Sulphur
Ephemerella invaria
- Hook size
- #14-16
- On the water
- May 15 – Jun 30
- Peak
- May 25 – Jun 20
- Time of day
- Evening
This is June's big evening hatch, the one that brings everyone out between 7 p.m. and dark. But a large share of the insects that matter most never really float free — they stay half-stuck in their old nymphal skin, and that single detail decides which fly actually works.
Telling it apart
- Size 14–16, a pale yellow to yellow-olive body — the classic "sulphur" colour.
- Pale grey wings, three tails.
- Emerges reliably in the evening, generally between 7 and 9 p.m.
- A "crawler"-type nymph: sturdy but not flattened, living among weeds and gravel in moderate, fertile current.
- A large share of duns never fully clear their nymphal shuck and drift half-trapped inside it.
Easily confused with
The Pale Evening Dun (Ephemerella dorothea), from the same genus: smaller (16–18 versus 14–16) and it only comes off in the last twenty minutes of daylight, while the Sulphur starts around 7 p.m. and runs longer.
A crawler, not a swimmer or a burrower
The Sulphur nymph has neither the flattened body of a fast-water insect nor the burrow of a silt-dweller: it crawls among aquatic weeds and fine gravel, in moderate, fertile current. That's an in-between profile, which is why it turns up across a wider range of water types than most of the mayflies in this chapter.
Look for it in long, shallow runs with aquatic vegetation, rather than the fastest riffles or the slowest pools.
Stuck in the shuck
This is the detail that changes everything. In the Sulphur, an unusually high share of duns fail to fully clear their nymphal skin at emergence. The insect stays caught by the abdomen, half out, and drifts like that — unable to fly, unable to sink.
To a trout, that's a helpless, trapped meal available for far longer than a clean dun ever is. The result: in a real Sulphur hatch, an emerger tied with a trailing shuck often out-fishes a perfectly good, high-floating dry, even while adult duns are visible everywhere on the water.
Racing the light
The hatch starts around 7 p.m. and builds as the light drops, often peaking in the last half hour before dark — exactly when it gets hard to see your fly. Rig your leader, flies and knots before it starts; there is no time to spare once trout begin rising steadily.
Fishing it, stage by stage
Nymph
All day, among weeds and gravel.A crawler nymph dead-drifted near bottom, in fertile, moderate current.
Emerger (trailing shuck)
From the start of the hatch, around 7 p.m.In the film, motionless or on a very slow drift. This is often the single most productive fly of the whole hatch.
Every other insect worth knowing, month by month: See the full hatch chart