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

Mosquito months in Philadelphia County

In a normal year mosquito season in Philadelphia County, PA starts around April 1 and ends around November 9, about 32 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 Philadelphia County, and when

• Start tipping out standing water weekly by March 18.
• Book any yard treatment for early April, ahead of the July peak.
• Repellent can go away after November 9.
• 69 West Nile cases since 2016: you should wear EPA-registered repellent at dusk June to September, and check PA spray notices.

Philadelphia County against the rest of PA

Philadelphia County has the 1st longest season of 67 PA counties, at 222 days against 222 in Philadelphia County and 154 in Clinton County. Its mean temperature is 56.8°F.

• Delaware County: April 4 to November 7 (217 days, 24 WNV)
• Berks County: April 9 to October 31 (206 days, 17 WNV)

West Nile in Philadelphia County

From 1999 to 2025, CDC ArboNET counted 147 human West Nile cases in Philadelphia County (123 neuroinvasive) across 22 years. The worst was 2003 with 24; the latest was 2025.

More: how the dates are worked out · reading the West Nile counts.