Why is my compost bin full of little flies?

What the cloud of flies actually is, why it means the heap is out of balance rather than diseased, and the two changes that clear it in a week.

Difficulty: beginner
Time: 20 min
Read: 3 min
Safety: caution

Short answer

They are fruit flies and fungus gnats, and they are breeding on wet, exposed, sugary green material — usually fruit and vegetable peelings sitting on the surface. Bury every addition under a layer of torn cardboard or dry leaves and the cloud disappears within about a week. They are a nuisance, not a problem with the compost.

A heap with flies is composting. The flies are there because the food is accessible, not because anything has gone wrong biologically. That distinction matters, because the instinct is to spray or to abandon the bin, and neither is necessary.

Safety

Turning a heap raises dust and bioaerosols. Wear gloves, turn it on a still day, stand upwind, and avoid putting your face over a heap you are disturbing. Wash your hands afterwards, and keep tetanus cover current.

What you'll need

Torn cardboard, shredded paper or dry leaves
Fork
A lidded caddy for kitchen waste (optional)

Step by step

Look at what is on the surface.Exposed fruit, melon rind, tomato waste and juicer pulp are the usual culprits. Fruit flies find them within hours in warm weather.
Cover every addition immediately.A good handful of torn cardboard, shredded paper or dry leaves over each bucket of kitchen waste. This single habit is most of the fix.
Bury food waste rather than tipping it on top.Pull the surface aside with a fork, put the waste into the hollow, and draw the material back over it. Nothing edible should be visible.
Add plenty of brown material and turn once.A fly-heavy heap is almost always too wet and too green. A thorough turn with a barrowload of cardboard mixed through raises the temperature and dries it, and both discourage the flies.
Check the lid and the ventilation.A missing lid, a cracked lid or a bin sitting under a fruit tree makes it far worse. A lid that fits keeps flies out and rain off.
Deal with the kitchen caddy too.Much of the population breeds indoors and travels out with the bucket. Empty it more often, rinse it, and keep it lidded.
Leave the tiny white and translucent creatures alone.Springtails, worms, woodlice and mites in a heap are the process working. Only the flying clouds are worth acting on, and only because they are annoying.
Do not spray anything into the heap.Insecticide kills the worms and the organisms doing the composting, and it ends up spread on your beds. The management change works and this does not.

Tips

Keep a sack of torn cardboard beside the bin. Almost every compost problem is solved by having browns to hand at the moment you add greens.
A layer of grass clippings makes flies worse; a layer of cardboard makes them better.
In hot weather, a slab or a piece of old carpet over an open heap smothers the surface and the flies with it.

Common mistakes

Tipping the food caddy on top and walking away — It is an open feeding station. Covering it takes ten seconds and removes the whole problem.
Adding water to a heap that already has flies — They are there because it is too wet. It needs dry brown material, not more moisture.

If it doesn't work

Cloud of tiny flies when the lid lifts

Cause: Exposed fruit and vegetable waste — Fix: Bury each addition under torn cardboard or dry leaves.

Flies and a sour, vinegary smell

Cause: Anaerobic, too wet, too green — Fix: Turn it thoroughly with plenty of dry brown material mixed through.

Flies indoors around the kitchen caddy

Cause: Breeding in the caddy itself — Fix: Empty and rinse it more often and keep it lidded.

Small black flies rising from houseplant pots too

Cause: Fungus gnats in permanently wet compost — Fix: Let the surface of the pots dry between waterings and top them with a layer of grit.

Questions people ask

Are compost flies harmful?

No. They are a nuisance in the kitchen and around the bin, and they do no harm to the compost, the garden or you. They disappear when the food they breed on is covered.

Will the flies get into my house?

Some will, particularly if the bin is near a door. Moving the bin further away and keeping the kitchen caddy lidded and emptied deals with most of it.

Should I stop composting food waste?

No. Cover it instead. Raw fruit and vegetable waste is the most valuable nitrogen source most households have.