The Hawthorn Fly (Biblo marci) also known as St. Mark's fly is a species of fly from the family Bibionidae. It is found across mucho of Europe. Their common name comes from the fact that the adults usually emerge around St. Mark's Day (25th 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.