Spotted my first plasterer bee ( Colletes sp.)of the year the other day in the neighbour's garden feeding on ox-eye daisies. It soon became obvious that it was being followed around by a medium sized fly which stayed around 6" away at all times. Turns out the fly was a Sarcophagid Miltogramma species which are kleptoparasites of the bees. It presumably follows the bee back to it's nest and then lays it's eggs in there.