Mosquito season in Chaffee County, CO: when it starts and ends

Mosquito months in Chaffee County

In a normal year mosquito season in Chaffee County, CO starts around June 9 and ends around September 8, about 13 weeks.

1. Jan
2. Feb
3. Mar
4. Apr
5. May
6. Jun
7. Jul
8. Aug
9. Sep
10. Oct
11. Nov
12. Dec

What to do in Chaffee County, and when

• Start tipping out standing water weekly by May 26.
• Book any yard treatment for early June, ahead of the July peak.
• Repellent can go away after September 8.
• 11 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July, and check CO spray notices.

Chaffee County against the rest of CO

Chaffee County has the 58th longest season of 64 CO counties, at 91 days against 200 in Mesa County and 69 in Summit County. Its mean temperature is 35.5°F.

• San Miguel County: June 15 to September 16 (93 days, 2 WNV)
• Jackson County: June 8 to September 8 (92 days, 0 WNV)
• Gilpin County: June 12 to September 7 (87 days, 0 WNV)
• Gunnison County: June 14 to September 8 (86 days, 1 WNV)

West Nile in Chaffee County

From 1999 to 2025, CDC ArboNET counted 18 human West Nile cases in Chaffee County (5 neuroinvasive) across 3 years. The worst was 2023 with 10; the latest was 2023.

See how the dates are worked out, and reading the West Nile counts.