The late Charlie Fox introduced me to the trico mayflies over 30 years ago and ever since then it has been one of my favorite hatches both in the East and in the Rockies. One morning as I was speculating on the density of the hatch I noticed that a spider web in a down fall over the stream had caught many flies. For a rough count I held my hand up beside the spider web and counting by tens I estimated that under the span of my hand there were over 200 flies from just that days hatch. You can imagine how the many flies on the stream will turn the trout on.
For example, one morning on a small Pennsylvania stream a very large brown trout which was feeding on natural tricos kept refusing my fly. In frustration I stopped fishing and went back to sit down in the shade to evaluate my tactics. While sitting there my arm fell across my knee, setting my wristwatch up in line with my trout so I timed him. In one measured minute he took 67 natural tricos. Vince Marinaro who was sitting beside me speculates that I was getting drag that we could not discern but that turned the trout off. He suggested going back to my trout but trying a different casting position. I did this and took the trout on the first drift.
This shows how important it is to get a natural presentation on trout feeding on a heavy trico hatch. Sometimes I use an up and across cast, sometimes down and across and sometime straight across stream cast. I never cast straight upstream to them because this would allow the leader to spook the trout. I also often use a slack line cast and fish 7X or 8X.
I use to carry eleven trico patterns but now I find I can fool most of the trout on a size 22 down wing spinner if I get a natural drift.