These emerger patterns can be adapted to any hatch, mostly by the colors you use & the size of the fly. As with all patterns knowledge of local insects trumps everything else. Emergers not only work well for imitating the emerging bugs they were initially designed to imitate, they also work well when fished as a wet fly, too.
You can study the rise forms of the trout to determine what stage of insect the fish are taking, or see where in the water column they're eating the bugs. However, if you're paying close attention, you'll notice the very first insects of the hatch beginning to come off the water. It's at this very early stage of the hatch that you should tie on an emerger - before you see any large number of bugs in the air.
The common school of thought is that once the fully formed adult insects begin to appear on the surface, you should switch over to some form of dry fly. Well, next time try this: fish the emerger fly all the way through the hatch. Depending on the hatch you're fishing, this can work extremely well for you. The trout seem to continue to recognize the emerging insects as an easy meal - even after your fly is the only remaining form of emerger in the water.
Throughout the hatch, experiment with how deep you put your pattern in the water. Emergers usually float in, or just under, the surface film, but there's no reason why you can't try to sink your fly or treat it with floatant to keep it high up on the surface. You can even swing these patterns like a traditional wet fly both during & before the hatch. The fly rising up through the water at the end of the swing will fool even the most careful of trout. The main thing is that you experiment with the depth of your fly throughout the hatch, because the trout will feed at different depths as the hatch progresses.
You might be surprised at what you discover doing this. Of course, all this experimenting takes great discipline while the trout are rising all around you. Still, you might be richly rewarded for continuing to fish the emerger pattern long after the hatch has come off, & you'll learn more about the feeding habits of trout & a few new tricks to catch trout.