Gray or Slate Drake Mayflies

Gray or Slate Drake Mayflies

isonychia_dun2There seems to be some debate as to what genus that the larger Gray or Slate colored mayflies we come across on the Lower Yuba really are. The consensus is that they are Drakes. Are they the genus Siphlonurus or are they the genus Isonychia? I’m not an entomologist so the best I can do is research.

According to the information in the book “Flyfishers Guide to Northern California” put together by Seth Norman, the chapter authored by our local Ralph Wood, states that they are Gray Drakes or Siphlonurus occidentalis.

Hogan Brown has remarked that there is a minor Isonychia hatch on the Lower Yuba of which he has created a pattern the “Isonychia Nymph” to match.

Dave Sloan who regularly guides on the Lower Yuba has also come up with a fly to match the Isonychia mayfly, Sloan’s Bead Head Mighty May Hendrickson fly pattern He states that it is an extremely effective choice when imitating mature Hendrickson (or Isonychia) nymphs.

The Siphloeurus mayfly is commonly referred to as the “Gray Drake”. The Isonychia mayflies are commonly referred to as the “Slate Drake”.

When I looked up the Slate Drale or Isonychia mayfly on the “Trout Nut” he states ” In the West, Isonychia hatches are not very important, and they may be attributed to several minor species, especially Isonychia Velma “.

Well for now I’ll refer to them as Gray Drakes, until someone who knows more about this than me can confirm which mayfly or both are present on the Lower Yuba.

As far as matching the Drake that hatches on the Yuba and catching fish it probably doesn’t matter that much. I’ll provide the entomology on both. If anyone can shed some light on this please let me know.

Enttomology of the Gray Drake, “Siphlonurus” Mayfly

gray-drake_spinnerScientific Name – genus Siphlonurus
Common Names – Gray drake, black drake
Nymph Size – 12-20 mm (1/2 to 3/4 in)
Nymph Color – Grays
Spinner Size – 12-20 mm (1/2 to 3/4 in)
*Spinner Color – Wing: clear. Body: dark gray, burgundy, yellow-olive; bottom lighter than top
Other Characteristics – Nymph: three fringed tails; large gills; antennae less than twice as long as the width of the head. Duns and spinners: two tails.

About Gray Drakes

Gray drakes are swimmer-type mayflies. They inhabit weedy sections of slow moving streams and some lakes. In rivers, the mature nymphs move near shore, then crawl out of the water to emerge. Hatches are in the summer through September.

Nymph imitations should be fished like a small streamer, retrieving the fly near the bottom in short strips of a few inches (pause between each strip).

Because the duns don’t emerge in the water, they are seldom available to trout during a hatch. Spinners patterns are very useful, however, and the large size of the natural insect piques the interest of trout. Egg-laying occurs over slow, flat water, and that is where your spinner imitation should be cast; it can happen at almost any time of day. Unfortunately the spinner falls can be so massive that it’s difficult to get a trout’s attention or for you to distinguish your fly from the hundreds of naturals.

The Slate Drake, “Isonychia” Mayfly

isonychia_dun2Scientific Name – genus Isonychia
Common Name – Isonychia, Heindrickson



Nymph Size – 13-19 mm (1/2 to 3/4 in)
Nymph Color – Black, dark red-brown
Dun Size – 13-19 mm (1/2 to 3/4 in)
Dun Color – Wing: dark gray with mottling. Body: gray, red-brown
Spinner Size – 13-19 mm (1/2 to 3/4 in)
Spinner Color – Wing: clear. Body: dark red-brown or gray
Other Characteristics – Nymphs: slender shape; long hairs on front legs; three fringed tails. Duns and spinners: two tails; large hind wings

About Isonychias

Isonychia mayflies are strong swimmers, and live in moderate to fast flows. They are not important everywhere; in fact, they are of no importance whatsoever on most rivers. But where they occur, they are often found in good numbers and can generate aggressive and selective feeding from trout. They hatch during the summer months.

