Mosquito season in Boulder County, CO: when it starts and ends
Mosquito months in Boulder County
In a normal year mosquito season in Boulder County, CO starts around May 25 and ends around September 25, about 18 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Boulder County, and when
- Start tipping out standing water weekly by May 11.
- Book any yard treatment for late May, ahead of the July peak.
- Repellent can go away after September 25.
- 191 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July, and check CO spray notices.
Boulder County against the rest of CO
Boulder County has the 45th longest season of 64 CO counties, at 123 days against 200 in Mesa County and 69 in Summit County. Its mean temperature is 42.2°F.
- Larimer County: May 23 to September 26 (126 days, 225 WNV)
- Pitkin County: May 23 to September 25 (126 days, 1 WNV)
- Ouray County: May 26 to September 23 (120 days, 0 WNV)
- Teller County: May 27 to September 24 (120 days, 6 WNV)
West Nile in Boulder County
From 1999 to 2025, CDC ArboNET counted 908 human West Nile cases in Boulder County (189 neuroinvasive) across 23 years. The worst was 2003 with 421; the latest was 2025.
Background: reading the West Nile counts; how the dates are worked out.