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

Mosquito months in Grant County

In a normal year mosquito season in Grant County, SD starts around May 1 and ends around October 8, 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 Grant 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.
• 7 West Nile cases since 2016 (last 2025): you should use repellent at dusk July to July, more so if over 60.

Grant County against the rest of SD

Grant County has the 40th 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.

• Hanson County: April 30 to October 7 (160 days, 5 WNV)
• Spink County: April 30 to October 7 (160 days, 21 WNV)
• Marshall County: May 1 to October 8 (160 days, 10 WNV)
• Potter County: May 1 to October 7 (160 days, 9 WNV)

West Nile in Grant County

From 1999 to 2025, CDC ArboNET counted 28 human West Nile cases in Grant County (3 neuroinvasive) across 14 years. The worst was 2012 with 5; the latest was 2025.

See how the dates are worked out, and reading the West Nile counts.