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How much damage can they do?

I have a vigorous ginger and pumpkin wine bubbling away, I topped up the airlock with water which has quite a bit of gunk in it.

I noticed a few drops of the water dripped into the demijohn and with closer inspection quite a few dead fruit flies were in the airlock.

Do you think the wine is spoiled?

Probably not - one of the functions of the airlock is to do exactly what you've seen.

Fruit flies (also called vinegar flies, by the way) don't do any damage per se, but they have a good chance of carrying acetobacter. That's the micro-organism which turns alcohol into vinegar. A hefty dose of metabisulphite would stop that happening, but it would take quite a while to dissipate from the wine. If you do have an acetobacter infection, I wouldn't worry - how much does a gallon of wine vinegar cost?

For future reference, it isn't a good idea to simply top up the airlock. Take the whole lot out, empty the water out, rinse everything, adjust the water level and then put it all back into the neck of the demijohn.

Mike

Good stuff! Cheers Mike