Philadelphia sits between the Delaware and Schuylkill rivers, and a typical home has long been the row house. Shared walls and shared basements mean a pest in one house often has a path into the next.
The 2020 census put Philadelphia at 1,603,797 people, and the city is the seat of a county that shares its name. About 134 square miles of land sit between the Delaware River on the east and the Schuylkill River through the middle. Most of it lies low, near 39 feet above sea level, while the highest ground, in Chestnut Hill, reaches about 446 feet. For much of its history the typical home here has been the row house.
That mix of old attached housing and river weather shapes the pest year. Wikipedia places the city on the northern edge of the humid subtropical zone, with a July average near 78.7 degrees, about 44 inches of precipitation a year and roughly 22 inches of snow. Warm, damp summers favor mosquitoes and roaches, and cold snaps push rodents toward heated cellars.