Why cool weather makes this a slower, longer problem
In hot cities a fruit fly problem arrives like a storm and passes. Here it behaves differently, and understanding why saves you from expecting a quick end. Development speed depends on temperature. Outdoors, with summer highs around 67F, the cycle stretches out. Indoors it does not, because a heated flat sits well above that most of the year. So instead of a summer explosion and a winter collapse, San Francisco gets a low, steady population that persists as long as the food does. That has two consequences. First, there is no season to wait out. A source ignored in October is still producing in February. Second, small numbers are more meaningful here than they would be elsewhere: five flies that never go away point to a permanent breeding site, not to a bad week. The practical response is to treat any persistent low level cloud as a search problem rather than a nuisance. Find the material, remove it, and the line finally drops instead of leveling off again.
Compost pails, green bins, and small kitchens
Separating food scraps is normal here, and it is also the single most common breeding site in the city's kitchens. That is not an argument against composting, it is an argument for washing the container. The mechanism is simple. Scraps begin fermenting within a day. When you tip the pail out, a film stays in the bottom, and that film alone will support a generation. A pail emptied faithfully but never washed produces flies indefinitely. The routine that works: empty daily, wash with hot soapy water, dry before it goes back under the sink, and wipe the lid and the rim where residue collects. Do the same for the green bin outside, particularly the underside of the lid, which almost nobody cleans. Small kitchens raise the stakes because there is nowhere to isolate anything. The pail sits under the sink beside the pipe, the recycling sits next to the pail, and the fruit bowl is a foot away on the only free counter. In a compact San Francisco kitchen, one unwashed container puts flies within reach of every surface you use.
When the source is in the building, not your flat
This is the situation that wastes the most time. You clean thoroughly, the flies persist, and you start wondering what you missed. Sometimes nothing. Older San Francisco buildings share more than walls. Waste rooms hold containers from every unit, light wells collect debris and have drains that rarely run, and horizontal waste lines connect several kitchens. Any of those can support a population that redistributes itself through the building. The way to establish this is with a record. For two weeks, do everything inside your control and write down the daily catch from a vinegar cup. If the count falls, the source was yours. If it stays flat while your kitchen is spotless, you have evidence pointing outward. Take that to the building manager in writing and ask for something specific: an inspection of the waste room containers, the light well drain, and the shared waste line. A hallway spray is not a response to a plumbing source, and asking for the inspection by name usually gets a better outcome than a general complaint.
Coastal gardens and the fly that gets there first
If you have a back garden, a light well planter, or a shared yard with berries, part of your indoor problem may be arriving fully grown. Spotted-wing drosophila is the reason. UC IPM counts up to ten generations a year on coastal berries and cherries, and unlike the common vinegar fly it lays into fruit that is still firm. That means fruit picked and brought inside can already carry eggs, and fruit left on the plant or dropped underneath keeps producing adults. UC IPM also notes it overwinters as prepupae or pupae on the ground, so a mild coastal winter leaves the population intact and ready in spring. There is no freeze here to reset it. The yard work is straightforward: pick fruit at ripeness rather than a few days later, clear drops weekly during the season, and keep green waste containers away from the door. Indoors, treat garden fruit like something that needs using or refrigerating quickly rather than displaying in a bowl.
Questions worth asking before you hire
Fly work is priced on inspection time and visits, so the questions that matter are about scope rather than chemicals. Ask whether the provider will identify the species and show you the breeding site before applying anything. Ask whether the inspection includes the building's waste area and drains or stops at your door. Ask whether drain line treatment is inside the number. Ask how many visits are covered and what happens if flies persist at three weeks. In a multi-unit property, add one more: who is authorizing work in the shared spaces? A treatment limited to one apartment while the waste room goes untouched is money spent on a symptom. Expect the cheapest quote to be one visit and an aerosol. That approach ends with a second call, because whatever was pupating on service day emerges within a couple of days. Two visits about two weeks apart line up with the biology and usually cost less in total. Get the plan in writing, including which sources you are expected to handle.