Blue flies buzzing around your house are almost certainly bluebottle flies, a type of blow fly with a distinctive metallic blue abdomen. A few drifting in through an open window is normal. But if you’re seeing a sudden cluster of them indoors, something is attracting them, and that something is usually decaying organic matter you haven’t found yet.
What Bluebottle Flies Look Like
Bluebottle flies are noticeably larger than common houseflies, measuring 10 to 14 mm long. Their most recognizable feature is a bright, shiny metallic blue abdomen with black markings. The head and upper body are dull gray, the eyes are red, and the wings are clear. Up close, you can spot tufts of orange-yellow hair on their cheeks and face, which distinguishes them from other similar-looking blow fly species that have darker facial hair. Their bodies are covered in short black bristles, and they make a loud, low-pitched buzzing sound that’s hard to miss in a quiet room.
Why They’re in Your House
Bluebottle flies are scavengers. They feed and lay eggs on decaying organic material, so their presence indoors almost always traces back to one of two sources: rotting food or a dead animal.
If you’re seeing just one or two flies, they likely came in through a door or window, drawn by the smell of food waste in your kitchen bin. This is routine and not a sign of a problem. But if you’re suddenly seeing five, ten, or more blue flies appearing over several days, especially in a room with no obvious food source, a dead animal is the most likely explanation. Mice, rats, birds, and squirrels that die inside wall cavities, lofts, under floorboards, or behind appliances create an ideal breeding site. A single carcass can produce dozens of flies.
Timing matters too. Bluebottle flies become active once temperatures rise above 10°C (50°F). If you turn on your heating system in cooler months, the warmth inside wall cavities can wake dormant insects and draw them toward a carcass that’s been there for weeks. This is why people sometimes see a burst of blue flies in autumn or early spring with no obvious cause.
How to Find the Source
Eliminating the breeding source is the only way to stop more flies from emerging. Start by noting which room the flies concentrate in, since they tend to stay near where they hatched. Then check the most common hiding spots for a dead animal or forgotten food waste:
- Loft and attic spaces, where birds or squirrels may have entered and died
- Behind kitchen appliances like refrigerators, ovens, and dishwashers
- Under floorboards, particularly near exterior walls where rodents travel
- Behind bath panels and in cupboards that aren’t opened often
- Inside old fireplaces or chimney cavities, where birds sometimes become trapped
- Wall cavities near where you’ve had signs of mice or rats
A dead animal inside a wall cavity is not always reachable, and sometimes you simply cannot find it. In that case, the fly problem is self-limiting. Once the carcass dries out and the larvae have nothing left to feed on, no new flies will develop. This typically takes two to three weeks, depending on the size of the animal and the temperature.
How Quickly They Breed
Bluebottle flies develop fast. A female lays eggs directly on decaying material, and the larvae (maggots) feed aggressively before entering a pupal stage. At typical indoor temperatures between 18°C and 25°C, larvae reach the pupal stage in roughly 80 to 83 hours, just over three days. In warmer conditions around 33°C, that drops to about 71 hours. After pupation, adult flies emerge within one to two weeks. This rapid cycle is why a small, hidden food source can produce a noticeable wave of flies in your home within days.
Health Risks
Bluebottle flies are not just a nuisance. Because they land on and feed on decaying animal tissue, feces, and garbage before entering your home, they carry bacteria on their bodies and legs. Blow flies are known to transmit Salmonella, E. coli, and Campylobacter, all of which cause food poisoning. They contaminate food simply by landing on it. Any uncovered food that a bluebottle fly has touched should be discarded.
In rare cases, blow fly larvae can cause a condition called myiasis, where maggots develop in open wounds or broken skin. This is uncommon in healthy people in temperate climates but is worth knowing about if you’re dealing with a large infestation near someone with compromised skin.
Getting Rid of Blue Flies Indoors
The single most effective step is removing the breeding source. Without it, no spray or trap will solve the problem long-term, because new flies will keep hatching. Once you’ve found and removed whatever they’re feeding on (or confirmed it’s in an inaccessible wall cavity and will dry out on its own), you can deal with the adult flies already inside.
For small numbers, a standard fly swatter or a vacuum cleaner works fine. Sticky fly traps hung in the affected room catch adults passively without chemicals. Longer ribbon-style traps work well in garages, basements, and attics where aesthetics matter less. If you’re dealing with a larger wave of flies and want faster results, a pyrethrin-based aerosol spray provides a quick contact kill. These sprays knock down flying insects on contact but don’t leave long-lasting residue, so they work best as a one-time knockdown rather than ongoing control.
For persistent problems where flies keep entering from wall cavities or exterior gaps, applying a residual insecticide around entry points (window frames, door frames, cracks in walls) can reduce numbers over several weeks. Reapplication once a month maintains the barrier.
Preventing Blue Flies From Returning
Prevention comes down to two things: keeping them out and removing what attracts them.
On the exclusion side, tight-fitting window screens are your first line of defense. But bluebottle flies also exploit small gaps that screens don’t cover. Cracks around window frames, gaps under soffits, and openings around ventilation ducts are common entry points. Sealing these with caulk prevents flies from working their way behind walls. If you live in an area where cluster flies or blow flies are a recurring seasonal problem, do your sealing by the end of August, before insects start seeking shelter for winter.
On the sanitation side, your kitchen garbage is the biggest attractant. Cover bins tightly and remove household garbage every four to five days during warm months. Flies can complete a full breeding cycle in less than a week, so a bag of food waste sitting for longer than that in summer gives them enough time to lay eggs and produce larvae. Store meats and perishables in the refrigerator rather than leaving them on counters, and keep outdoor garbage bins sealed with tight lids. If you have pets, clean up any animal waste from indoor areas promptly.
Addressing rodent problems is also indirect fly prevention. If mice or rats are entering your home and dying in wall cavities, that cycle will keep producing bluebottle flies. Sealing rodent entry points and managing any existing infestation cuts off the most common hidden food source for blow flies indoors.