Which fly do you actually have?
Start by matching the fly to its breeding site, because that decides the fix. The four household groups each come from a different place, and treating the wrong one wastes weeks. House flies, little house flies, blow flies, flesh flies, and stable flies are the outdoor filth group. They grow up in manure, garbage, grass clippings, pet waste, and carcasses, then come inside through open doors and torn screens. If your flies are the classic gray kind and you have a garbage can, a dog, or a lawn, start there. Drain flies are fuzzy, hold their wings like a tiny moth, and rest on walls near a shower or floor drain. They breed in the gelatinous biofilm coating drain pipes, overflows, septic systems, and condensate pans. Phorid flies are smaller, dark, humpbacked, and run rather than fly when disturbed. They will use any rotting organic matter, and the hard cases involve a sewer leak under a slab or a houseplant that never dries. Fruit flies, the red-eyed ones over the bananas, get their own page on this site. If you can name the group, you can usually name the source, and the state pages walk through which group dominates where.
Why does the calendar change so much from state to state?
Because fly development is driven by temperature, the same house fly that takes about 45 days from egg to adult at 60°F finishes in 7 days at 95°F, per Texas A&M AgriLife Extension. That single fact explains the map. In Florida, Hawaii, south Texas, and the desert Southwest, outdoor temperatures stay in the fast-development range for most of the year, so filth flies cycle continuously and sanitation never gets a break. A garbage can that skips a pickup in Phoenix in August is a new generation by the following week. In the northern tier, from Maine across to Idaho and Washington, cold weather shuts outdoor breeding down for months. The University of New Hampshire Extension still counts six to eight generations a year there, but they pile up in July and August, which is when nearly every complaint arrives. The Midwest and Mid-Atlantic run a longer season, roughly May into October, with the peak in the hottest weeks. Drain flies and phorid flies ignore all of this. Plumbing is warm and wet in Colorado in January just as it is in Georgia in July, so those two groups are year-round indoor pests in every state.
What should the first visit look like?
A good first visit is mostly looking, not spraying. The provider should walk the outside first: garbage cans and dumpsters, the pet-waste area, compost, grass-clipping piles, any animal pens, and the ground under downspouts. Inside, they should check drains, overflows, the condensate pan on the air handler, under-sink cabinets for leaks, and wet potted soil. They should also identify the fly. Sticky traps or a quick look at the specimens on a windowsill tell them whether they are dealing with house flies, blow flies, drain flies, or phorids, and that identification should be part of what they tell you. Expect a written finding that names the breeding source and who fixes it. If the source is your garbage schedule or the dog run, that is on you and they should say so plainly. If it is a carcass in a wall or a sewer break under the slab, they should explain the access or plumbing work involved. Only after that should products come up: an insect growth regulator for a larval site, a bacterial drain treatment, a light trap for a commercial door, or a short pyrethrin knockdown for the adults. If the quote is a spray with no source named, get a second opinion.
Why a dead animal produces a fly swarm, and what to do
A sudden crowd of sluggish, shiny blue or green flies inside a closed house is the signature of a carcass. Blow flies find a dead squirrel, rat, bird, or raccoon within hours of death and lay eggs on it. UC IPM puts blow fly development at as little as 7 days from egg to adult, with adults emerging one to two weeks after the maggots appear, which is why the swarm often arrives just as the smell starts to fade. The usual hiding spots are wall voids, attics, chimneys, crawlspaces, and the space under a bathtub. Older homes with unlined chimneys and crawlspaces see this more often than newer slab homes, and it happens in every climate zone because the animal died indoors. You can wait it out: once the carcass dries, breeding stops and the adults die within a few weeks. Most people would rather not. A provider can locate the carcass by smell and fly concentration, open the wall or access the attic, remove it, and clear the remaining adults with a space spray. Vacuuming the adults off windows each morning speeds things up. The prevention piece is exclusion: screen attic vents, cap the chimney, and close crawlspace openings so the next animal does not get in to die there.
Renters, landlords, and who pays for fly control
Who pays depends on where the flies breed. If the source is inside the unit and under the tenant's control, meaning overflowing trash, a dirty litter box, or produce left out, most leases put that on the renter. If the source is structural, meaning a sewer leak under the slab, a dead animal in the wall, a leaking condensate line, or shared dumpsters that are not emptied, that is the landlord's problem in most states because it falls under habitability and maintenance. The practical move for a tenant is to document. Photograph the flies, note the date and where they gather, and put the request in writing. Ask the landlord to have the source identified rather than just sprayed, because a spray crew that visits every two weeks without finding the breeding site is a sign nobody has looked. For landlords with multiple units, phorid flies in several ground-floor bathrooms at once usually mean one broken drain line, and drain flies across a building usually mean the floor drains or a condensate pan have been ignored. A single plumbing repair beats a standing spray contract. Nothing here is legal advice; lease terms and state landlord rules vary, and the state pages note where local rules apply.
How to compare two fly control quotes
Lay the two quotes side by side and look for the source first. The better quote names what the provider found, even if it is something as ordinary as a garbage can without a lid, and says what happens to it. A quote that only lists products and visit frequency is selling adult knockdown, which lasts about as long as one generation, three days to four weeks depending on temperature according to the University of New Hampshire Extension. Next, compare the follow-up plan. Drain fly and phorid work needs repeat visits until a taped drain stays clean overnight; carcass removal needs one visit plus adult cleanup; a commercial door program needs trap maintenance. The number of visits should match the source, not a standard package. Then check what is on you. A good provider tells you plainly that the dog waste, the clippings pile, or the compost bin is the problem and that treatment will not hold unless you change it. That honesty is a good sign, not a dodge. Finally, compare exclusion. Screens, sweeps, and vent covers cost more up front and are the only part of the job that prevents next year's problem. Prefer the quote that includes them over the one that is cheaper because it skips them.