Re: Yellowstone Park - Northeast Portion - early Sept.
Hoppers, beetles and ants and you are pretty much covered.
Don't overlook beetles and ants for the much easier to see and fish hoppers.
Slough Creek fish can get VERY selective that time of year, especially as you move later into September.
Sometimes very small beetles and ants are key to getting into fish and will require 5x or lighter tippet.
Sometimes later in the day they will be taking only small spinners (18-20) and will refuse everything else.
The lower water near to the access road to the campground on Slough can be very challenging and rewarding and the hybrids get quite large for park fish.
They see a lot of big flies and will often take a lazy look and swim away.
Walk around and spot cruising risers. Often times a well presented size 18 or 20 ant will draw more takes than a hopper.
Same goes for beetles size 16 to 18 maybe even 20.