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

Mosquito months in Pennington County

In a normal year mosquito season in Pennington County, SD starts around May 2 and ends around October 9, about 23 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 Pennington 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 9.
• 32 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check SD spray notices.

Pennington County against the rest of SD

Pennington County has the 43rd longest season of 66 SD counties, at 160 days against 179 in Yankton County and 139 in Lawrence County. Its mean temperature is 47°F.

• Marshall County: May 1 to October 8 (160 days, 10 WNV)
• Potter County: May 1 to October 7 (160 days, 9 WNV)
• Brown County: April 30 to October 6 (159 days, 71 WNV)
• Sanborn County: April 30 to October 6 (159 days, 6 WNV)

West Nile in Pennington County

From 1999 to 2025, CDC ArboNET counted 227 human West Nile cases in Pennington County (62 neuroinvasive) across 18 years. The worst was 2003 with 139; the latest was 2025.

More: how the dates are worked out · reading the West Nile counts.