When is the Hawthorn Fly most available to trout?

April–May, typically 12:00–16:00. It picks up from about 4°C, peaks around 12°C and tails off by 22°C.

What flies imitate the Hawthorn Fly?

Black Gnat, Hawthorn, Bibio, typically on a #12–10 hook.

How big is the Hawthorn Fly?

Adult stage: 12–14mm. That matches a #12–10 hook.

Terrestrial · Terrestrial

Hawthorn Fly

Bibio marci

Size

12–14mm

Hook size

#12–10

Hatch window

Season April–MayPeak time 12:00–16:00

Water temperature

12° peak

4°22°

How fish feed on it

St Mark's Fly, named for turning up around the saint's day in late April, and a strong flier rather than a windfall — you know it before you see it by the dangling back legs it trails in flight, a silhouette nothing else on the water shares. It needs real warmth in the air rather than the water to get up at all, around 14 to 15°C, so a mild April afternoon can have it swarming thick over the hedgerows while the river underneath is still cold from winter. It's the wind that does the damage from there: gusts catch the swarms over margins and slow water and put them down in numbers, mating pairs going in together and struggling rather than lying still. Black Gnat, Hawthorn Fly and Bibio patterns in size 12 to 10 cover it; look for the fall wherever hedgerow meets water on a breezy, mild spring afternoon.

Flies that imitate this

Black Gnat

Hawthorn

Bibio

Ecology

Rivers & stillwaterLimestone loughsLowland lochsReservoirsTolerates poor water

Where you'll find this

British IslesCommon
Atlantic fringeOccasional
Continental EuropeOccasional
The AlpsOccasional
IberiaScarce
ScandinaviaScarce

Common Occasional

Identification

Bibio marci or St. Mark's fly is a fly from the family Bibionidae. Their common name comes from the fact that the adults usually emerge around St Mark's Day, 25 April.