Mosquito season in Clay County, SD: when it starts and ends

Mosquito months in Clay County

In a normal year mosquito season in Clay County, SD starts around April 24 and ends around October 11, about 24 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 Clay County, and when

• Start tipping out standing water weekly by April 10.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 11.
• 8 West Nile cases since 2016 (last 2025): you should use repellent at dusk June to August, more so if over 60.

Clay County against the rest of SD

Clay County has the 12th longest season of 66 SD counties, at 170 days against 179 in Yankton County and 139 in Lawrence County. Its mean temperature is 47.6°F.

• Minnehaha County: April 24 to October 13 (172 days, 86 WNV)
• Brule County: April 25 to October 14 (171 days, 4 WNV)
• Lincoln County: April 24 to October 11 (170 days, 39 WNV)
• Gregory County: April 26 to October 13 (170 days, 3 WNV)

West Nile in Clay County

From 1999 to 2025, CDC ArboNET counted 44 human West Nile cases in Clay County (9 neuroinvasive) across 12 years. The worst was 2003 with 20; the latest was 2025.

See how the dates are worked out, and reading the West Nile counts.