Mosquito season in St. Louis County, MO: when it starts and ends

Mosquito months in St. Louis County

In a normal year mosquito season in St. Louis County, MO starts around March 26 and ends around November 6, about 32 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 St. Louis County, and when

• Start tipping out standing water weekly by March 12.
• Book any yard treatment for late March, ahead of the July peak.
• Repellent can go away after November 6.
• 41 West Nile cases since 2016: you should wear EPA-registered repellent at dusk June to September, and check MO spray notices.

St. Louis County against the rest of MO

St. Louis County has the 14th longest season of 115 MO counties, at 225 days against 242 in Pemiscot County and 187 in Crawford County. Its mean temperature is 57.1°F.

• Newton County: March 27 to November 8 (226 days, 0 WNV)
• Miller County: March 26 to November 6 (225 days, 0 WNV)
• Stoddard County: March 27 to November 7 (225 days, 1 WNV)
• Jasper County: March 27 to November 5 (223 days, 0 WNV)

West Nile in St. Louis County

From 1999 to 2025, CDC ArboNET counted 176 human West Nile cases in St. Louis County (134 neuroinvasive) across 23 years. The worst was 2002 with 60; the latest was 2025.

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