Source : Webster's Revised Unabridged Dictionary (1913)

Vinegar fly \Vin"e*gar fly\
   Any of several fruit flies, esp. {Drosophila ampelopophila},
   which breed in imperfectly sealed preserves and in pickles.

Source : WordNet®

vinegar fly
     n : flies whose larvae feed on pickles and imperfectly sealed
         preserves