On many rivers in Southwestern Montana, In September and into October, Pseudocloens or Pseudo’s as many angler refer to them, are present on the Madison, and Missouri Rivers. This tiny (size 20-24) pale yellowish to olive mayflies can make things very interesting for the dry fly fishermen.
There can be great hatches of these small insects that can have anglers scratching their heads. Usually, long leaders and light tippets are required. With the small hooks needed, keeping a good size fish on is difficult to say the least. I often disregard them on the Madison River, as I find the trout will take a much larger dry fly in the faster broken water, but on the Missouri River they are a staple of many of the fish rising on the surface in the fall.
10-12 foot leaders down to a 5x tippet are required, along with a natural presentation. These mayflies hatch in abundance and can even be found on bright sunny days, when other insects are less apt to emerge.
I typically fish a bigger attractor in front of a parachute style fly tied in the appropriate color and size, but when they get picky, I go to a comparadun style fly fished alone. A soft hackle or a small unweighted pheasant tail tied on a short tippet below a bigger parachute Adams or similar attractor can also yield results.
If you are looking to challenge yourself, seek out some great fall dry fly fishing, on small dry flies on the Missouri River.