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

Mosquito months in Richland County

In a normal year mosquito season in Richland County, MT starts around May 2 and ends around October 6, 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 Richland County, and when

• Start tipping out standing water weekly by April 18.
• Book any yard treatment for early May, ahead of the July peak.
• Repellent can go away after October 6.
• 13 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check MT spray notices.

Richland County against the rest of MT

Richland County has the 6th longest season of 56 MT counties, at 157 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 44.8°F.

• Valley County: April 30 to October 5 (158 days, 12 WNV)
• Yellowstone County: April 30 to October 4 (157 days, 26 WNV)
• Rosebud County: May 3 to October 6 (156 days, 3 WNV)
• Prairie County: May 2 to October 5 (155 days, 1 WNV)

West Nile in Richland County

From 1999 to 2025, CDC ArboNET counted 36 human West Nile cases in Richland County (14 neuroinvasive) across 11 years. The worst was 2003 with 12; the latest was 2025.

Background: reading the West Nile counts; how the dates are worked out.