The Dark Olive is just one common name used to describe some, but not all, members of the insect family groups called Baetis (pronounced beet-us) Ephemerella, Baetidae, Drunella and Diphetor. Other names include Bluewing, Tiny Olives, Iron Blue Dun, Iron Blue Quill, Little Slate Winged Olive, Little Slate Winged brown quill, Tiny Western Olive and Little Western Iron Blue Quill. There are many species of Dark Olive mayflies.
There is often confusion among fly fishermen because the species are difficult to distinguish. One fisherman will report success fishing a great hatch of Olives another on the same river will be discussing the pattern he used to imitate a Little Slate Winged Olive hatch. Quite often they are discussing the same insect. It is very difficult to distinguish one species from another. Part of the reason for identification confusion is that the scientific names for major hatches of Baetis have changed recently. But who cares what the scientific name of a sub species of mayfly is. The trout do not care. To them food is food what ever it is called. When the trout are taking these early season mayfly, getting the size right matters more than an exact imitation. Hatches begin as early as February and continue until November. Those fly fishermen that only pick up their rod in the late spring and summer months really miss some fine fishing. Hatches normally start just after lunch. Search out back-eddies on the river. This is where I normally find olives circle endlessly around and around.
Olive mayflies imitations can catch trout throughout the flyfishing season. The disadvantage is that you have to carry a variety of hook sizes as one Blue Winged Olive will emerge in the morning and needs a hook size 12 dark olive fly whilst another emerges at dusk and requires a hook size 20 fly. The presence of rising trout gently sipping something from the water's surface with no duns visible on the water generally means that the trout are taking the suspended nymphs and emerging duns. The Pheasant Tail Nymph and Gold Ribbed Hare's Ear nymphs are ideal imitations for the olive's early stages.