Are fruit flies seasonal?

Why late summer is the peak

Two things line up. First, there's more ripe fruit about than at any other time of year, outdoors and in your kitchen. Kentucky names tomatoes, melons, squash, grapes and other produce brought in from the garden as a common start to an indoor problem. The eggs come in on the fruit, which is why fruit flies seem to appear from nowhere. Adults also fly in through open windows and doors.

Second, the numbers have had all summer to build. Researchers studying a wild population in an orchard worked on the assumption of about ten generations over a summer, against one or two over a winter. Each female lays 25 to 35 eggs a day, according to the Montreal Insectarium. Stack ten generations of that and late summer is simply where the curve has got to.

Why a warm kitchen keeps them going

Temperature sets the speed. The Montreal Insectarium gives the range: the life cycle takes 50 days at 12°C and just eight days at 30°C. Test Valley Borough Council puts egg to adult at eight to ten days under summer conditions. That's the difference between a handful of flies and a cloud.

The species started out in the tropics and spread round the world with people. The Insectarium says it breeds year round in heated homes. A warm kitchen with a fruit bowl in it is everything a fruit fly needs, so for them it never stops being summer.

That speed is also why a few days of swatting gets you nowhere. A new batch is always a week or so behind the one you can see, and how long fruit flies live does the maths on how long you need to keep at it.

Fruit flies in winter mean the source is indoors

This is the useful bit. Outside in a British winter, it's far colder than the temperatures where the cycle runs in about a week. So a steady stream of fruit flies in December is hatching somewhere warm, and that means inside.

So go looking. Kentucky lists the usual places beyond the fruit bowl: drains, bins, empty bottles and cans, mops and cleaning cloths. Test Valley's advice is the same: make a careful search for where the larvae are breeding, and get rid of any overripe fruit or veg. Finding the breeding source in a house goes room by room.

One thing to rule out first. If the tiny flies are hanging round a houseplant rather than the fruit, they're probably fungus gnats. The RHS says those breed in damp compost and are about all year on houseplants. They look similar, they need a different fix, and telling fruit flies from fungus gnats comes down to where they gather.

When do fruit flies go away?

Outdoors, as it gets cold. At 12°C their cycle has already slowed to 50 days, so the autumn crowd thins out as the weather turns. Indoors, they don't go on their own while there's something to breed in. Bin the source and the eggs stop. The adults already flying don't go with it, so they need catching.

Cold works in your favour in the kitchen too. Test Valley suggests keeping fruit in cold storage to slow the larvae down, and a fridge should run at 0 to 5°C, well below the 12°C where the cycle already takes seven weeks. The full start-to-finish plan is in how to get rid of fruit flies, and what makes a fruit fly trap work covers the catching.

If it's autumn and you've got hundreds of bigger, slower flies upstairs or in the loft, that's a different seasonal fly altogether. A sudden mass of flies in the house explains which fly that is and what it means.