Mosquito season in Custer County, MT: when it starts and ends
Mosquito months in Custer County
In a normal year mosquito season in Custer County, MT starts around May 11 and ends around September 25, about 20 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Custer County, and when
- Start tipping out standing water weekly by April 27.
- Book any yard treatment for mid May, ahead of the July peak.
- Repellent can go away after September 25.
- 6 West Nile cases since 2016 (last 2025): you should use repellent at dusk July to August, more so if over 60.
Custer County against the rest of MT
Custer County has the 32nd longest season of 56 MT counties, at 137 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 46.2°F.
- Toole County: May 11 to September 26 (138 days, 4 WNV)
- Powder River County: May 14 to September 29 (138 days, 1 WNV)
- Hill County: May 12 to September 26 (137 days, 2 WNV)
- Stillwater County: May 13 to September 26 (136 days, 1 WNV)
West Nile in Custer County
From 1999 to 2025, CDC ArboNET counted 56 human West Nile cases in Custer County (10 neuroinvasive) across 12 years. The worst was 2003 with 14; the latest was 2025.
More: how the dates are worked out · reading the West Nile counts.