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

Mosquito months in Granite County

In a normal year mosquito season in Granite County, MT starts around May 29 and ends around September 12, about 15 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 Granite County, and when

• Start tipping out standing water weekly by May 15.
• Book any yard treatment for late May, ahead of the July peak.
• Repellent can go away after September 12.
• No West Nile cases since 2016, so bites in Granite County are a nuisance; check window screens.

Granite County against the rest of MT

Granite County has the 54th longest season of 56 MT counties, at 106 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 42.1°F.

• Powell County: May 26 to September 11 (108 days, 0 WNV)
• Beaverhead County: May 30 to September 14 (107 days, 0 WNV)
• Sanders County: June 9 to September 18 (101 days, 1 WNV)
• Gallatin County: June 11 to September 8 (89 days, 1 WNV)

West Nile in Granite County

CDC ArboNET lists no human West Nile cases for Granite County from 1999 to 2025.

See how the dates are worked out, and when the season starts across the US.