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

Mosquito months in Hamlin County

In a normal year mosquito season in Hamlin County, SD starts around May 3 and ends around October 5, about 22 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 Hamlin County, and when

• Start tipping out standing water weekly by April 19.
• Book any yard treatment for early May, ahead of the July peak.
• Repellent can go away after October 5.
• 12 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to July, and check SD spray notices.

Hamlin County against the rest of SD

Hamlin County has the 58th longest season of 66 SD counties, at 155 days against 179 in Yankton County and 139 in Lawrence County. Its mean temperature is 42.5°F.

• Edmunds County: May 2 to October 5 (156 days, 10 WNV)
• Deuel County: May 3 to October 5 (155 days, 5 WNV)
• Lyman County: May 3 to October 5 (155 days, 4 WNV)
• Fall River County: May 5 to October 7 (155 days, 1 WNV)

West Nile in Hamlin County

From 1999 to 2025, CDC ArboNET counted 32 human West Nile cases in Hamlin County (10 neuroinvasive) across 16 years. The worst was 2016 with 4; the latest was 2025.

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