Female DunMales generally hatch first sometime between sunset and sunrise and they are of no use to an angler. Females, however, hatch closer to the spinner fall in the early morning hours, so if you get up early you'll can have a crack at catching trout on an upright wing fly. Females have olive to bright green bodies, with grey/brown/black thorax. They are a short and stocky mayfly.
Male spinners will start to fall first, and sometimes they will be falling while the females are still hatching. This transitional period can be really frustrating to fish, with some trout selecting the duns and others feeding on spinners. On my stream a #24 BWO spinner also falls around the same time making the fly selection that much harder. Watch a trout rise and see if you can tell what it's taking before tying on your fly.
Female Dun and Male Spinner.
During the major spinner fall casting and presentation trumps every other factor. Fish often feed in a quick rhythm gorging on many bugs at once. If you are not hooking up, try focusing on making an accurate and perfectly timed cast instead of tying on a new fly.
When all else fails go for a fly that will stand out from the masses of Tricos. My last Trico spinner fall was the most frustrating of all, so I clipped off my tiny mayfly and tied on #14 cinnamon ant... the fish loved it!