Fruit flies, the rubbish bin and the compost bucket

Every summer there is a week when the fruit bowl seems to grow a cloud. Tiny brownish flies, two or three millimetres long, drift up when you pick up a banana and settle back on the bench when you walk away. They are not dangerous, but they are annoying, and once they arrive they seem to multiply by the hour.

They nearly do. Understanding why is the key to getting rid of them.

What they are

The flies that gather in New Zealand kitchens are usually vinegar flies, small flies in the genus Drosophila, which are often just called fruit flies. They are brown to yellowish, with clear wings and, if you look closely, red eyes. The University of California’s integrated pest management programme describes them well in its fruit fly guide: their larvae feed in damaged, fermenting or overripe fruit, and most species do not attack sound, undamaged fruit.

That last point matters. The flies are not ruining your good fruit. They are finding the one soft peach, the split tomato, the forgotten potato in the bottom of the bag, and breeding there.

They are not the same as the exotic fruit flies, such as the Queensland fruit fly, that New Zealand’s biosecurity rules are designed to keep out. Those are larger pests that lay eggs inside sound fruit on the tree. If you ever find maggots in fruit picked from a backyard tree and suspect an exotic fruit fly, the Ministry for Primary Industries wants to hear about it through its biosecurity reporting channels.

Why a warm kitchen breeds them

Vinegar flies develop fast when it is warm. Standard references give an egg-to-adult time of about eight and a half days at 25°C for the common species, rising to around 19 days at 18°C. Each female can lay hundreds of eggs over her life. So a kitchen that sits in the mid-twenties through a humid Auckland or Tauranga February is close to ideal for them, and one overripe banana left for a week can turn into a cloud.

The same numbers explain why they are scarce in winter, and why a cleaning blitz that leaves one soft onion in the pantry seems to work for a week and then fail.

Where they are breeding

Before you do anything else, find the source. In our experience it is almost always one of these:

• The fruit bowl. One bruised or split piece is enough. Check the bottom layer.
• The vegetable bin or potato bag. A single rotting potato or onion smells faintly sweet and sour and is a favourite.
• The benchtop compost caddy. Food scraps start to ferment within a day or two in summer.
• The rubbish bin, especially the lid seal and the bottom of the bin under the liner, where juices pool.
• The recycling. Unrinsed juice bottles, wine bottles, beer cans and yoghurt pottles.
• Drains and the sink overflow. A film of food and grease in the trap or overflow can be enough.
• Spills you cannot see: under the toaster, behind the fridge, in the bottom of the dish rack.

The routine that stops the next generation

Because a new generation can arrive in about a week, the aim is to remove every breeding site and keep it that way for a fortnight or so. After that the adults die off on their own.

1. Eat it, chill it or bin it. Ripe fruit goes in the fridge once it is ripe. Bananas and stone fruit that are past their best go in the freezer for baking and smoothies.
2. Empty the compost caddy every day or two in warm weather, and rinse it with hot soapy water each time. A caddy with a tight lid, lined with a sheet of newspaper, helps.
3. Take the rubbish out more often in summer, and wash the bin and its lid properly once a week while the flies are around. Look under the liner.
4. Rinse recycling before it goes in the crate.
5. Clean the sink drain and overflow. Pull out the plug basket, scrub it, and run a bottle brush through the overflow. Pour a kettle of hot water through afterwards. Do not reach for drain cleaner for this job; physical cleaning is what removes the film.
6. Wipe under and behind things on the bench once.

Traps: useful for counting, not for fixing

A small jar of cider vinegar or a splash of wine with a drop of dish soap, covered with cling film pierced a few times, will catch vinegar flies. The soap breaks the surface tension so they sink. A piece of very ripe banana in a jar with a paper cone in the top works too.

These are worth setting because they tell you whether the numbers are going down. They will not solve the problem while there is a breeding site somewhere, because the flies emerging from it will outnumber the ones you catch. The University of California guide makes the same point about fruit fly traps: they can reduce numbers, but they will not give complete control.

Compost that does not attract flies

If you keep an outdoor compost bin or worm farm, a few habits stop it feeding flies near the back door:

• Bury fresh food scraps under a layer of dry material, such as leaves, shredded paper or old compost, every time.
• Keep the lid on and the bin a few metres from the kitchen door if you can.
• Do not let the benchtop caddy become a second compost bin. It is a short-stay container.

When it is not fruit flies

If the flies are dark, slightly furry and hang around the shower or a basin rather than the fruit bowl, they may be drain flies, also called moth flies, which breed in the film inside drains. Cleaning the drain physically is the fix. If small moths rather than flies are rising from the pantry, you are dealing with pantry pests, which are a different job altogether, and we cover them in a separate piece.

A fortnight of empty caddies, clean bins and fruit in the fridge is usually all it takes. Then the bowl can go back on the bench.