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.
I have no experience with important fishing to this species, but it is probably the Baetisca species that Ernest Schwiebert discussed in Nymphs as locally abundant on one stretch of a Catskill river.
This is the most common species of Baetisca in the East. Its official range extends into the Midwest, but populations there are overshadowed by the more prolific Baetisca laurentina.
I was fishing the Vermillion River in Minnesota south of the metro area and found a nymph similar to the Armored Mayfly Nymph. It did appear to be much more green in color. It was moving through a really muddy part of the stream, which I found interesting.