of MT
Jefferson County has the 45th longest season of 56 MT counties, at 122 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 41.3°F.
- Ravalli County: May 22 to September 24 (125 days, 1 WNV)
- Lincoln County: May 16 to September 15 (122 days, 0 WNV)
- Missoula County: May 23 to September 22 (122 days, 0 WNV)
- Deer Lodge County: May 25 to September 23 (121 days, 0 WNV)
West Nile in Jefferson County
From 1999 to 2025, CDC ArboNET counted 1 human West Nile case in Jefferson County (0 neuroinvasive) across 1 year. The worst was 2025 with 1; the latest was 2025.
More: how the dates are worked out · when the season starts across the US.