Prior to emergence, the nymphs migrate to slower water. During the migration, many drifting in the flow and are taken by trout. This creates the most productive fly fishing opportunity: dead-drifting nymph imitations near the bottom.

Nymphs crawl out of the water and onto to dry land to emerge, so there is little point in fishing a dun imitation. However, spinner falls can be important.



Isonychia Spinner


Gray and Slate Drake Tactics:

  • Use Hendrickson patterns that imitate the mature nymph life stage
  • Use an impressionistic searching fly or as a realistic imitation when matching the hatch
  • Hatches in eastern North America occur from spring (April) to early summer (June)
  • Nymphs are active throughout the day, making this nymph a fantastic searching pattern when the current hatch is unknown
  • Fish on a dead drift in medium to fast water
  • Use a high stick nymphing technique through really fast riffles
  • When targeting fish during a prolific hatch, cast upstream and drift through the main current
  • Hendrickson nymphs can work quite well during heavy Isonychia
  • When using the Hendrickson nymph to fish an Isonychia hatch, vary your presentation styles as these nymphs are quite mobile and could be anywhere within the river environment
  • Trout strike quickly and aggressively when feeding on Isonychia, so be prepared to set the hook
  • The typical nymph is size 14

Clay’s Fly Box – The Flies I carry for the Slate Drake “Isonychia” Patterns – Updated November 2015

I don’t carry many patterns to match this hatch, as i consider it to be of minor importance on the Lower Yuba River.

With that said this year I’m going to pay more attention to the hatch cycle and attempt to target it. Look for the cases along the shore below the riffles and see I can make these patterns work.

Isonychia Nymphs

hogans_isonychia_nymphHogan’s Isonychia Nymph
Hogan developed the Isonychia Nymph to match hatches on the Lower Yuba River and the North Fork of the Feather River. He noticed that the nymphs were longer than most nymphs, trimmer and swam very well. His fly is longer, trimmer and the marabou tail gives the illusion of swimming. Hogan recommends dead drifting the fly, but swinging it towards the bank prior or through runs prior to the main hatch.

sloans_isonychia_nymphSloan’s Isonychia Nymph
Dave Sloan has developed a lot of patterns while fishing Northern California rivers. His Sloans Isonychia Nymph is a good one.

mercers_poxyback_isonychiaMercer’s Poxyback Isonychia Nymph

Isonychia Emergers

nemes_soft_hackle_pheasant_tail_isonychiaNemes Soft Hackle Pheasant Tail Isonychia
Sylvester Nemes’ Soft Hackle Pheasant Tail Isonychia – Hendrickson is a mashed-up pattern, combining the tyer’s American take on a classic English North Country fly pattern tied in the drab colors of an Isonychia or Hendrickson cripple with a Sawyer’s classic Pheasant Tail nymph. This fly’s long, soft partridge hackle feathers give it lots of enticing movement and its profile is perfect for attracting hungry fish.

Isonychia Dries

gray_drake_comparadunGray Drake Comparadun
The Gray Drake Comparadun provides a great mix of lifelike silhouette, visibility, and buoyancy. Many Yellowstone area fly fishing guides go straight to comparaduns when a hatch brings up actively rising trout. This fly pattern floats in all but the fastest and roughest stream currents and trout seem to prefer this fly to many of the traditional hackled patterns. Fly fishers around the world should consider this a staple fly where large gray mayflies are present.

gray_drake_parachuteGray Drake Parachute
This Parachute Gray Drake dry fly is very visible to the fly fisher and very realistic to rising trout. It closely resembles a hatching or spent Gray Drake and lands upright more often than some other traditionally hackled trout fly patterns.

The Others – Flies that I don’y carry but look like good ones!

gray_drake_nymphSlate Gray Drake Nymph

gray_drake_crippleGray Drake Cripple

adams_parawulffAdams Parawulff – March Brown

Clay Hash

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