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CausesModerate risk

Why Are There Suddenly Lots of Flies in My House?

Written by PestMan Editorial TeamTechnical review pendingPublished Updated

Quick answer

A sudden burst of flies indoors almost always means they are breeding nearby. Large metallic blowflies appearing in numbers usually point to a dead animal, such as a rat or possum, in the roof, a wall or under the floor. Grey house flies suggest a bin, compost or pet waste. Tiny flies point to fruit or drains.

Why this happens

Flies develop from egg to adult in about one to two weeks in warm weather, so a single breeding source can produce dozens or hundreds of flies at once. When they emerge inside the building, they gather at windows trying to get out.

Blowflies find a carcass by smell within hours. If a rodent dies in a wall void after baiting, or a possum or bird dies in the roof, a burst of blowflies often follows a week or two later, sometimes with a bad smell and maggots crawling from cornices or skirting boards.

Other sources include a forgotten bin, meat scraps, pet food, compost, a dead bird in a gutter or chimney, and rubbish under the house.

What you may be seeing

  • Many large, shiny, noisy flies at windows (blowflies).
  • A bad smell in one room or area.
  • Maggots crawling across floors, often at night.
  • Grey house flies around the kitchen and bins.

What you can do

  • Check bins, compost, pet food and recycling first.
  • Follow the smell and where flies concentrate to find a possible dead animal.
  • Check the roof void, sub-floor, wall cavities and chimney if it is safe to do so.
  • Remove the source wearing gloves, double-bag it and dispose of it in the general waste.
  • Vacuum up flies and maggots, then clean the area.

What not to do

  • Do not rely on fly spray alone; more flies will emerge until the source is gone.
  • Do not handle dead animals with bare hands.

When professional help may be needed

If the source is in a wall or ceiling you can’t reach, a pest manager or builder may need to open a small access point. If the dead animal was a rodent, it is also a sign to deal with the rodent problem. If the flies come from a dead possum, remember possums are protected wildlife.

Flies

Two-winged insects that breed in rotting organic matter. House flies, blowflies, vinegar flies and drain flies each point to a different breeding source, and finding that source is the key to getting rid of them.

Rats

Introduced rodents that live alongside people, nest in roofs, walls and gardens and gnaw through building materials and wiring. Roof rats are the most common household rat in many Australian cities.

Species mentioned

Blowflies

Family Calliphoridae

Large, often metallic green, blue or bronze flies that breed in meat and dead animals. A sudden swarm indoors usually means something has died nearby.

House Fly

Musca domestica

The common grey fly found indoors worldwide. It breeds in rubbish, compost, manure and pet waste, and can carry bacteria onto food.

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Sources & further reading

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