Mosquito season in Roosevelt County, MT: when it starts and ends
Mosquito months in Roosevelt County
In a normal year mosquito season in Roosevelt County, MT starts around May 14 and ends around September 22, about 19 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Roosevelt 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 22.
- 1 West Nile case since 2016 (last 2018): you should use repellent at dusk July, more so if over 60.
Roosevelt County against the rest of MT
Roosevelt County has the 38th longest season of 56 MT counties, at 131 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 40.6°F.
- Wibaux County: May 17 to September 26 (132 days, 0 WNV)
- Fergus County: May 20 to September 28 (132 days, 0 WNV)
- Sheridan County: May 15 to September 22 (130 days, 2 WNV)
- Lewis and Clark County: May 20 to September 26 (128 days, 6 WNV)
West Nile in Roosevelt County
From 1999 to 2025, CDC ArboNET counted 44 human West Nile cases in Roosevelt County (12 neuroinvasive) across 6 years. The worst was 2003 with 24; the latest was 2018.
Background: when mosquitoes go away for the winter; how the dates are worked out.