Mosquito season in Chester County, PA: when it starts and ends

Mosquito months in Chester County

In a normal year mosquito season in Chester County, PA starts around April 12 and ends around October 27, about 28 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 Chester County, and when

• Start tipping out standing water weekly by March 29.
• Book any yard treatment for mid April, ahead of the July peak.
• Repellent can go away after October 27.
• 23 West Nile cases since 2016: you should wear EPA-registered repellent at dusk June to August, and check PA spray notices.

Chester County against the rest of PA

Chester County has the 10th longest season of 67 PA counties, at 198 days against 222 in Philadelphia County and 154 in Clinton County. Its mean temperature is 52.6°F.

• Adams County: April 11 to October 27 (200 days, 2 WNV)
• Allegheny County: April 12 to October 27 (198 days, 23 WNV)
• Fulton County: April 12 to October 27 (198 days, 0 WNV)
• Montgomery County: April 12 to October 27 (198 days, 41 WNV)

West Nile in Chester County

From 1999 to 2025, CDC ArboNET counted 38 human West Nile cases in Chester County (22 neuroinvasive) across 13 years. The worst was 2003 with 9; the latest was 2025.

Background: reading the West Nile counts; how the dates are worked out.