Mosquito season in Jefferson County, CO: when it starts and ends
Mosquito months in Jefferson County
In a normal year mosquito season in Jefferson County, CO starts around May 13 and ends around October 2, about 20 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Jefferson County, and when
- Start tipping out standing water weekly by April 29.
- Book any yard treatment for mid May, ahead of the July peak.
- Repellent can go away after October 2.
- 132 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July, and check CO spray notices.
Jefferson County against the rest of CO
Jefferson County has the 35th longest season of 64 CO counties, at 142 days against 200 in Mesa County and 69 in Summit County. Its mean temperature is 45.5°F.
- Montrose County: May 11 to October 4 (146 days, 81 WNV)
- Rio Blanco County: May 12 to October 1 (142 days, 4 WNV)
- Costilla County: May 13 to September 30 (140 days, 1 WNV)
- Conejos County: May 13 to September 29 (139 days, 0 WNV)
West Nile in Jefferson County
From 1999 to 2025, CDC ArboNET counted 378 human West Nile cases in Jefferson County (149 neuroinvasive) across 23 years. The worst was 2003 with 157; the latest was 2025.
More: how the dates are worked out · reading the West Nile counts.