Mosquito season in Lewis and Clark County, MT: when it starts and ends
Mosquito months in Lewis and Clark County
In a normal year mosquito season in Lewis and Clark County, MT starts around May 20 and ends around September 26, about 18 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Lewis and Clark County, and when
- Start tipping out standing water weekly by May 6.
- Book any yard treatment for mid May, ahead of the July peak.
- Repellent can go away after September 26.
- 6 West Nile cases since 2016 (last 2023): you should use repellent at dusk July, more so if over 60.
Lewis and Clark County against the rest of MT
Lewis and Clark County has the 40th longest season of 56 MT counties, at 128 days against 163 in Dawson County and 89 in Gallatin County. Its mean temperature is 42.4°F.
- Roosevelt County: May 14 to September 22 (131 days, 1 WNV)
- Sheridan County: May 15 to September 22 (130 days, 2 WNV)
- Madison County: May 22 to September 26 (127 days, 1 WNV)
- Meagher County: May 22 to September 25 (126 days, 0 WNV)
West Nile in Lewis and Clark County
From 1999 to 2025, CDC ArboNET counted 16 human West Nile cases in Lewis and Clark County (4 neuroinvasive) across 7 years. The worst was 2007 with 9; the latest was 2023.
Background: reading the West Nile counts; how the dates are worked out.