Mosquito season in Denver County, CO: when it starts and ends
Mosquito months in Denver County
In a normal year mosquito season in Denver County, CO starts around April 22 and ends around October 18, about 26 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Denver County, and when
- Start tipping out standing water weekly by April 8.
- Book any yard treatment for late April, ahead of the July peak.
- Repellent can go away after October 18.
- 189 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check CO spray notices.
Denver County against the rest of CO
Denver County has the 8th longest season of 64 CO counties, at 179 days against 200 in Mesa County and 69 in Summit County. Its mean temperature is 51.3°F.
- Pueblo County: April 19 to October 18 (182 days, 37 WNV)
- Logan County: April 19 to October 17 (181 days, 19 WNV)
- Las Animas County: April 23 to October 18 (178 days, 2 WNV)
- Baca County: April 24 to October 19 (178 days, 3 WNV)
West Nile in Denver County
From 1999 to 2025, CDC ArboNET counted 442 human West Nile cases in Denver County (221 neuroinvasive) across 24 years. The worst was 2003 with 162; the latest was 2025.
Background: reading the West Nile counts; how the dates are worked out.