Hello –
We have a whole section on combatting fruit flies in the worm bin over at our COMPOSTERconnection website (scroll down to the bottom). Here’s the link https://www.planetnatural.com/composter-connection/indoor-composting/vermicomposting/.
Bottom line: Buried food should not attract fruit flies. These flying pests indicates that food is exposed or rotting, which may mean that there is too much of it. Citrus fruit, especially, will attract flying insects. Try not to overload your worms with too much food and consider covering the bin with a plastic sheet or a piece of old carpet. You can also use our Fruit Fly Trap to get rid of the little buggers.
Good luck!