The first Sulphur activity is usually caused by invaria & rotunda not dorothea.
NYMPHS
Nymphs swim laboriously with a peculiar wiggle.
Anglers should seek appropriate sites along banks as well as behind boulders and dead falls. Use deep running, weighted nymphs during hatch less hours
Some hatch into sub-imago’s a foot below the surface. Fish Little Marryatt or Pale Evening Dun patterns wet style with jerks.
Try parachute patterns, in the surface film, during emergergence.
Joe Kohler introduced us to a nymph pattern which is a killer. Joe uses olive thread to tie in a few wisps of Lemon Wood Duck, 1/3 the length of the hook, for the tail. He dubs a cigar shaped body with dark brown angora rabbit and whip finishes. The fly is complete.
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TACTICS
Be observant. During this hatch trout are extra selective to silhouette, size and color. Size difference is considerable. Are Trout taking dorothea (18) or the larger invaria or rotunda? Are they taking duns or spinners?
Use size #16, if you get refusals go to #18. If the dun imitation is pale yellow and about #16, species doesn't matter to you.
Duns take off quickly so emerger patterns (parachutes) and spinners may be better fishing. Tie some orange at the thorax, try to wind the hackle so the orange is apparent.
Position downstream of the target and cast up and across into the fading light.
Cast to one side or the other so the fish has to turn his head to suck in the imitation.
A little twitch will sometimes put an extra "something" into the drift
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