Why the San Francisco wasp season looks different
If you moved here from almost anywhere else, the wasp calendar will feel off, and the reason is heat, or the lack of it. In most of the country colony growth tracks summer temperature closely. Hot months mean fast larval development, more workers, and a sharp peak. San Francisco's average summer high sits near 67 degrees, which is enough for a colony to function but not enough to push it hard. Nests build steadily instead of exploding. The other end is just as unusual. With winter lows averaging 46 degrees, there is no hard freeze to end things on a date. Colonies wind down when food and daylight run out rather than dying overnight. The practical effects are worth planning around. Queens emerge earlier here, in late winter, so the spring inspection window opens in February rather than April. The peak arrives later, often in September and October. And a nest that would be obviously finished by November in the Midwest may still have active traffic here, which is why a wall colony should be confirmed dead by observation rather than assumed dead by the calendar.
A nest in an attached building is a different problem
Most wasp advice assumes a detached house on its own lot. A great deal of San Francisco housing is not that, and the difference matters. When buildings share framing, a wall cavity is not neatly bounded by your deed. A German yellowjacket colony can enter through a gap in your trim and build its comb in a space that runs behind a neighboring wall. Treating one entry point while the rest of the cavity stays open sometimes just relocates the traffic a few feet sideways. That has three consequences. Tell the neighbor as soon as you suspect a colony, because they may be seeing wasps indoors without knowing why. Use one company across both sides rather than two working separately, since a coordinated inspection is what actually finds every opening. And expect the inspection to involve access you would not normally give a stranger: a roof hatch, a light well, a back stair, a shared garage. With a median home value around $1,300,000, this is also a case where the repair after treatment deserves as much attention as the treatment itself.
What a first visit should look like
The valuable part of a wasp visit here is the search, not the spray. Expect a technician to spend the bulk of the appointment watching traffic and working the exterior. On a San Francisco building that means the roof edge and parapet, trim joints around bay windows, stucco cracks and control joints, vent screens, the light well if there is one, and the back stair structure. Garden beds and the base of any retaining wall get a look for ground nests. Application into a cavity is a dust at the entry point, done at dusk or after dark once foragers are home. It is undramatic by design. Traffic drops over the next day or two, and because there is no frost here to confirm the outcome for you, a follow-up visit is genuinely part of the job rather than an upsell. Settle two questions before work starts. Is removing the nest included once the colony dies, and is closing the opening part of the price. In older buildings the second question is the expensive one, and it is the one that determines whether you do this again next year.
Renters, building managers, and who pays
A nest inside the building envelope is normally the owner's expense, because the wasps entered through a defect in the structure: a torn vent screen, a gap in trim, a crack in stucco. Those are maintenance items, not tenant housekeeping, and in a multi-unit building they are usually a common-area issue rather than a unit issue. A paper wasp comb on a back deck chair or a planter the tenant owns is fairly the tenant's to handle, and at 15 to 200 wasps that is a small job. Report it in writing the same day you notice it. Note the entry point, the floor and side of the building, roughly how many wasps a minute are going in and out, whether wasps are appearing inside any unit, and whether anyone in the building has a sting allergy. That last item raises the priority, and the American College of Allergy, Asthma and Immunology reports a 60 percent chance of a similar or worse reaction for someone who has already had one. Do not treat a wall cavity yourself in a rented flat. Beyond the sting risk, opening or foaming a wall is damage you may be asked to pay for.
Comparing quotes without guessing
Two quotes for the same building often describe very different work, so line them up on scope first. Start with access. One company may be pricing a treatment from the sidewalk and another may be including roof access or a light well entry. That difference explains most of the gap between numbers, and the cheaper one is not always the one that reaches the nest. Then ask whether the search itself is included. On a cavity nest, finding every opening is the job, and a quote that assumes one entry point may not survive contact with an older building. Then ask about removal and repair as separate items. Killing a colony, pulling out the paper, and sealing the gap are three pieces of work, and in San Francisco the sealing is often carpentry or stucco rather than a squirt of caulk. Then ask about the callback in plain terms: if wasps use that opening again in two weeks, who returns and at what cost. Given the lack of a hard freeze here, ask how long that window runs. Finally, confirm the license through the state pesticide agency directory.
The safety math worth knowing
Most people can take a wasp sting and be uncomfortable for a day. The reason wasp nests get treated as a safety matter rather than a nuisance is the minority who cannot. The University of Kentucky estimates about two million Americans are hypersensitive to stinging insect venom, roughly one person in a hundred, and puts stinging insect reactions at around 225,000 emergency room visits a year. The American College of Allergy, Asthma and Immunology reports potentially life-threatening reactions in 0.4 to 0.8 percent of children and about 3 percent of adults, which is the detail people miss: adults are more at risk than children, so a household with no kids is not automatically a low-risk household. The number that should change behavior is the repeat rate. Anyone who has already had a serious reaction faces a 60 percent chance of a similar or worse one. That person should not be near a nest, should not be the one holding the can, and should carry whatever their doctor prescribed during the months when colonies are at full strength. In a dense city, add one more consideration: a nest in a shared wall exposes people you have not met.