How big these nests actually get
Size is the single most useful thing to know, because it decides whether you are looking at a five minute job or a job for someone in a suit. Penn State provides real numbers for the species on Pennsylvania's list. Paper wasps sit at the small end: a few dozen adults, occasionally growing past 200. Their nest is an open comb on a stalk with no outer covering, and the whole colony is visible at once. Bald-faced hornets run upwards of 300 individuals inside a fully enclosed gray envelope. You cannot see them, and disturbing the nest brings a coordinated response from the whole colony. European hornets peak at 200 to 400 workers, with large nests reaching 1,000, usually tucked inside a hollow tree or a wall where the comb is invisible. Yellowjackets are the wide category. Penn State says colonies range from only a few hundred individuals to thousands of members, and Ohio State records mature nests at roughly 4,000 to 5,000 workers with 10,000 to 15,000 cells by late September. That range is exactly why a hidden yellowjacket nest is treated differently from everything else on the list.
The only wasp in Pennsylvania that works at night
Most wasp encounters happen in daylight, near a nest, in the middle of the afternoon. The European hornet breaks that pattern, and it is why so many Philadelphia calls come in after dark. Penn State describes it as the only nocturnal wasp in Pennsylvania and as well established across the eastern United States, including all of the state. It is large, brown and yellow, and it flies toward lit windows and porch lights, which puts it face to face with people who were not anywhere near a nest. The nest itself is usually invisible. European hornets favor hollow trees, wall voids, attic spaces, and occasionally an outbuilding, so the comb sits behind something solid. Peak size runs 200 to 400 workers, with large nests hitting 1,000, and Penn State places the peak in mid September. Two clues help you confirm it before anyone gets stung. First, the timing, since no other local wasp is flying at eleven at night. Second, stripped bark on lilac and shrub stems, which they chew for material and sap. Turning off exterior lights reduces the encounters but does not address the nest.
A May to October timeline for a Philadelphia house
The season here is fixed at both ends, which makes planning easy if you know the dates. May is founding month. Penn State records Pennsylvania paper wasps establishing nests in May, and yellowjacket queens are doing the same work out of sight. Nests are tiny, populations are single digit, and one pass around the house with a broom handles more than any later intervention will. June and July are growth. Workers hunt caterpillars and flies to feed larvae, so wasps are busy and largely indifferent to people. This is when a hidden colony expands inside a wall without anyone noticing. August and September are peak and peak trouble. Ohio State describes the late summer switch from proteins to sweets across August through October, and Penn State puts European hornet nests at their maximum in mid September. October and November end it. Penn State notes paper wasps are essentially gone by the end of September and hard frosts kill the hornets. With winter lows near 26 degrees, that reset is reliable, and the right move is to take the paper down and close whatever hole it used.
Attached houses, shared walls, and the neighbor problem
Most cities have some attached housing. In Philadelphia it is the default, and it changes wasp control in ways that catch people out. A colony in a shared wall cavity is not confined to one address. The comb sits in a void that both houses touch, and workers use whatever openings exist on either side. Treating one entrance while a second opening stays active next door leaves the colony functioning, and both households keep seeing wasps. The same is true vertically. A parapet, a cornice run, or a continuous soffit can carry a cavity across several houses, so the flight line you are watching may not be above your own rooms at all. What helps is coordination and photographs. Tell your attached neighbors what you found, where the traffic goes, and what time of day it is heaviest. Ask the technician to inspect the neighbor facing side of the wall rather than only the elevation you called about. If any of the houses are rented, get the report to the owner in writing, since exterior and structural nests fall on the building rather than on a tenant's belongings.
The wasp that looks worst and matters least
Every July somebody calls about a wasp the size of a thumb hovering low over a lawn or a bare patch of soil. Cicada killers are on Pennsylvania's species list, and they are almost the opposite of the hornets people confuse them with. They are solitary. There is no colony, no shared nest, and no group defense. A female digs a burrow, provisions it with a paralyzed cicada, lays an egg, and repeats. The males that patrol aggressively over a lawn and fly at people cannot sting at all, and females sting only when handled or stepped on barefoot. What they do cause is cosmetic. A stretch of dry, sparsely planted, well drained soil can end up with dozens of burrows and small mounds of excavated dirt, which looks alarming on a front lawn. The practical response is landscaping rather than pesticide: thicken the turf, mulch bare beds, and water the area so the soil stops being attractive for digging. They are gone by late summer, and by then most of the noise around them has ended too.