Mosquito season in Williams County, ND: when it starts and ends

Mosquito months in Williams County

In a normal year mosquito season in Williams County, ND starts around May 11 and ends around September 28, about 20 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 Williams County, and when

• Start tipping out standing water weekly by April 27.
• Book any yard treatment for mid May, ahead of the July peak.
• Repellent can go away after September 28.
• 31 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July, and check ND spray notices.

Williams County against the rest of ND

Williams County has the 47th longest season of 53 ND counties, at 140 days against 160 in Sheridan County and 135 in Rolette County. Its mean temperature is 41.8°F.

• Divide County: May 9 to September 27 (141 days, 2 WNV)
• Mercer County: May 11 to September 28 (140 days, 25 WNV)
• Mountrail County: May 11 to September 27 (139 days, 20 WNV)
• Burke County: May 10 to September 25 (138 days, 2 WNV)

West Nile in Williams County

From 1999 to 2025, CDC ArboNET counted 84 human West Nile cases in Williams County (17 neuroinvasive) across 16 years. The worst was 2003 with 29; the latest was 2025.

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