Mosquito season in Sheridan County, MT: when it starts and ends
Mosquito months in Sheridan County
In a normal year mosquito season in Sheridan County, MT starts around May 15 and ends around September 22, about 19 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Sheridan County, and when
- Start tipping out standing water weekly by May 1.
- Book any yard treatment for mid May, ahead of the July peak.
- Repellent can go away after September 22.
- 2 West Nile cases since 2016 (last 2023): you should use repellent at dusk July, more so if over 60.
Sheridan County against the rest of MT
Sheridan County has the 39th longest season of 56 MT counties, at 130 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 39.6°F.
- Fergus County: May 20 to September 28 (132 days, 0 WNV)
- Roosevelt County: May 14 to September 22 (131 days, 1 WNV)
- Lewis and Clark County: May 20 to September 26 (128 days, 6 WNV)
- Madison County: May 22 to September 26 (127 days, 1 WNV)
West Nile in Sheridan County
From 1999 to 2025, CDC ArboNET counted 19 human West Nile cases in Sheridan County (8 neuroinvasive) across 6 years. The worst was 2003 with 11; the latest was 2023.
See how the dates are worked out, and when the season starts across the US.