Mosquito season in Grand County, CO: when it starts and ends
Mosquito months in Grand County
In a normal year mosquito season in Grand County, CO starts around June 4 and ends around September 15, about 15 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Grand County, and when
- Start tipping out standing water weekly by May 21.
- Book any yard treatment for early June, ahead of the July peak.
- Repellent can go away after September 15.
- 3 West Nile cases since 2016 (last 2023): you should use repellent at dusk July, more so if over 60.
Grand County against the rest of CO
Grand County has the 52nd longest season of 64 CO counties, at 102 days against 200 in Mesa County and 69 in Summit County. Its mean temperature is 36.8°F.
- Moffat County: June 1 to September 19 (110 days, 0 WNV)
- Garfield County: June 4 to September 20 (108 days, 2 WNV)
- Clear Creek County: June 7 to September 14 (99 days, 2 WNV)
- Mineral County: June 10 to September 16 (98 days, 0 WNV)
West Nile in Grand County
From 1999 to 2025, CDC ArboNET counted 4 human West Nile cases in Grand County (2 neuroinvasive) across 4 years. The worst was 2023 with 1; the latest was 2023.
See how the dates are worked out, and when mosquitoes go away for the winter.