Blue Winged Olive
Serratella ignita — BWO, Small Olive, Evening Olive
The most important small olive on British and European chalk streams — reliable, widespread, and technically demanding.
Mayflies — Ephemeroptera
Ephemerellidae
Body 7–9 mm / Hook size 14–18
Late afternoon & evening
Chalk streams, fast riffles, and gravel-bedded rivers across Europe
Lifecycle
The Blue Winged Olive nymph is a crawler, living among stones and weed on the river bed. Unlike the burrowing Ephemera nymphs, BWO nymphs are agile and free-living, with a visible olive abdomen and distinctive three tails. They are active feeders on algae, diatoms, and fine organic matter throughout their larval period of 40–60 days.
Emergence typically begins in late afternoon and peaks in the evening, triggered by falling light levels. The nymph swims to the surface and the dun hatches rapidly through the film — in choppy water, hatching occurs faster than on still glides, which is why BWO hatches on riffles can be very productive. The blue-grey wings and olive abdomen of the freshly hatched dun are distinctive.
Spinner falls (Sherry Spinner) occur most reliably in the hour before dark, the female carrying a distinctive ball of bright orange or rust-coloured eggs before depositing them on the surface. After ovipositing, the spent spinner lies flat in the film with spent wings outstretched — a crucial stage for evening surface-feeding trout.
Peak months
The Blue Winged Olive is arguably the most important hatch for European chalk stream anglers. While the Green Drake grabs the headlines, the BWO provides consistent and technically demanding fly fishing from April through October across a far wider range of rivers. It is the hatch that teaches an angler to really observe fish behaviour, to tie perfect drag-free presentations, and to match a small fly precisely.
The dun's slate-blue wings contrast with its pale olive abdomen making it identifiable when drifting on the surface. The spinner — historically called the Sherry Spinner — is a rich rusty-orange, quite different in appearance from the dun, which catches many anglers off guard when fish switch from dun to spinner.
The species is widespread across the UKl, Central Europe, and Scandinavia. It thrives equally in classic English chalk streams and in faster, more turbulent freestone rivers. In Slovenia, Czech Republic, and the Alpine rivers of Austria and Germany, BWO hatches are prolific throughout summer.
Technically, the BWO tests every skill a fly fisher has: drag detection in broken lies, presentation angle, tippet choice, and the ability to distinguish between a fish feeding on duns versus spent spinners at last light.
Fishing tips
Dun Presentation
Start with a CDC Dun or Parachute BWO in size 14–16 for the main dun emergence. Position yourself below and to the side of rising fish and cast slightly upstream with a slack-leader presentation to avoid drag. Trout feeding on BWO duns can be very rhythmic — time your cast to arrive just before the next predicted rise.
Switching to the Spinner
As light fades and rises become subtler and more frequent, the fish have often switched to the Sherry Spinner lying in the film. Carry a few size 16 spent-wing spinner patterns (rusty-orange body, spent CDC or poly wings flat on the surface). You may need to change from dun to spinner more than once during a long evening rise.
Low Light Tactics
The last 30 minutes before dark often produce the largest fish, but visibility is minimal. Use a larger parachute post on your CDC dun so you can see it, or switch to a pattern you can track by sound and feel alone. Tying on a size 14 instead of 16 in failing light loses only a marginal edge but saves fumbled knots.
Fly patterns
Other hatches
MayfliesGreen Drake
Ephemera danicaThe iconic mayfly of British chalk streams — its annual emergence signals the most celebrated fortnight in fly fishing.
MayfliesMarch Brown
Rhithrogena germanicaThe first major hatch of the European fly fishing season — a welcome sight on cold, rocky rivers in early spring.
MayfliesIron Blue Dun
Nigrobaetis nigerThe fly fisher's dark secret — a small, near-black mayfly that hatches prolifically in cold, blustery conditions when nothing else is on the water.
MayfliesCaenis / Angler's Curse
Caenis horariaThe infuriatingly tiny mayfly that hatches in such vast numbers that fish refuse to look at anything larger — the tying and presentation challenge of a lifetime.
CaddisfliesGrannom
Brachycentrus subnubilisThe first major caddis of the year — appearing in huge April swarms, the Grannom kick-starts the surface fly fishing season on chalk streams.
