Hawthorn fly
Bibio marci
Window
Typical: 01 May – 20 May (2026)
Coincides with hawthorn blossom. St Mark's fly — emerges around 25 April (St Mark's Day) traditionally.
Relevance to fishing
Big black terrestrial; falls onto water in wind and trout slam it. Three-week window only. Imitate with a size 12 black hackle.
Phenology chain
Triggered by
- Hawthorn (May) blossom typically 0d before (range -3 – 7d)
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Observation history (0)
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