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

Mosquito months in Gallatin County

In a normal year mosquito season in Gallatin County, MT starts around June 11 and ends around September 8, about 13 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 Gallatin County, and when

• Start tipping out standing water weekly by May 28.
• Book any yard treatment for mid June, ahead of the July peak.
• Repellent can go away after September 8.
• 1 West Nile case since 2016 (last 2017): you should use repellent at dusk July, more so if over 60.

Gallatin County against the rest of MT

Gallatin County has the 56th longest season of 56 MT counties, at 89 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 39.7°F.

• Granite County: May 29 to September 12 (106 days, 0 WNV)
• Sanders County: June 9 to September 18 (101 days, 1 WNV)

West Nile in Gallatin County

From 1999 to 2025, CDC ArboNET counted 5 human West Nile cases in Gallatin County (2 neuroinvasive) across 3 years. The worst was 2006 with 3; the latest was 2017.

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