Mosquito season in Fremont County, WY: when it starts and ends
Mosquito months in Fremont County
In a normal year mosquito season in Fremont County, WY starts around May 4 and ends around October 4, about 22 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Fremont County, and when
- Start tipping out standing water weekly by April 20.
- Book any yard treatment for early May, ahead of the July peak.
- Repellent can go away after October 4.
- 12 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to July, and check WY spray notices.
Fremont County against the rest of WY
Fremont County has the 3rd longest season of 23 WY counties, at 153 days against 163 in Big Horn County and 75 in Teton County. Its mean temperature is 44.5°F.
- Big Horn County: April 27 to October 6 (163 days, 1 WNV)
- Campbell County: May 5 to October 7 (155 days, 3 WNV)
- Niobrara County: May 5 to October 5 (153 days, 0 WNV)
- Sheridan County: May 8 to October 5 (150 days, 3 WNV)
West Nile in Fremont County
From 1999 to 2025, CDC ArboNET counted 224 human West Nile cases in Fremont County (36 neuroinvasive) across 15 years. The worst was 2007 with 117; the latest was 2025.
Background: reading the West Nile counts; how the dates are worked out.