Mosquito season in Harrison County, MO: when it starts and ends

Mosquito months in Harrison County

In a normal year mosquito season in Harrison County, MO starts around April 10 and ends around October 25, about 28 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 Harrison County, and when

• Start tipping out standing water weekly by March 27.
• Book any yard treatment for early April, ahead of the July peak.
• Repellent can go away after October 25.
• 1 West Nile case since 2016 (last 2025): you should use repellent at dusk June to August, more so if over 60.

Harrison County against the rest of MO

Harrison County has the 101st longest season of 115 MO counties, at 197 days against 242 in Pemiscot County and 187 in Crawford County. Its mean temperature is 51.8°F.

• Gentry County: April 11 to October 25 (198 days, 0 WNV)
• Daviess County: April 10 to October 24 (197 days, 0 WNV)
• Clark County: April 11 to October 24 (196 days, 0 WNV)
• DeKalb County: April 11 to October 24 (196 days, 0 WNV)

West Nile in Harrison County

From 1999 to 2025, CDC ArboNET counted 2 human West Nile cases in Harrison County (1 neuroinvasive) across 2 years. The worst was 2025 with 1; the latest was 2025.

Background: when mosquitoes go away for the winter; how the dates are worked out.