Foods that you wet yourself like worm chow, chicken scratch, egg layer mash and such should be fed on top. They can sour quickly and this allows you to see what has been eaten. Foods like vegies and scraps can be fed either way but burying them under a little bedding keeps the flies away and other potential invaders. basically if you have a problem with flies bury it somehow even if it is just a piece of wet cardboard over it to keep the flies from getting to it.