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

Mosquito months in Yellowstone County

In a normal year mosquito season in Yellowstone County, MT starts around April 30 and ends around October 4, about 22 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 Yellowstone County, and when

• Start tipping out standing water weekly by April 16.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 4.
• 26 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to July, and check MT spray notices.

Yellowstone County against the rest of MT

Yellowstone County has the 5th longest season of 56 MT counties, at 157 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 46.1°F.

• Phillips County: April 30 to October 6 (159 days, 4 WNV)
• Valley County: April 30 to October 5 (158 days, 12 WNV)
• Richland County: May 2 to October 6 (157 days, 13 WNV)
• Rosebud County: May 3 to October 6 (156 days, 3 WNV)

West Nile in Yellowstone County

From 1999 to 2025, CDC ArboNET counted 113 human West Nile cases in Yellowstone County (48 neuroinvasive) across 12 years. The worst was 2003 with 36; the latest was 2025.

More: how the dates are worked out · reading the West Nile counts.