Mosquito season in Day County, SD: when it starts and ends
Mosquito months in Day County
In a normal year mosquito season in Day County, SD starts around May 2 and ends around October 6, about 22 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Day County, and when
- Start tipping out standing water weekly by April 18.
- Book any yard treatment for early May, ahead of the July peak.
- Repellent can go away after October 6.
- 10 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to July, and check SD spray notices.
Day County against the rest of SD
Day County has the 55th longest season of 66 SD counties, at 157 days against 179 in Yankton County and 139 in Lawrence County. Its mean temperature is 42.8°F.
- Clark County: May 2 to October 6 (157 days, 11 WNV)
- Codington County: May 2 to October 6 (157 days, 25 WNV)
- Edmunds County: May 2 to October 5 (156 days, 10 WNV)
- Deuel County: May 3 to October 5 (155 days, 5 WNV)
West Nile in Day County
From 1999 to 2025, CDC ArboNET counted 38 human West Nile cases in Day County (7 neuroinvasive) across 13 years. The worst was 2003 with 11; the latest was 2023.
See how the dates are worked out, and reading the West Nile counts.