Mosquito season in Union County, SD: when it starts and ends
Mosquito months in Union County
In a normal year mosquito season in Union County, SD starts around April 22 and ends around October 15, about 25 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Union County, and when
- Start tipping out standing water weekly by April 8.
- Book any yard treatment for late April, ahead of the July peak.
- Repellent can go away after October 15.
- 13 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check SD spray notices.
Union County against the rest of SD
Union County has the 5th longest season of 66 SD counties, at 176 days against 179 in Yankton County and 139 in Lawrence County. Its mean temperature is 47.3°F.
- Douglas County: April 21 to October 16 (178 days, 11 WNV)
- Charles Mix County: April 22 to October 17 (178 days, 18 WNV)
- Hutchinson County: April 22 to October 13 (174 days, 18 WNV)
- Turner County: April 22 to October 13 (174 days, 5 WNV)
West Nile in Union County
From 1999 to 2025, CDC ArboNET counted 31 human West Nile cases in Union County (14 neuroinvasive) across 13 years. The worst was 2018 with 7; the latest was 2023.
See how the dates are worked out, and reading the West Nile counts.