Mosquito season in Larimer County, CO: when it starts and ends
Mosquito months in Larimer County
In a normal year mosquito season in Larimer County, CO starts around May 23 and ends around September 26, about 18 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Larimer County, and when
- Start tipping out standing water weekly by May 9.
- Book any yard treatment for late May, ahead of the July peak.
- Repellent can go away after September 26.
- 225 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July, and check CO spray notices.
Larimer County against the rest of CO
Larimer County has the 43rd longest season of 64 CO counties, at 126 days against 200 in Mesa County and 69 in Summit County. Its mean temperature is 43.2°F.
- Archuleta County: May 23 to September 30 (130 days, 4 WNV)
- Routt County: May 20 to September 23 (126 days, 0 WNV)
- Pitkin County: May 23 to September 25 (126 days, 1 WNV)
- Boulder County: May 25 to September 25 (123 days, 191 WNV)
West Nile in Larimer County
From 1999 to 2025, CDC ArboNET counted 1,122 human West Nile cases in Larimer County (189 neuroinvasive) across 23 years. The worst was 2003 with 546; the latest was 2025.
See how the dates are worked out, and reading the West Nile counts.