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

Mosquito months in Delaware County

In a normal year mosquito season in Delaware County, PA starts around April 4 and ends around November 7, about 31 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 Delaware County, and when

• Start tipping out standing water weekly by March 21.
• Book any yard treatment for early April, ahead of the July peak.
• Repellent can go away after November 7.
• 24 West Nile cases since 2016: you should wear EPA-registered repellent at dusk June to August, and check PA spray notices.

Delaware County against the rest of PA

Delaware County has the 2nd longest season of 67 PA counties, at 217 days against 222 in Philadelphia County and 154 in Clinton County. Its mean temperature is 55.8°F.

• Philadelphia County: April 1 to November 9 (222 days, 69 WNV)
• Berks County: April 9 to October 31 (206 days, 17 WNV)
• York County: April 8 to October 30 (205 days, 14 WNV)

West Nile in Delaware County

From 1999 to 2025, CDC ArboNET counted 55 human West Nile cases in Delaware County (44 neuroinvasive) across 16 years. The worst was 2012 with 7; the latest was 2025.

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