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What Are the Small Flies in Compost Bins and Are They a Problem?

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Many gardeners are familiar with the experience of lifting the lid off a compost bin and seeing a cloud of small flies rise into the air in front of them.

These little flies are vinegar flies or drosophila, and the reason why they’re found in the compost bin or worm farm is because they feed on and breed in rotting fruit and vegetable matter, and their larvae (maggots) feed on micro-organisms. They’re short-lived, with a lifespan of around 50 days.

They can be identified by their bright brick-red eyes and yellow-brown bodies with distinct black rings running across the top of the abdomen, much like on a bee. These features are not easy to see without a magnifying glass as an adult vinegar fly only measures around 3mm (1/8”) in length.

There are around 1,500 different Drosophila species, and one species, Drosophila melanogaster, is extensively used in genetic research. It’s chosen for this purpose because many generations can be produced in a very short period of time. It has a very short life cycle of 8.5 days (and a bit longer in higher temperatures) to develop from an egg into an adult, is easy to breed, and lots of offspring can be produced because the female lays about 100 eggs per day.

Are Drosophila a Fruit Fly?

Even though drosophila are often called small fruit flies, they are not related to the pests known as fruit flies, and they do not cause any harm in the garden.

Drosophila, the genus of flies commonly known as vinegar flies, belong to the family Drosophilidae, and tend to gather around overripe or rotting fruit.

On the other hand, true fruit flies, such as Queensland fruit fly (Bactrocera tryoni) and Mediterranean fruit fly (Ceratitis capitata) belong to the family Tephritidae, which mainly feed on unripe or ripe fruit, making them highly destructive agricultural pests.

There’s an exception from the Drosophila genus that’s worth mentioning. It has been reported in the US that the spotted wing drosophila or cherry vinegar fly (Drosophila suzukii), an invasive vinegar fly species from Asia, has made its way into that country. The larvae of Drosophila suzukii can also feed on fresh fruit and can sometimes be a pest. They can attacks more than 100 different fruit crops, including cherries, blackberries, blueberries and grapes.

How do you distinguish Drosophila suzukii from other Drosophila species?

Drosophila suzukii male identifying characteristics:

  1. Light yellow or brown fly with red eyes.
  2. Dark spot on leading edge of the wing near the tip, centred on the first major wing vein
  3. Markings consist of bands at the ends of abdominal segments, bands are unbroken.
  4. Front feet with two combs each. Combs with 3-6 teeth. Teeth of combs parallel to the length of the foot.

Drosophila suzukii female identifying characteristics:

  1. Light yellow or brown fly with red to red-brown eyes.
  2. Large, hardened, saw-like ovipositor with dark teeth.
  3. Markings consist of bands at the ends of abdominal segments, bands are unbroken.
  4. No dark areas around wing veins.
  5. No combs on front feet.
  6. Smaller than 1/8” or 4mm.

How to Control Vinegar Flies in Compost Bins

The tiny vinegar fly, of the Drosophila genus, is often found in compost bins

Vinegar flies are totally harmless, but can become annoying when their populations grow to very large numbers.

To minimise their numbers, here are some actions that you can take:

On the bright side, the nuisance factor of these little flies can serve as a reminder to clean up any fallen fruit, which will attract them when it becomes overripe or starts rotting.

Good garden hygiene isn’t the only reason to collect fallen fruit, because if unripe or ripe fruit falls and is left there, it can become a breeding site for the real fruit flies, which are insidious pests, so keep your garden clean!

More articles on Garden Pests, Diseases and Problems

References

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