Mosquito season in Jefferson County, MO: when it starts and ends
Mosquito months in Jefferson County
In a normal year mosquito season in Jefferson County, MO starts around March 31 and ends around November 3, about 31 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Jefferson County, and when
- Start tipping out standing water weekly by March 17.
- Book any yard treatment for late March, ahead of the July peak.
- Repellent can go away after November 3.
- 3 West Nile cases since 2016 (last 2025): you should use repellent at dusk June to August, more so if over 60.
Jefferson County against the rest of MO
Jefferson County has the 34th longest season of 115 MO counties, at 217 days against 242 in Pemiscot County and 187 in Crawford County. Its mean temperature is 55.8°F.
- Douglas County: March 31 to November 3 (217 days, 0 WNV)
- Franklin County: March 31 to November 3 (217 days, 0 WNV)
- Ripley County: March 31 to November 3 (217 days, 0 WNV)
- Iron County: March 31 to November 2 (216 days, 0 WNV)
West Nile in Jefferson County
From 1999 to 2025, CDC ArboNET counted 17 human West Nile cases in Jefferson County (14 neuroinvasive) across 10 years. The worst was 2003 with 3; the latest was 2025.
See how the dates are worked out, and when mosquitoes go away for the winter.