Bug FAQHawthorn Fly
Bibio marci · Order: Diptera · Family: Bibionidae
Also called: Saint Mark's Fly, St Mark's Fly, St Marks Fly
Source: Wikipedia
Adult Hawthorn Flies are 12-14 mm long, shiny black with dark hairs and translucent wings marked by dark veins. They swarm harmlessly in meadows and parks in late April and May, but their larvae feed on plant roots and can damage a wide range of garden and crop species.
Source: Wikipedia
How to recognize it
The body is glossy black with dark hairs; the wings are see-through or milky-white with dark veins along the front edge.
Source: Wikipedia
Size
Adults are typically 12 to 14 mm long.
Source: Wikipedia
Is it a pest in the house?
They also feed on many other plant species that are not of commercial importance.
Source: Wikipedia
Where it lives
Larvae develop in grassy areas, feeding on dead plant material and living roots.
Source: Wikipedia
What it eats
Adults pollinate by feeding on nectar and pollen from fruit-tree blossoms such as apple, pear, and hawthorn. Larvae are herbivores and scavengers that eat dead vegetation and plant roots.
Source: Wikipedia
Life cycle
Females lay their eggs in soil or in decaying plant material.
Source: Wikipedia
When you see it
Adults usually appear around St Mark’s Day, 25 April. Swarming peaks in late April and May, but the insects are harmless to humans.
Source: Wikipedia
Where it comes from
The species occurs across much of Europe.
Source: Wikipedia
More photos
Questions people ask
Are they harmful to gardens?
The larvae feed on plant roots and can damage a variety of garden and crop plants.
Source: Wikipedia
When do they appear?
Adults emerge around 25 April and are most noticeable in late April and May.
Source: Wikipedia
Same family (Bibionidae)
Sources
- Wikipedia: https://en.wikipedia.org/wiki/Bibio_marci
Look it up elsewhere: GBIF · iNaturalist · BugGuide · Wikipedia
Text written 2026-10-24 from the sources above and checked sentence by sentence against them. Data fetched 2026-10-22.