Large Blue-Winged Olive

Telling it apart

• Distinctly bigger than a typical Baetis, size 14–16 versus 16–22 for the Blue-Winged Olive.
• Small horn-like tubercles visible on the nymph's thorax — hence the name cornuta.
• Dark olive to olive-brown body, darker than a Baetis.
• Emerges in the morning rather than evening, in a narrow three-week window in June–July.

Not just a big Baetis

Anglers often lump any olive-toned mayfly under the generic "BWO," but Drunella cornuta isn't even in the same family as the Baetis — it belongs to the Ephemerellidae, the same family as the Sulphur and the Pale Evening Dun, rather than the Baetidae. The nymph carries small horned tubercles on its thorax, a distinguishing trait of the Ephemerellidae not found on Baetis.

The mix-up mostly comes down to colour and the fact that the two hatches can overlap in June — but size alone (14–16 versus 16–22) should settle it before you ever reach for a hand lens.

Why morning, in the heart of summer

Most of our big summer mayflies come off in the evening, as light fades. Drunella cornuta does the opposite: it emerges in the morning, generally mid-June into early July, right as the water starts warming up seriously.

Emerging early in the day, before water hits its daily temperature peak, keeps the nymph and newly emerged dun in a window where dissolved oxygen stays higher — a real advantage for an insect living in water that warms quickly at this time of year.

A short window not to miss

Three weeks, mornings only: a far narrower window than the Baetis, which runs across nearly the whole season. If you fish a river you know well in June and spot an unusually large olive in mid-morning, there's a good chance it's this one rather than a late-running Baetis.

Because the window is short and morning-only, it's an easy hatch to miss entirely if your summer habit is fishing evenings instead — worth getting out early at least once during those three weeks.