Flies in the house: which fly it is, and why it's there
Published Updated
By John Stopka, Co-Founder & CEO · Reviewed by John Stopka
Which fly you have tells you where it is coming from. Ordinary house flies breed in garbage and other rotting matter and come in through doors and torn screens. Slow, sluggish flies on sunny windows in fall and late winter are cluster flies that spent the winter in the walls. A sudden burst of shiny green or blue flies usually means something has died nearby — often a mouse in a wall. And big horse flies are outdoor biters that follow you in by accident.
Which fly is it?
| Fly | Looks like | Where it comes from |
|---|---|---|
| House fly | about 1/4 inch, dull gray, four dark stripes on the back | garbage, pet waste, compost and other rotting matter outside |
| Cluster fly | a little larger, slow, with golden hairs on the back | overwinters in walls and attics; out on sunny windows |
| Blow fly (bottle fly) | shiny metallic green or blue | a dead animal, meat scraps or garbage |
| Horse fly / deer fly | large, often with patterned eyes or wings | outdoors near water, woods and livestock |
| Small flies and gnats | tiny | drains, fruit, houseplants — see our gnats and fruit flies article |
How long do flies live?
Not long, but they breed fast. In summer, a house fly can go from egg to adult in about a week to ten days, and the adult lives a few weeks. A single female lays batches of eggs over her life, so a garbage can left for a week in July can produce a houseful. In cold weather the whole cycle slows right down.
What do fly eggs and maggots look like?
Fly eggs are tiny, white and laid in clumps on whatever the larvae will eat — rotting food, garbage, pet waste or a dead animal. They hatch in about a day into maggots: pale, legless, carrot-shaped larvae that feed for several days, then crawl away to a drier spot to turn into a hard brown pupa.
Maggots in a garbage can or on a porch mean flies have been breeding right there. Maggots crawling across a floor or ceiling indoors usually mean something is decaying in a wall, ceiling or crawlspace above or beside them.
Why are there so many flies in my house all of a sudden?
Usually one of three things:
- Something died. A burst of shiny green or blue bottle flies indoors, often at one window or in one room, almost always means a dead mouse, rat, bird or other animal in a wall, attic or crawlspace. The flies stop when the source is found and removed — and a dead rodent is worth a look at how it got in; see rats vs mice.
- A breeding spot close to the door. An overfull or dirty garbage can, compost, pet waste or a recycling bin near an entrance will keep supplying house flies.
- A way in. Torn screens, doors propped open and gaps around windows and vents let them straight in.
Why are there slow flies on my windows in fall and winter?
Those are cluster flies. They are a little larger than house flies and move slowly and clumsily. Their larvae live in the soil as parasites of earthworms, and the adults look for a sheltered place to spend the winter — the walls, attic and window frames of a house. On warm, sunny days in fall, winter and early spring they wake up and gather on the sunniest windows.
They do not breed indoors and do not feed on food or garbage, so they are a nuisance rather than a sanitation problem. Sealing gaps around windows, siding, soffits and vents before late summer is what keeps them out.
Do horse flies bite?
Yes — female horse flies and deer flies bite, and it hurts, because they cut the skin rather than pierce it. They are outdoor flies of woods, water and pastures, active on hot summer days, and one indoors has simply followed someone in. There is nothing inside the house for them to breed on. If a bite concerns you, contact a healthcare provider.
How do you keep flies out?
- Keep garbage cans lidded, clean and away from doors, and take trash out often in summer.
- Pick up pet waste and keep compost away from the house.
- Repair torn screens and keep screen doors closing properly.
- Seal gaps around windows, doors, vents and siding — before late summer for cluster flies.
- If shiny flies appear suddenly, look for a dead animal rather than spraying the flies.
How does Liberty handle flies?
Liberty treats flies, alongside the everyday pests our pest control plans cover. The first step is always finding what they are breeding in or coming out of, because treating the flies without removing the source only buys a few days. Text us about what you are seeing, or book a free inspection. For flies in a restaurant or other food business, see restaurant pest control.
When to call a professional
Call when flies keep coming back after you have dealt with the garbage and screens, when cluster flies show up every fall, or when a sudden burst of bottle flies points to something dead that you cannot find.
Common questions
How long do flies live?
A house fly lives a few weeks as an adult, and in summer it can go from egg to adult in about a week to ten days — which is why flies build up so quickly around garbage.
Why are there suddenly so many flies in my house?
Most often a dead animal in a wall, attic or crawlspace (shiny green or blue flies), a breeding spot near a door such as a garbage can, or torn screens letting them in.
What are the slow flies on my windows in winter?
Cluster flies. They spend the winter in walls and attics and come out on sunny windows. They do not breed indoors.
What do fly eggs look like?
Tiny, white and laid in clumps on rotting food, garbage, pet waste or a dead animal. They hatch into maggots in about a day.
Does Liberty treat flies?
Yes. Liberty treats flies, starting by finding what they are breeding in. Text us or book a free inspection.
Sources
- University of Kentucky Entomology. Michael F. Potter. "How to Pest-Proof Your Home" (ENTFACT-641). https://entomology.mgcafe.uky.edu/ef641 (opens in a new tab)