Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
There are grannoms aplenty in the Bald Eagle, however by May 20th there are no grannoms on any of our central Pa streams. More likely Ryacophila (green rock worms)size 14/12, which can emerge throughout late April and May, or Hydropsyche,size 16/18 with a tan wing and a cinnamon body. The trout behavior you describe is common on the Little Juniata. In either case, fishing a pupa in the film is the way to go, especially for Hydropsyche. They spend hours struggling in the film before emerging.
I was on Bald Eagle Creek around Milesburg last year in May. It was mid morning and fish were rising everywhere. I saw caddis in the air but the rises weren't splashy. Every caddis pattern I tried, fish would rise to and refuse so I'm fairly certain they were taking caddis. Do the trout up there get selective to egg sac or spent wings because they were the only types I didn't have to try. I'm not even sure they were Grannoms, so does anyone know what other caddis hatch around May 20th? This year I'll be there from May 12 to 19, so any ideas would be terrific and much appreciated. Thanks George.