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

Mosquito months in Arapahoe County

In a normal year mosquito season in Arapahoe County, CO starts around May 2 and ends around October 10, 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 Arapahoe 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.
• 113 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check CO spray notices.

Arapahoe County against the rest of CO

Arapahoe County has the 29th longest season of 64 CO counties, at 161 days against 200 in Mesa County and 69 in Summit County. Its mean temperature is 50.2°F.

• Elbert County: May 2 to October 11 (162 days, 6 WNV)
• Adams County: May 2 to October 10 (161 days, 146 WNV)
• Fremont County: May 2 to October 10 (161 days, 24 WNV)
• La Plata County: May 7 to October 10 (156 days, 28 WNV)

West Nile in Arapahoe County

From 1999 to 2025, CDC ArboNET counted 315 human West Nile cases in Arapahoe County (132 neuroinvasive) across 20 years. The worst was 2003 with 138; the latest was 2025.

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