lastchance:

With the variety of species of mayflies that hatch in PA that are referred to as "olives"; nothing is absolute when it comes to the behavior of insects. Heck I've seen Tricos mating over a road that parallels a creek.

I carry a few generic poly wing olive spinners with me...

...just in case.

They are so easy to tie that it isn't a big deal. I have caught a few fish on them but my bread and butter during the fall olive hatches where I fish are emergers because with the colder temps; the duns often take a while to pop. Any good emerger pattern will work; just vary the color to suit the natural.