We at the Trout University store are introducing a new fly into our Perfect Fly line up and it will be available very soon. It is the Sulphur dun fly pattern. They are very similar to the Eastern Pale Evening Duns. They are very important where they exist and they can provide some excellent dry fly action when they hatch.
The Sulphurs are very plentiful, in fact, the most plentiful of all mayflies that exist in the Clinch River and the South Holston River. This is true even though the Eastern Pale Evening Duns exist in plentiful quantities in both of these streams and are usually incorrectly identified as Sulphurs. The Sulphurs are slightly smaller than the Eastern Pale Evening Duns (EPED). The abdomens of the Sulphur mayflies are a true sulfur color while the EPED are more tan in appearance. The most important difference is the two mayflies prefer slightly different types of water. You will find the EPEDs in faster water than the Sulphurs. Don't misunderstand this to mean they prefer fast water. It just means they prefer faster water than the Sulphurs which prefer moderate to slow water. The EPEDs will inhabit the moderately flowing riffles and runs, while the Sulphurs prefer the slower moving water of the pockets and pools of the streams.
These Sulphurs should start hatching about two weeks past the start of the Pale Evening Duns. The hatches often overlap. Normally they would start about the middle of May here in the East, but things are running at least two weeks ahead of schedule. I think they will begin to emerge in the Smokies as early as the last week of April to no later than the first week of May.