Overview
Blow flies — also called bottle flies, bluebottles, or greenbottles — are the large, shiny, metallic flies whose sudden appearance indoors is often a message rather than a nuisance. Where a house fly is dull grey, a blow fly gleams metallic blue, green, or bronze, and it buzzes loudly against the window. Outdoors they’re nature’s clean-up crew, among the first insects to find and breed on a dead animal. That’s exactly why a sudden swarm of metallic flies inside a home usually means something has died in the structure — a mouse or rat in a wall, a bird or squirrel in the attic or chimney, or a rodent caught in a trap that was never cleared. Understanding blow flies is less about the fly and more about what it’s telling you: find the carcass, and the flies take care of themselves.
Identification
Blow flies are 6–12 mm — larger than a house fly — with a robust body and an unmistakable metallic sheen in blue, green, or bronze. The eyes are large and reddish, and in flight they’re loud and fast. Different species carry different colours: bluebottles are deep metallic blue, greenbottles bright metallic green. The metallic shine is the single best field mark separating them from other filth flies.
| Feature | Blow Fly | House Fly |
|---|---|---|
| Size | 6–12 mm, larger | 4–7 mm, smaller |
| Colour | Metallic blue, green, bronze | Dull grey |
| Thorax | Shiny, no stripes | Four dark stripes |
| Sound | Loud, buzzing | Quieter |
| Indoors often means | A dead animal in the structure | Garbage or waste nearby |
They’re occasionally confused with cluster flies at winter windows, but cluster flies are non-shiny grey with golden thorax hairs and signal overwintering, not a carcass.
Life Cycle
Blow flies develop with striking speed on a carcass. Females lay clusters of eggs directly on decaying flesh — a single female can lay hundreds — and the eggs hatch into maggots within about a day in warm conditions. The maggots feed voraciously on the soft tissue for several days, growing quickly, then leave the carcass to pupate in nearby drier material or soil. Adults emerge days later. In warm weather the entire egg-to-adult cycle can complete in one to two weeks. This speed is why a dead animal in a wall produces a sudden burst of adult flies at the windows a week or two after the animal died, and why removing the carcass ends the problem so cleanly — there’s no new food source to sustain another generation.
Habitat & Behaviour
Outdoors, blow flies are strong, wide-ranging fliers that locate carrion, garbage, and waste from a distance and arrive fast. They’re active by day and drawn to decaying meat, dead animals, pet and animal waste, and garbage with organic scraps. Indoors, the behaviour that matters is the emergence pattern: when maggots have developed on a carcass hidden in a wall void, attic, or chimney, the new adults are drawn to light and gather at windows, trying to get out. A cluster of loud metallic flies concentrated at the windows of one room — often with a faint odour nearby — is the tell-tale indoor sign.
Diet
Adult blow flies feed on a range of liquids: the fluids of decaying flesh, nectar, sap, and sugary or protein-rich material. The larvae are specialists on decaying animal tissue, which is what makes the group such efficient carrion recyclers. It’s this carrion habit that drives every practical point about them — they find, feed on, and breed in dead animals, and their presence indoors points straight back to one.
Signs of Infestation
- A sudden swarm of large metallic flies at indoor windows — the primary sign, and a strong clue to a carcass in the structure.
- A faint or growing odour of decay near the fly activity, from the dead animal itself.
- Maggots or pupae near a wall void, attic space, chimney base, or an old rodent trap.
- Clustering in one room or area rather than spread through the house, pointing to the carcass location.
- Flies appearing days after rodent or wildlife activity was noticed — the timing of a carcass decomposing.
Damage Caused
Blow flies cause no structural damage themselves. The damage in a blow-fly situation comes from the underlying cause: a decomposing animal inside the building envelope creates odour, staining, and a potential secondary pest problem as other scavengers arrive. If the carcass is a rodent, it also confirms an active or recent rodent problem that needs its own attention, and if it’s wildlife such as a squirrel or bird in the attic or chimney, it points to a wildlife entry issue that will recur until the access point is sealed.
Health Risks
Blow flies are a moderate health concern for the same reason house flies are: they move between carrion, faeces, and garbage and then onto food and surfaces, mechanically carrying bacteria such as Salmonella and E. coli. They don’t bite. The more pressing concern in most indoor cases is the decomposing animal producing the flies — a health and sanitation issue in its own right, particularly if it’s in a wall or duct near living space. In a food business, any filth fly including blow flies is treated as a contamination risk that has to be managed.
Seasonal Activity in Ontario
Outdoors, blow flies are active from spring through fall — roughly April to October — peaking in the warm months when carcasses and garbage break down fastest and the life cycle runs quickest. Indoors, however, a blow-fly swarm can happen any time of year, because it’s tied to a dead animal in the structure rather than to outdoor weather. In fact, many indoor cases show up in late fall and winter, after a rodent that moved inside for warmth dies in a wall, or after a squirrel or bird that entered an attic or chimney doesn’t get out. Cottage and rural properties — around Barrie, Muskoka, and the lakes — see more wildlife-carcass cases given the surrounding animal activity.
Where They Hide
The larvae develop on the carcass, wherever it is: inside wall and ceiling voids, in attic insulation, at the base of a chimney, in a duct, under a porch or deck, or in an uncleared trap. Adults about to emerge move toward light and gather at windows and light fixtures near the carcass. Outdoors they hide and breed in garbage bins with organic waste, compost with meat scraps, and any dead animal on the property.
How They Enter Homes
Blow flies enter the way other flies do — through open doors and windows, torn screens, and gaps — but the indoor swarms that prompt most calls don’t come from flies flying in. They come from eggs laid on an animal that died inside the structure, with the adults developing and emerging within the home. That animal got in first: a mouse or rat through a foundation gap, a squirrel or bird through a roof or chimney opening, or a bat into an attic. The fly problem is downstream of a wildlife or rodent entry problem.
Prevention Tips
- Keep rodents out — seal foundation gaps, and address any mouse or rat activity before an animal dies inside a wall.
- Screen and cap chimneys, roof vents, and attic openings to keep birds, squirrels, and bats from entering and dying inside.
- Clear and check rodent traps promptly so a caught animal doesn’t decompose and breed flies.
- Manage garbage tightly — sealed bins, no exposed meat scraps, emptied regularly.
- Cover and remove pet waste from the yard, a common outdoor breeding source.
- Fit and maintain door and window screens to block adults entering from outside.
- Investigate any unexplained odour of decay promptly, before a hidden carcass produces a swarm.
The through-line: blow flies indoors are a wildlife-and-rodent-exclusion problem as much as a fly problem.
DIY vs. Professional Treatment
If you can locate and remove the dead animal, a blow-fly problem often resolves itself — the emerging adults finish their cycle and die off with no new source. The difficulty is that the carcass is frequently in an inaccessible wall void, attic, chimney, or duct, and locating it means following the flies and the odour to the right cavity, sometimes opening a wall. That’s where professional help pays off: Sani IQ can trace the source, remove or manage the carcass, treat the area, and — crucially — identify and seal the rodent or wildlife entry point that let the animal in, so it doesn’t recur. Our work is backed by the Pest-Free-Or-It’s-Free guarantee. If you’re seeing a swarm of metallic flies indoors, book an inspection.
References
- Purdue Extension — Filth Flies (House, Blow, Bottle, Soldier and Flesh Flies)
- Utah State University Extension — Blow Flies
Last updated: July 16, 2026 · Reviewed by Sani IQ licensed technicians