Mosquito season in St. Charles County, MO: when it starts and ends
Mosquito months in St. Charles County
In a normal year mosquito season in St. Charles County, MO starts around March 31 and ends around November 2, about 31 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in St. Charles 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 2.
- 9 West Nile cases since 2016 (last 2025): you should use repellent at dusk June to August, more so if over 60.
St. Charles County against the rest of MO
St. Charles County has the 37th longest season of 115 MO counties, at 216 days against 242 in Pemiscot County and 187 in Crawford County. Its mean temperature is 55.4°F.
- Ripley County: March 31 to November 3 (217 days, 0 WNV)
- Iron County: March 31 to November 2 (216 days, 0 WNV)
- Ozark County: April 2 to November 4 (216 days, 0 WNV)
- Barry County: April 3 to November 5 (216 days, 0 WNV)
West Nile in St. Charles County
From 1999 to 2025, CDC ArboNET counted 29 human West Nile cases in St. Charles County (24 neuroinvasive) across 17 years. The worst was 2002 with 7; the latest was 2025.
More: how the dates are worked out · reading the West Nile counts.