Mosquito season in Fremont County, CO: when it starts and ends
Mosquito months in Fremont County
In a normal year mosquito season in Fremont County, CO starts around May 2 and ends around October 10, about 23 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Fremont County, and when
- Start tipping out standing water weekly by April 18.
- Book any yard treatment for early May, ahead of the July peak.
- Repellent can go away after October 10.
- 24 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July, and check CO spray notices.
Fremont County against the rest of CO
Fremont County has the 30th longest season of 64 CO counties, at 161 days against 200 in Mesa County and 69 in Summit County. Its mean temperature is 47.5°F.
- Adams County: May 2 to October 10 (161 days, 146 WNV)
- Arapahoe County: May 2 to October 10 (161 days, 113 WNV)
- La Plata County: May 7 to October 10 (156 days, 28 WNV)
- Weld County: May 3 to October 5 (155 days, 228 WNV)
West Nile in Fremont County
From 1999 to 2025, CDC ArboNET counted 135 human West Nile cases in Fremont County (32 neuroinvasive) across 14 years. The worst was 2003 with 77; the latest was 2025.
More: how the dates are worked out · reading the West Nile counts.