Are fruit flies harmful?

Can fruit flies make you sick?

They can move germs about. Nobody has shown them making anyone ill at home. That gap is the whole answer.

You will have read that they carry E. coli, Salmonella and Listeria. That comes from lab studies, and here is what they did. In 1999, researchers collected fruit flies on a compost heap of rotting apples and peaches. In the lab, the flies picked up a harmless strain of E. coli easily, inside and out, and carried it into cuts on clean apples, where it multiplied over the next 48 hours. The authors wrote that more studies under real conditions were needed to confirm it.

A 2018 study went further. It used a related species, Drosophila repleta, in purpose-built lab enclosures, and showed the flies moving E. coli O157 and Salmonella from contaminated food onto surfaces, and onto a doughnut and lettuce. The bacteria stayed on the flies for the full 48 hours they were watched. The Listeria in that study was a harmless stand-in, a relative used because the real thing is dangerous to work with. The paper itself is written for food businesses.

So a fly can carry what it walked through. A family doctor quoted by Cleveland Clinic puts the home risk plainly: fruit flies are a nuisance but not really a health threat, and researchers have yet to prove the transfer raises your chances of food poisoning.

Where they are breeding is what matters

A fly that grew up in a soft banana has been in a soft banana. A fly that came out of the sink drain has been somewhere else. Kentucky lists drains, food bins, empty bottles and cans, mops and cleaning rags as places they breed, and Oregon State says the film of muck they breed in builds up quickly in sink drains and overflows.

So if you are seeing them round the plughole or the food caddy, wipe the chopping board before you use it and keep cut fruit covered. Test Valley's advice is not to leave cut or spoiling fruit, juice, jam or pickles open in the kitchen. Then go and find the source, because the Scottish guidance says one rotting potato or an unwashed bin can breed thousands. Where fruit flies breed in a house goes through every spot worth checking.

Is food they have landed on safe to eat?

Usually, yes. The same Cleveland Clinic piece says you are unlikely to get physically sick from food a fruit fly has been on. Fruit with eggs or maggots in it is a different question. Kentucky says the damaged or over-ripe part can be cut away without binning the rest, and what to do with fruit that has maggots in it covers when to keep it and when to bin it.

Do not eat the mushy bit, though. Oregon State notes that eating food with fly eggs or larvae in it can cause short periods of enteric myiasis, where the larvae survive for a while in your gut. It is the one real reason to cut generously.

Is it safe to sleep in a room with fruit flies?

Yes. Oregon State files them under nonbiting flies. They feed by bringing up enzymes onto food and mopping up the result with sponging mouthparts. They are not in your bedroom for you.

They are there for something in the room. Minnesota names fermenting drinks such as beer and wine, and soft drink containers, and Kentucky adds empty bottles and cans. A half-finished glass of wine on the bedside table, a juice carton in the bin or an apple core under the bed is usually it. Take it out, and the flies have no reason to stay.

When it stops being a normal number

There is one official yardstick, in that Scottish guidance. It says a person will normally feel some distress if there are 50 or more flying fruit flies in any one room at one time, on three days running. That is guidance written for councils dealing with insect nuisance, not a rule for your kitchen. It is still a fair line between a few round the fruit bowl and something breeding nearby.

If you are past it, something in the house is breeding them. The guidance has a simple test for a drain: tape a clear food bag over the plughole overnight, and if they are breeding down there, the new adults end up in the bag. It also says the problem will carry on however often you spray the adults until the breeding spot is gone, which is the honest limit of fly spray on fruit flies. Clearing them means finding what the fruit flies are breeding in first.