Bibio marci or St. Mark’s fly or Hawthorn Fly is a species of flies from the family Bibionidae.
Their common name comes from the fact that the adults usually emerge around St Mark’s Day, 25 April.
Like most bibionid larvae, they grow up in grassy areas and are herbivores and scavengers feeding on dead vegetation or living plant roots. Bibio marci larvae are known to be root damage pests of celery, asparagus, roses, saxifrages, lawn grass, lettuce and Polyanthus. They also feed on a very large number of plant species that are commercially unimportant.