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

Mosquito months in Washington County

In a normal year mosquito season in Washington County, CO starts around April 27 and ends around October 16, about 25 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 Washington County, and when

• Start tipping out standing water weekly by April 13.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 16.
• 6 West Nile cases since 2016 (last 2025): you should use repellent at dusk July to August, more so if over 60.

Washington County against the rest of CO

Washington County has the 15th longest season of 64 CO counties, at 172 days against 200 in Mesa County and 69 in Summit County. Its mean temperature is 50.1°F.

• El Paso County: April 24 to October 15 (174 days, 66 WNV)
• Delta County: April 25 to October 16 (174 days, 111 WNV)
• Morgan County: April 27 to October 15 (171 days, 23 WNV)
• Sedgwick County: April 27 to October 14 (170 days, 4 WNV)

West Nile in Washington County

From 1999 to 2025, CDC ArboNET counted 23 human West Nile cases in Washington County (9 neuroinvasive) across 9 years. The worst was 2003 with 10; the latest was 2025.

Background: reading the West Nile counts; how the dates are worked out.