Mosquito season in Flathead County, MT: when it starts and ends
Mosquito months in Flathead County
In a normal year mosquito season in Flathead County, MT starts around May 8 and ends around September 28, about 20 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Flathead County, and when
- Start tipping out standing water weekly by April 24.
- Book any yard treatment for early May, ahead of the July peak.
- Repellent can go away after September 28.
- 1 West Nile case since 2016 (last 2023): you should use repellent at dusk July, more so if over 60.
Flathead County against the rest of MT
Flathead County has the 23rd longest season of 56 MT counties, at 143 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 44°F.
- Pondera County: May 10 to October 2 (145 days, 1 WNV)
- Garfield County: May 8 to September 29 (144 days, 1 WNV)
- Chouteau County: May 9 to September 29 (143 days, 1 WNV)
- Big Horn County: May 10 to September 28 (141 days, 4 WNV)
West Nile in Flathead County
From 1999 to 2025, CDC ArboNET counted 3 human West Nile cases in Flathead County (1 neuroinvasive) across 2 years. The worst was 2006 with 2; the latest was 2023.
Background: when mosquitoes go away for the winter; how the dates are worked out.