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

Mosquito months in Douglas County

In a normal year mosquito season in Douglas County, CO starts around May 1 and ends around October 12, about 23 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 Douglas County, and when

• Start tipping out standing water weekly by April 17.
• Book any yard treatment for early May, ahead of the July peak.
• Repellent can go away after October 12.
• 49 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to July, and check CO spray notices.

Douglas County against the rest of CO

Douglas County has the 24th longest season of 64 CO counties, at 164 days against 200 in Mesa County and 69 in Summit County. Its mean temperature is 49.4°F.

• Dolores County: May 2 to October 15 (166 days, 0 WNV)
• Otero County: April 29 to October 11 (165 days, 12 WNV)
• Phillips County: May 1 to October 12 (164 days, 3 WNV)
• Lincoln County: May 1 to October 10 (162 days, 2 WNV)

West Nile in Douglas County

From 1999 to 2025, CDC ArboNET counted 106 human West Nile cases in Douglas County (41 neuroinvasive) across 15 years. The worst was 2003 with 39; the latest was 2025.

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