Mosquito season in Powder River County, MT: when it starts and ends
Mosquito months in Powder River County
In a normal year mosquito season in Powder River County, MT starts around May 14 and ends around September 29, about 20 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Powder River County, and when
- Start tipping out standing water weekly by April 30.
- Book any yard treatment for mid May, ahead of the July peak.
- Repellent can go away after September 29.
- 1 West Nile case since 2016 (last 2025): you should use repellent at dusk July to August, more so if over 60.
Powder River County against the rest of MT
Powder River County has the 31st longest season of 56 MT counties, at 138 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 46.3°F.
- Liberty County: May 11 to September 26 (138 days, 0 WNV)
- Toole County: May 11 to September 26 (138 days, 4 WNV)
- Custer County: May 11 to September 25 (137 days, 6 WNV)
- Hill County: May 12 to September 26 (137 days, 2 WNV)
West Nile in Powder River County
From 1999 to 2025, CDC ArboNET counted 8 human West Nile cases in Powder River County (2 neuroinvasive) across 4 years. The worst was 2003 with 4; the latest was 2025.
Background: when the season starts across the US; how the dates are worked out.