Fruit flies or drain flies: how do I tell?

You do not need to swat one to find out. Watch where they settle and how they move, then do one overnight test. Three small flies cause most of the trouble indoors: fruit flies, drain flies and fungus gnats. They look alike in the air, but each breeds in a different place, and treating the wrong one is how people lose weeks.

None of them says anything about how clean your home is. One drain, one plant pot or one forgotten potato is enough.

The quick tells

• Fruit fly: tan or light brown, a rounded little body, and often bright red eyes. Hovers in lazy loops over the fruit bowl, the bin, the recycling, a glass of wine. Mostly in the kitchen.
• Drain fly: grey or brown and fuzzy, with broad wings like a tiny moth’s. Sits still on bathroom tiles and walls near the sink, bath or shower, and makes short, weak hops when disturbed.
• Fungus gnat: dark, slim, with long legs and antennae, a bit like a very small mosquito. Hangs round houseplants and the windows near them, and runs across the compost. A little cloud lifts off when you water.

A fly that sits flat on the bathroom wall and barely moves when you get close is very likely a drain fly. Small flies in a bathroom with no food in it point the same way.

Where each one breeds

• Fruit flies breed in anything fermenting: soft fruit, an onion going off at the back of a cupboard, juice or wine left in the recycling, a damp mop, the seal of a bin, gunk in the kitchen drain.
• Drain flies breed in the slime film inside pipes, traps and overflows, especially drains that are slow or rarely used.
• Fungus gnats breed in damp compost. The larvae live in the top layer of the pot.

Settle it in one night

1. Tape over the drains. Before bed, lay clear tape over each plughole, sticky side down, with a small gap for air. Flies on the tape in the morning mean drain flies, and tell you which drain.
2. Put out a small dish in the kitchen with a little cider vinegar and a drop of washing up liquid. Fruit flies go to it. If the dish stays empty while the bathroom fills up, think drain.
3. Push a yellow sticky card into a plant pot. Dark, slim flies on it point to fungus gnats.
4. Look at a caught one up close. A phone camera, zoomed in, shows fuzzy moth wings or red eyes clearly enough to tell.

You can also photograph one and scan it: Shoo checks fruit flies against drain flies, and says “not sure” and asks for another photo rather than guess. Either way, the tape and the dish usually settle it.

Kids and pets: sticky cards and tape have no poison, but glue sticks to fingers and fur. Hang or stand them up high, out of a toddler’s reach and away from cats.

The first step for each

• Fruit flies: find and remove what they breed in, brush out the kitchen drain, keep fruit in the fridge for a week, and trap the adults that are left. See where they come from in a spotless kitchen and the fruit fly guide.
• Drain flies: scrub the slime film out of the pipe with a brush, then use an enzyme or foaming drain cleaner by the label. Bleach runs straight past the film. See why bleach does not work.
• Fungus gnats: let the top of the compost dry between waterings, catch adults on yellow cards, and treat the larvae with an established biological control. See fungus gnats in houseplants.

It is common to have two at once: fruit flies in the kitchen and drain flies in the bathroom. That is fine. Treat each source on its own and both will drop.

If none of these fit

If the flies are bigger, if they bite, or if they gather at a window in a room with no drain and no plants, it may be something else, such as mosquitoes or flies coming from a hidden source like a damp void. Take a clear photo of one on the tape and compare it with the fruit fly guide and the mosquito guide.