Large Dark Olive

Large Dark Olive

The Large Dark Olive (LDO) is a member of the Baëtis family (Baetis rhodani) and only found in running water. It does not seem to matter the strength of the flow, these Olives are found throughout the British Isles in all types of river and stream. They are easily identified in April (and March) as they will usually be the only up-winged fly on the water. The emerging LDO usually measures around 15mm (a size 14).

Where To Find Them

Large Dark Olives are found in every part of the British Isles. They can be especially prolific in Central and Southern England.

As with most of the up-winged family of flies, the Large Dark Olive is found in all running water.

When To Find Them

Large Dark Olives are unusual as far as hatch timings go, where the fly angler is concerned there are two major hatches through the year (in Spring and in Autumn). LDO will hatch all year round, usually starting in earnest towards the middle of March and continuing through until the end of April.

They will then start again in earnest from September through until the end of October.

What To Look For

Large Dark Olives are renowned for their timing, you can virtually guarantee that if it's a nice, warm day the hatch will start between 11am and 12pm and continue until around 4pm. Initially, hatches will be quite sporadic, gradually increasing through to a crescendo between 1pm and 2pm. It's like ringing the dinner bell for the trout!

Hatches of LDO can be very localised, one pool may be on fire - and the one just a few yards upstream completely devoid of life.

It is also worth noting that you may find very heavy hatches of LDO's when it is very warm and sunny. you may also struggle to catch and wonder why? This may be because the newly hatched fly is not resting on the surface of the water for long (as the sun is virtually instantaneously drying out its wings) and therefore the trout has less chance to intercept it.


The lifecycle of the Large Dark Olive is very similar to that of the Mayfly (below), the only difference being that the female adult LDO will climb down bankside vegetation to lay her eggs, this is why the duns are not of too much interest to the fly angler.

LDO nymphs are often called agile darters, they are small, thin and can be found darting around the riverbed where the trout can intercept them. The best imitation for these is the famous Sawyers Pheasant Tail Nymph (beaded when used in heavier flows). When imitating the LDO nymph, please take note of the body thickness (most commercially bought flies have far too much material in the body), the body should be very thin. LDO nymphs are roughly 12 to 15mm long and only 3 or 4mm wide (just about the same as a size 14 Pheasant Tail Nymph):

Once the nymph decides the time is right to hatch into a Dun (usually between 12pm and 2pm) the nymph swims through the water column to the surface. The nymph then transforms into the dun in open water and takes a few moments for its wings to unfold and dry out (it is at this point where it is able to fly). From the time the nymph arrives at the surface to when its wings dry out is the point that trout usually take them (either just below the surface or as they emerge), so don't be too quick to retrieve your fly if it should start to sink!

Female LDO's return to the backside vegetation and crawl down them to lay their eggs. Trout are generally not interested in this stage.

From a fly fishing perspective the stages which we must try to imitate are:

  • Emergers
  • Nymphs


The most popular imitations for emerging LDO's are either of the dry flies below. Don't worry if you don't have these exact patterns, as anything which has the same general colour, profile and size will do. These flies should be fished as you would any other dry fly, and they should be used as soon as you start to see the Duns sailing down the river; this can be anytime from about 12pm until 4pm.


If you are fishing during the afternoon, then emergers are the way to go. Take a look at these for some inspiration:


Olive CdC Emerger



Image Sources

LDO Adult Images: Courtesy of Ben Lupton (thanks Ben, you’re a star)

LDO Nymph: Courtesy of


This blog is brought to you with the sole intent to spread the information around so we can all maybe learn something. If you would like to buy any of the flies contained within this email, we do hope that you will consider us and see what flies we have to offer. If you would like any further details on any of the above flies, they can be found using these links:

Olive CdC Emerger
CdC Quill Emerger

Red-Neck Pheasant Tail Nymph
Pheasant Tail Mary

Previous issues of Hatch Chat can be found on our Hatch Chat blog, here:

The next issue of Hatch Chat will cover the Hawthorn Fly.