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

Mosquito months in Adams County

In a normal year mosquito season in Adams 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 Adams 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.
• 146 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check CO spray notices.

Adams County against the rest of CO

Adams County has the 28th longest season of 64 CO counties, at 161 days against 200 in Mesa County and 69 in Summit County. Its mean temperature is 49.9°F.

• Lincoln County: May 1 to October 10 (162 days, 2 WNV)
• Elbert County: May 2 to October 11 (162 days, 6 WNV)
• Arapahoe County: May 2 to October 10 (161 days, 113 WNV)
• Fremont County: May 2 to October 10 (161 days, 24 WNV)

West Nile in Adams County

From 1999 to 2025, CDC ArboNET counted 511 human West Nile cases in Adams County (208 neuroinvasive) across 22 years. The worst was 2003 with 232; the latest was 2025.

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