Small flies in my worm bin

Question ...

My worm composting bins are infested with small flies, presumably fruit flies, which swarm up in a cloud whenever the bins are opened. Would adding lime (ground limestone) help? I assume that derris would have a harmful effect on the worms. The bins appear to be operating normally otherwise and are not anaerobic.

Answer ...

From your description it does sound like fruit flies. They do tend to increase when the weather is warm and they are feeding on the fruit waste in your bin. As your bin appears to operating normally, in this instance adding ground limestone would not eradicate the problem. I would recommend burying the fresh waste when you add it to your bin as well as covering the top of the compost with a damp newspaper or cardboard. This will help to reduce access to the fruit that they feed on.

The more quickly the waste is processed, the fewer the flies – so you might like to consider setting up a second worm bin, so you have two operating through the summer. Whilst Derris is a permitted pesticide in an organic garden it is not recommended for use against fruit flies and could have a detrimental effect on the functioning of your worm bin. Fruit flies are harmless and not causing a health hazard in this situation.

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