Mosquito season in Weld County, CO: when it starts and ends
Mosquito months in Weld County
In a normal year mosquito season in Weld County, CO starts around May 3 and ends around October 5, about 22 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Weld County, and when
- Start tipping out standing water weekly by April 19.
- Book any yard treatment for early May, ahead of the July peak.
- Repellent can go away after October 5.
- 228 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check CO spray notices.
Weld County against the rest of CO
Weld County has the 32nd longest season of 64 CO counties, at 155 days against 200 in Mesa County and 69 in Summit County. Its mean temperature is 48.3°F.
- Fremont County: May 2 to October 10 (161 days, 24 WNV)
- La Plata County: May 7 to October 10 (156 days, 28 WNV)
- Montrose County: May 11 to October 4 (146 days, 81 WNV)
- Rio Blanco County: May 12 to October 1 (142 days, 4 WNV)
West Nile in Weld County
From 1999 to 2025, CDC ArboNET counted 957 human West Nile cases in Weld County (311 neuroinvasive) across 22 years. The worst was 2003 with 402; the latest was 2025.
More: how the dates are worked out · reading the West Nile counts.