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

Mosquito months in Sully County

In a normal year mosquito season in Sully County, SD starts around April 29 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 Sully County, and when

• Start tipping out standing water weekly by April 15.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 11.
• 3 West Nile cases since 2016 (last 2022): you should use repellent at dusk July to August, more so if over 60.

Sully County against the rest of SD

Sully County has the 21st longest season of 66 SD counties, at 165 days against 179 in Yankton County and 139 in Lawrence County. Its mean temperature is 46°F.

• Haakon County: April 29 to October 11 (165 days, 1 WNV)
• Jerauld County: April 29 to October 10 (165 days, 5 WNV)
• Oglala Lakota County: April 30 to October 12 (165 days, 9 WNV)
• Beadle County: April 27 to October 8 (164 days, 32 WNV)

West Nile in Sully County

From 1999 to 2025, CDC ArboNET counted 15 human West Nile cases in Sully County (3 neuroinvasive) across 7 years. The worst was 2003 with 8; the latest was 2022.

More: how the dates are worked out · when mosquitoes go away for the winter.