This mayfly, Ephemerellidae Drunella includes a couple of species, grandis, doddsi, g.flavitincta and g.grandis. These can be lumped together and considered one hatch since the traits of each are so similar.
In this area Green drakes are large greenish may flies size #12 to a size #10. The 2 principal drakes here are the grandis and doddsi. The grandis is the drake which is fished in the area until later in the year when the slightly smaller doddsi appears on the upper Pan in September. The doddsi prefers colder water hence its location closer to Reudi Reservoir.
The Roaring Fork and Frying Pan enjoy one of the longest periods for green drakes hatches in the country. The sheer size of the insect, in all stages, nymph, emerger, dun & spinner draws trout’s attention. Only the large stoneflies and the brown drake or possibly the hexagenia mayflies are capable of producing better results.
The green drake nymph is a rather large "crawler", generally a flatter and wider abdomen than swimmers or burrowing types. They inhabit moderate to fast riffles and runs. They are stout and strong, and therefore are seldom swept away in the current. Consequently anglers don't begin fishing the nymphs until they begin to emerge from July through to September depending on the area.
The emerging nymph is not a good swimmer and tends to drift helplessly downstream as it rises in the water column. Green drake nymphs frequent rocky, cobbled stream bottoms where the current is somewhat stronger, adding more futility to the swimming efforts of the nymph.
Prior to emerging the nymph migrates to a calm section of the stream and repeatedly swims up and down to the surface. When the conditions are exactly right the nymph arches its abdomen forward over the thorax which either causes or enhances the wing pad to split.
Because the duns are large, it takes a long time for their wings to dry. So after the dun emerges on the surface, it floats for long distances on quiet flows. That means the insect is available to the fish for a longer period of time.
Once hatched the Green Drakes are on the water for a period of two to three weeks. Generally the emergence begins on warmer waters. That is why in this area the hatch starts on the Colorado and the the Roaring Fork at Glenwood Springs late June early July and occurs up the Roaring Fork and into the Frying Pan over July into August as the water warms. The Roaring Fork hatch occurs generally right before dusk. However on cloudy days the drakes can come off during the afternoon depending on the degree of cloud cover.
The Frying Pan's "Drake" hatch begins later in the season, and occurs almost right at noon.
Drake nymphs can be fished in the period before the adults emerge so in faster water it is not unusual to fish large mayfly nymphs in May and June even though the actual hatch is some time away.
The most common stages to catch a higher number of fish occurs in either the emergence stage or the spinner stage, or fishing crippled dun patterns. On cooler or cloudy days the emerged dun may float on the surface of the water for a longer period, making it a more reliable food source. On warm days the wings dry faster, allowing the dun to fly away more quickly. This tends to deter trout from pursuing the dun after spinner falls have begun, as the spent spinners are a large, easy, stationary meal, while quick drying duns tend to fly away.
Even after the duns start hatching, trout will ignore them and continue to feed on the nymphs for a few more days. A Poxyback Green Drake nymph can be very effective when dead drifted near the bottom, especially near dusk.
Using green drake cripples, either to represent the emerger or a crippled dun often works well. Trout will key in on drifting flies that aren't likely to be going anywhere as they are easy prey. We have discussed cripples separately. They are all too often an overlooked pattern. Many a time I have found the cripple far more productive than the dun. So it is essential that the fly fisherman maintains a healthy range of cripple patterns in this fly box.
For variety it is also a tactic to fish the cripples under the water where it is a little more turbulent. This is consistent with the drake being unable to emerge and getting caught in faster water and drawn under.
The spinner stage of the green drake hatch is also very reliable. Trout will gorge themselves when conditions are right during this spinner fall. Trout may choose to feed exclusively on spent adults.
Because green drakes rarely hatch in large numbers, it is difficult to know where feeding trout are lying until you see a trout rise. Furthermore, hatches tend to occur on quiet runs, so a lot of blind casting will only spook the fish. Therefore the best strategy is to refrain from casting until you see a natural insect taken by a trout, then cast to that fish.
Trout feeding during a Green Drake emergence seem to switch abruptly from hatching nymphs to duns, and the angler need to watch rise forms carefully. Fish bulge (sometimes maybe a dorsal fin will break the water) when taking nymphs and suck when they take duns. Trout retain a ‘memory’ of these large insects for several weeks after the hatch and will still accept an imitation of the Green Drake. That is why on the Frying Pan drakes will work well into September.