Mosquito season in Valley County, MT: when it starts and ends
Mosquito months in Valley County
In a normal year mosquito season in Valley County, MT starts around April 30 and ends around October 5, about 23 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Valley 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 5.
- 12 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check MT spray notices.
Valley County against the rest of MT
Valley County has the 4th longest season of 56 MT counties, at 158 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 43.7°F.
- Treasure County: April 30 to October 7 (160 days, 0 WNV)
- Phillips County: April 30 to October 6 (159 days, 4 WNV)
- Yellowstone County: April 30 to October 4 (157 days, 26 WNV)
- Richland County: May 2 to October 6 (157 days, 13 WNV)
West Nile in Valley County
From 1999 to 2025, CDC ArboNET counted 52 human West Nile cases in Valley County (17 neuroinvasive) across 9 years. The worst was 2003 with 17; the latest was 2024.
Background: reading the West Nile counts; how the dates are worked out.