Mosquito season in Walworth County, SD: when it starts and ends
Mosquito months in Walworth County
In a normal year mosquito season in Walworth County, SD starts around May 1 and ends around October 7, about 23 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Walworth County, and when
- Start tipping out standing water weekly by April 17.
- Book any yard treatment for early May, ahead of the July peak.
- Repellent can go away after October 7.
- 17 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check SD spray notices.
Walworth County against the rest of SD
Walworth County has the 47th longest season of 66 SD counties, at 159 days against 179 in Yankton County and 139 in Lawrence County. Its mean temperature is 44°F.
- Sanborn County: April 30 to October 6 (159 days, 6 WNV)
- Moody County: May 1 to October 7 (159 days, 3 WNV)
- Butte County: May 2 to October 8 (159 days, 3 WNV)
- Brookings County: May 1 to October 6 (158 days, 27 WNV)
West Nile in Walworth County
From 1999 to 2025, CDC ArboNET counted 37 human West Nile cases in Walworth County (13 neuroinvasive) across 18 years. The worst was 2003 with 8; the latest was 2025.
More: how the dates are worked out · reading the West Nile counts.