Which fly is in your Boston kitchen?
Start with size and behavior, because that narrows it faster than any picture. A quarter-inch gray fly with four dark stripes on its back that flies in straight lines and lands on food is a house fly, and its source is outdoors in garbage, pet waste, or wet organic material. A slightly larger metallic blue or green fly is a blow fly, and it is telling you something is dead nearby. In Boston that is usually a squirrel or a bird in an attic, a chimney chase, or a wall cavity, often found weeks after it went quiet. A small fuzzy gray fly holding still on tile is a drain fly, and it came from the film in a drain rather than from outside. A tiny fly that runs across the counter in erratic bursts before flying is a phorid fly, and it points at decaying material in wet soil, a potted plant, or a leaking sewer line under a floor. Four flies, four completely different jobs to do.
A month-by-month fly timeline for Boston
January through March belongs entirely to indoor flies. With average winter lows near 22F nothing is breeding in the alley, so any fly in the house in February came from a drain, a heated basement, or something dead in the structure. April and May bring the first outdoor adults, but development is slow. Around 60F a house fly needs about six weeks from egg to adult, so numbers build quietly and most people never notice the first generation. June tightens the cycle. July and August are the peak, when average highs near 82F cut development to roughly a week or ten days and one neglected bin produces a visible problem in a fortnight. This is the window where weekly trash discipline matters most. September still runs warm enough to complete generations, and October usually brings the last outdoor wave. From November on, the flies you see are indoor flies again, and the honest answer is that they were probably there all summer too.
What a real first fly inspection looks like
A useful visit is mostly walking and looking, not spraying. Expect the inspector to ask where you see flies, at what time of day, and against which window they gather, because flies collect on the sunlit side nearest their source. They should pull the trap under a sink, check overflow openings, tape or trap drains overnight to confirm which pipe is producing, and look at floor drains in a basement or utility room. In a dense Boston building they should also ask about the shared trash room and the alley collection point, since the source is often not inside your unit at all. Outside they should check bins, the ground under bins, any compost, grass piles, pet waste, and standing water. If flies are large and metallic they should be sniffing along walls and up into an attic. What you want at the end is a sentence naming the source. If all you get is a description of the product used, you have bought a delay rather than a fix.
Why drain flies survive everything you pour down the pipe
Drain fly larvae live inside a gelatinous film on the pipe wall, above the water line, feeding on the organic matter caught in it. That single fact explains why every liquid remedy fails. Bleach, boiling water, and drain opener run straight down the middle of the pipe, touch the film for a second, and leave with it intact. What works is mechanical and repeated. A stiff long brush scrapes the sides, and a bacterial or enzyme foam expands to fill the pipe so it stays in contact with the vertical surfaces long enough to digest the film. Then you repeat weekly, because eggs already in the film hatch on their own schedule. Boston's average humidity around 68 percent works against you here, since damp basements and slow-drying traps keep the film wet all year. Rarely used drains are the usual culprit: a basement floor drain, a laundry standpipe, a spare shower, or a bar sink. Pour a cup of water into each once a month and treat them even when you have not seen a fly.
Renters, landlords, and the shared source problem
In a city as dense as Boston, the fly you are swatting frequently belongs to a source you do not control. A restaurant grease bin down the alley, a common basement drain, a neighbor's uncovered can, or a cracked stack shared by four units will all deliver adults to the unit with the warmest sunny window, which may well be yours. That changes what you should do first. Rather than buying products, build a short record: dated photos of the flies, the room and window they favor, the time of day, and whether they appear after someone runs water. If the flies spike when the building's laundry drains, you have found a shared cause. Hand that to the landlord or the management company in writing. Source removal in shared space is generally their responsibility, and a specific request to camera the basement line lands very differently than a complaint that there are flies. Ask to be told what was found, not just that someone treated the building.
Comparing two fly quotes without guessing
Two quotes for a fly job can differ by a factor of three and both be honest, because they may describe entirely different work. Read for the source, not the price. The first question is whether the estimate names what is breeding. A quote that says quarterly exterior treatment and interior fly service is describing adults. A quote that says foam three floor drains weekly for a month, camera the basement line, and remove the carcass in the north soffit is describing a fix. Only the second one has an end. Ask how many visits are included and what happens if flies return in week three, which matters in Boston during August when a generation completes in about a week. Ask whether drain work is mechanical scrubbing or just a poured product. Ask who handles the plumbing repair if a broken line is found, and whether the inspection fee comes off the treatment. Get the plan in writing, including the specific drains and rooms covered, so the follow-up visit is not a negotiation.