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

Mosquito months in Gregory County

In a normal year mosquito season in Gregory County, SD starts around April 26 and ends around October 13, 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 Gregory County, and when

• Start tipping out standing water weekly by April 12.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 13.
• 3 West Nile cases since 2016 (last 2025): you should use repellent at dusk July to August, more so if over 60.

Gregory County against the rest of SD

Gregory County has the 14th longest season of 66 SD counties, at 170 days against 179 in Yankton County and 139 in Lawrence County. Its mean temperature is 47.5°F.

• Clay County: April 24 to October 11 (170 days, 8 WNV)
• Lincoln County: April 24 to October 11 (170 days, 39 WNV)
• Jones County: April 27 to October 13 (168 days, 0 WNV)
• Aurora County: April 27 to October 11 (167 days, 5 WNV)

West Nile in Gregory County

From 1999 to 2025, CDC ArboNET counted 15 human West Nile cases in Gregory County (2 neuroinvasive) across 5 years. The worst was 2003 with 11; the latest was 2025.

See how the dates are worked out, and when mosquitoes go away for the winter.