The September math nobody expects
The single most useful fact about wasps in Baltimore is how steeply the growth curve runs at the end of the season. Ohio State's figures put a mature yellowjacket colony at roughly 4,000 to 5,000 workers with 10,000 to 15,000 cells by August and late September. Most of that construction happens in the final stretch. A nest that held a few hundred workers in early July is not twice that in September; it is many times that. That changes three practical things. It changes the risk of disturbing the nest, since the number of wasps that can respond at once is far larger. It changes the treatment, since a bigger colony takes longer to collapse and more often needs a follow-up check. And it changes the value of acting early, because the same nest addressed in June is a fraction of the work. It also explains something people find confusing: why a yard that was fine all summer becomes unusable within about two weeks. Nothing new moved in. The colony simply crossed the point where its numbers and its widened defended radius reach the places you walk.
Trash, back steps, and food outdoors
The complaint that brings most Baltimore households to this subject is not a nest at all. It is wasps at the trash cans and around anything being eaten outside from late August onward. The cause is the seasonal diet shift Ohio State describes: from proteins for the brood to sweets for the adults. A recycling bin with sugary residue, a trash can with food waste, a plate of crab or barbecue, and an open soda can are all high-value targets to a colony that is no longer raising larvae. In a dense city, where trash storage often sits close to a back door or an alley gate, that puts foraging wasps directly in the path people use every day. The fixes are ordinary and effective. Latch the bins and rinse them, particularly recycling. Move containers as far from seating and doors as the property allows. Serve food covered and cover it again immediately. Use lidded cups rather than open cans, and check any can that has been sitting, since a sting inside the mouth needs emergency care. Wipe tables and grill surrounds, because residue keeps drawing foragers back after the food is gone.
The four wasps on a Baltimore block
Knowing which one you have decides whether this is a fifteen-minute job or something to keep everyone away from. Native paper wasps build the open, stalked comb under porch ceilings, rails, and shutters. Penn State notes these nests are established in May here and are essentially finished by the end of September. Colonies stay in the dozens and the wasps defend the comb itself rather than a wide area. European paper wasps are marked like a yellowjacket but behave like paper wasps, choosing enclosed cavities: downspouts, pipe ends, mailboxes, gate posts. That is how a nest ends up somewhere you put your hand. Yellowjackets keep the nest hidden in soil, under slabs, or inside walls, reach thousands of workers, scavenge food, and pursue anyone who disturbs the entrance. This is the group behind serious stinging incidents. Bald-faced hornets build the gray aerial football, defend a wide radius, and are usually high enough that height rather than temper decides the approach. European hornets are the large brown ones active after dark.
Older exterior woodwork is what queens are looking for
wasps do not bore into a building. They use openings that already exist, and a queen needs a gap roughly the width of a pencil to get started. In a city founded in 1729, those gaps are plentiful: cornice and frieze openings, spaces behind loose or shrunken trim, unscreened gable and foundation vents, open masonry joints, uncapped downspouts, and the seams where an addition meets original framing. Behind each of them is a void that holds temperature well through a cool Maryland spring, which is exactly what a founding queen needs while she raises the first workers alone. That makes exterior maintenance the most effective wasp control available, and the best time for it is winter, when nothing is nesting. Screening a vent in January is a small task. Treating the colony behind it in September is not. One warning worth repeating because people get it wrong every year: never close an opening while wasps are using it. A blocked colony does not leave. It searches for a second exit, and interior drywall is softer than exterior wood.
What happens in the week after treatment
Knowing the normal course of a treatment stops people from concluding too early that it failed. Day one, the night of the application: activity may briefly increase as disturbed wasps come out, then subside. This is why treatment happens after dark and why nobody should be standing nearby. Days two and three: traffic at the entrance drops sharply. You will still see wasps arriving, because foragers that were away return to a nest that is now contaminated, and they carry material inward. Those returning wasps are the mechanism, not a sign the treatment missed. Days three through five: activity should be down to occasional stragglers. This is the window Kentucky extension has in mind when it advises waiting at least three to four days before removing a nest. Days five through seven: the entrance should be quiet. Now the opening can be sealed, and an exposed comb can be taken down safely. If meaningful traffic is still running after a week, that is the point to ask about a follow-up visit, which is why it is worth knowing in advance whether one is included.
Working out whether the nest can wait
Not every nest needs immediate attention, and knowing where yours falls saves money without pretending the risk away. A nest can usually wait when it is far from where people move, is above the first story with no walkway underneath, and is in a household with no sting allergies. A bald-faced hornet nest high in a back tree in October will end with the first hard frosts on its own, and the site will not be reused. A nest should not wait when it sits within a couple of steps of a door, a stair, an alley gate, or a play area, because ordinary daily movement reads as disturbance to a mature colony. Nor should it wait when it is inside a wall, since that colony can eventually reach living space, or when someone has already been stung on the property. And it should never wait in a home where anyone has had a systemic reaction. The CDC counted 788 sting deaths in the United States between 2011 and 2021, most of them in people with venom allergies, which is the whole argument in one number.