hi
fruit flies wont harm your worms or wormery, as they all help with the composting process.
however they can be really annoying when you open the wormery up and a cloud of them fly out.
The flies (fruit flies) are attracted to the fruit/ veg in the wormery
and often are placed in the wormery as eggs on the rinds on fruit.
1) always keep food buried / covered, flies will not burrow to the food so cover with newspaper, old carpet, vermicompost from another tray.
2) Flies are attracted to the acidic smell, add plenty of crushed egg shells to neutralise the bin.
3) break the cycle by not feeding for a few weeks, with the food buried, ensure that any food collected is kept in a fly proof container
4) Diatomaceous Earth is a product that soaks up moisture, and effectively kills flies. We have never used this product, but understand that it is effective in controlling flies
Hope that helps
Ronnie