Identify Your Pest Flying insects Blow fly (bottle fly)
Blow fly (bottle fly)
Calliphoridae · Also called: Bottle fly, green bottle fly, blue bottle fly
Blow flies are shiny green, blue or bronze flies that buzz loudly and are often larger than house flies. A sudden group of them indoors often means a mouse, bird or other animal has died in a wall, attic or crawl space.
Size About 6 to 14 mm long
Colour Metallic green, blue, bronze or blue-black
Key feature Shiny metallic body that catches the light, with a loud, heavy buzz
Season Spring to fall outside; indoors late winter or after an animal dies
Where Garbage bins, pet waste, windows and rooms near a hidden dead animal
What it looks like
Blow flies are sturdy flies, usually as big as or bigger than a house fly. Their clearest clue is colour. Part or all of the body shines like metal, in bright green, blue, bronze or blue-black, depending on the species. Green bottle flies and blue bottle flies are the ones most people notice. They fly with a loud, low buzz.
Adults gather on garbage, pet waste and dead animals, where females lay eggs. The eggs hatch into pale maggots that feed for several days, then crawl away to find a dry spot to turn into adults. Indoors, you may see many adults at one window, or maggots crossing the floor near a wall.
Often confused with
House fly
Cluster fly
Flesh fly
Signs to look for
Sudden swarm indoors
A strong, bad odour
Maggots on the floor
Flies on garbage or pet waste
Recent animal activity
Risks and damage
Blow flies do not bite. Because they breed in dead animals, garbage and pet waste, they can carry bacteria onto food and kitchen surfaces. Indoors, the bigger issue is what they point to. A large group inside often means a dead animal nearby, which can cause a strong odour and attract other pests such as larder beetles. New flies can keep hatching for a week or two after the source is removed, as pupae nearby finish developing.
In Alberta
Blow flies are common outdoors across Alberta from spring to fall. A few may shelter in homes over winter and appear at windows in late winter or early spring. A sudden swarm, in Edmonton, Calgary or rural homes, most often follows a mouse, bird or squirrel dying in a wall or attic.
Blow flies are often the first insects to reach a dead animal, sometimes within minutes. Forensic entomologists age the maggots on a body to work out the least amount of time since death.
How to keep them out
Seal entry points for animals
Keep garbage sealed
Pick up pet waste
Screen windows and vents
When it is time to bring in a professional
Call a professional if blow flies keep appearing inside or a bad odour will not go away. The same goes if you suspect a dead animal in a wall, attic or crawl space. Finding and removing the animal usually stops the flies. Technica’s licensed technicians can help locate the source and seal the gaps that let animals in.
Frequently asked questions
Why are there suddenly big, shiny flies in my house?
The most common reason is a dead animal, such as a mouse or bird, inside a wall, attic or crawl space. Flies lay eggs on it, and new adults can appear about two to three weeks later. A few blow flies at windows in late winter may simply be waking from winter shelter.
Will the flies go away on their own?
Once the dead animal dries out or the waste is removed, the flies usually stop coming. But the smell can linger, and the gap that let the animal in is still there. Finding the source is the most dependable fix.
Are blow flies dangerous?
They do not bite or sting. Because they feed on waste and dead animals, keep them off food and clean counters where they land. Wear gloves to clean up maggots or a dead animal, and wash your hands after. For a dead mouse, follow Alberta’s hantavirus cleanup advice and do not sweep or vacuum droppings.