Hey now!!!!...LOLWhen it gets this time of year up there, a size 18 is as large as I would fish.
And, your holdover fish have seen so many zebra midges with the bright beads and wire that they are reluctant to commit to one. A simple thread midge tied with two contrasting colors of thread (no bead, no wire) can really up your odds. There are some thread size braids out there that can give you some sparkle if you want a little more than the normal thread.