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

Mosquito months in St. Clair County

In a normal year mosquito season in St. Clair County, MO starts around April 4 and ends around November 1, about 30 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. Clair County, and when

• Start tipping out standing water weekly by March 21.
• Book any yard treatment for early April, ahead of the July peak.
• Repellent can go away after November 1.
• No West Nile cases since 2016, so bites in St. Clair County are a nuisance; check window screens.

St. Clair County against the rest of MO

St. Clair County has the 60th longest season of 115 MO counties, at 211 days against 242 in Pemiscot County and 187 in Crawford County. Its mean temperature is 56.3°F.

• Johnson County: April 3 to October 31 (211 days, 0 WNV)
• Warren County: April 3 to October 31 (211 days, 0 WNV)
• Stone County: April 4 to November 1 (211 days, 2 WNV)
• Audrain County: April 3 to October 30 (210 days, 1 WNV)

West Nile in St. Clair County

CDC ArboNET lists no human West Nile cases for St. Clair County from 1999 to 2025, though virus turned up in birds, mosquitoes or horses in 5 of those years.

Background: when the season starts across the US; how the dates are worked out.