Mosquito season in Marshall County, SD: when it starts and ends
Mosquito months in Marshall County
In a normal year mosquito season in Marshall County, SD starts around May 1 and ends around October 8, about 23 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Marshall 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 8.
- 10 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check SD spray notices.
Marshall County against the rest of SD
Marshall County has the 41st longest season of 66 SD counties, at 160 days against 179 in Yankton County and 139 in Lawrence County. Its mean temperature is 43.4°F.
- Spink County: April 30 to October 7 (160 days, 21 WNV)
- Grant County: May 1 to October 8 (160 days, 7 WNV)
- Potter County: May 1 to October 7 (160 days, 9 WNV)
- Pennington County: May 2 to October 9 (160 days, 32 WNV)
West Nile in Marshall County
From 1999 to 2025, CDC ArboNET counted 59 human West Nile cases in Marshall County (8 neuroinvasive) across 13 years. The worst was 2003 with 23; the latest was 2025.
Background: reading the West Nile counts; how the dates are worked out.