Mosquito season in Plymouth County, IA: when it starts and ends

Mosquito months in Plymouth County

In a normal year mosquito season in Plymouth County, IA starts around April 22 and ends around October 14, about 25 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 Plymouth County, and when

• Start tipping out standing water weekly by April 8.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 14.
• 10 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check IA spray notices.

Plymouth County against the rest of IA

Plymouth County has the 69th longest season of 99 IA counties, at 175 days against 198 in Des Moines County and 160 in Worth County. Its mean temperature is 46.9°F.

• Audubon County: April 22 to October 14 (175 days, 2 WNV)
• Buchanan County: April 22 to October 15 (175 days, 0 WNV)
• Guthrie County: April 23 to October 15 (175 days, 1 WNV)
• Hardin County: April 23 to October 15 (175 days, 2 WNV)

West Nile in Plymouth County

From 1999 to 2025, CDC ArboNET counted 29 human West Nile cases in Plymouth County (11 neuroinvasive) across 15 years. The worst was 2003 with 7; the latest was 2025.

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