top of page

The Slate Drake (Isonychia)

Updated: Sep 10, 2020

As spring transitions into summer, the angling pressure on popular trout streams begins to die down. It's during this transition period that the slate drake emergence begins.

The beginning of the slate drake hatch marks the end of the spring hatch season, especially as I see it in my home state of Pennsylvania. While these insects rarely provide the blanket hatches that make other mayflies such as the sulfur and green drake so famous, the slate drake hatch can still bring a lot of trout to the net!

A couple of my go-to patterns when fishing the slate drake hatch.

The slate drake is a hatch that occurs primarily in the East and Midwest. Although they can be found out West, it is a hatch that is not of any significant importance. In my region of the country, we refer to this mayfly under the common name - slate drake. The "slate drake" goes by several other common names as well that seem to differ by region. There is the mahogany dun, the dun variant, the leadwing coachman, and the maroon drake, all of which are common names to the same insect. To simplify things though, we are talking about the genus Isonychia. The genus level is about as detailed as I would like to go; I'm not going to try and get into species for fear of putting incorrect information out there.

Iso's, as they are often called, are a mayfly that I find particularly interesting: they have a unique appearance and behave in a way that seems to differ from most other mayflies. As this article delves into each insect stage, I'll go through some of these unique attributes and how you can apply that knowledge to fishing the slate drake hatch.

The Nymph

Mayfly nymphs are divided up into four categories: crawlers, clingers, burrowers, and swimmers. These groups help describe the way in which the nymphs move and where they can be found in a stream bed. The slate drake most-certainly fits into the swimmer category. In fact, Iso's are one of the fastest swimming nymphs in the world!

Slate drake nymph specimens. Note the white dorsal stripe, gilled abdomen, and frilled tails of the naturals and see how these features can be incorporated into your nymph pattern.

These agile swimmers are fairly easy to identify as well, too. They have long, slender bodies with gills that line the abdomen on both sides. Another defining characteristic is the white dorsal-stripe that runs from the wing case down the abdomen. This is a feature that I like to build into my nymphs as a sort-of trigger or hotspot for trout to key in on. The nymphs have setae, which are hair-like projections, on their forelegs to filter feed. While iso nymph will be primarily filter feeders (like other mayfly nymphs), they are among the few types of predatory mayflies and will sometimes

Hatching Behavior

In my opinion, the hatching behavior of the slate drake is one of the things that really set this insect apart from other mayflies. In my opinion, it's this hatching behavior specifically that is important to understand when fishing the slate drake.

Slate drakes emerge in two different fashions, one of which is similar to most mayflies and the other more so resembles the hatching behavior of a stonefly.

Let's start with the later. A few days before they are about to emerge, the nymphs migrate to the shallows and other quieter sections of the stream (suck as eddies). Once they are ready to emerge, the nymphs crawl onto shore or rocks/structure within the stream and shed their shucks like a stonefly. In late June, there is a stretch of water behind my camp in North-Central Pennsylvania where nymphal shucks cover every other rock and if you are really lucky, you may just witness the dun emerge and sit on the rock to dry its wings. I've found that a great way to time the hatch is just to observe the banks for indications that the hatch has begun. When fishing nymphs during the emergence, try targeting the shallows and in the eddies to take advantage of any fish that may be keying in on iso nymphs. I have experienced success with this technique and believe that this unusual hatching behavior partially correlates to this success.

The shuck from a slate drake nymph after it had emerged.

Alternatively, the slate drake also hatches as a mayfly traditionally would; swimming up through the water column and shedding it's shuck as it breaks the surface film. Some sources suggest that this behavior is exhibited in streams that may be larger and do not have in-stream structure for the nymph to crawl onto. Whether this is the case or not, I cannot say. One fact that I can attest to however, is that trout take full advantage of this and seem to be particularly apt to taking an emerger or dun on the surface.

The Dun

In discussing the dun and its importance for dry fly fishing applications, it's important to note the duration of the slate drake hatch. This hatch is one of the longest hatches that I know of, spanning from mid-June to mid-October on my local waters. Of course the heaviest emergence is during the earlier (June) and later (October) parts of the hatch.

The back legs of the slate drake are a different color than the front forelegs (as seen in the photo), making them fairly easy to identify.

Because of its sporadic, yet prolonged, hatching behavior, trout seem to key-in on the iso duns and are often eager to take a fly off the surface. This key characteristic makes slate drake imitations the ultimate dry-fly to fish over likely holding spots; even when trout aren't actively rising. Some resources I came across attribute this conditioning as to why larger trout are sometimes taken on slate drake imitations. Larger fish will tend to be more weary of potential food sources unless they believe it is a safe bet. If the same fish is continuously seeing the occasional slate drake float by with no "disturbance" associated with it, conditioning would indicate the food to be safe to eat.

I also really like using slate drake dry flies as my suspender for a dry-dropper rig. Due to the large size of this fly, it is able to support a fairly substantial nymph underneath it. Being later in the season, I like to fish green weenies, slate drake nymphs, or any other nymph #18 or smaller.

Slate drake parachute dry-fly stored on rigging foam for a dry-dropper rig. I often use a #12 parachute Adams which seems to work just as well.

The Spinner

The slate drake spinner, unlike the sporadically emerging dun, provides much more concentrated action for dry fly anglers. After the duns molt into spinners, they will wait for good conditions before gathering over riffled sections of stream. Here, they will drop their eggs before falling spent on the water. Because the iso hatch occurs during a warmer time of the year, spinners will wait for temperatures to cool slightly before gathering above the water. Due to this behavior, spinner falls generally occur later in the evening or after dark, making them difficult to hit, especially if you go to bed as early as I do! This is why earlier in the hatch (June) and later in the hatch (October) are your best opportunity to actually hit an iso spinner fall.

A spent slate drake spinner found in a swirling back-eddy the morning after a spinner fall.

As I wrap this up, I would like to say that I am no entomologist, but rather just some guy who is interested in bugs and the role they play in their environment. With that in mind, you can find the links I used to perform my research for this video and blog post at the end of the article. I hope you found this information interesting and if you'd like to learn more, I would encourage you to check out some of the links below as well as some books on the subject.

May the hatches always be heavy,



The Bug Book by Paul Weamer

The above links are Amazon Affiliate links in which we earn a small commission at no additional cost to you. By purchasing any of the above products through our links, you help support Backyard Angling and help us to continue to create good content.

Recent Posts

See All

1 Comment

excellent video i learned a lot i have a back ground in horticulture and learned some about insects the bother ornamentals i belong to TU and have done micro studies on Linn run here in Ligonier Pa. and just love to learn more about the life cycle of micros

bottom of page