Sporadic hatches are rarely as outstanding as those of Isonychia. On streams with good populations, they are reliably hatching in light numbers, here and there, for most of the evening through most of the mid- to late season.
The spinners, and occasionally the duns, produce more concentrated action, but the real value of the Isonychia hatch is its duration and the size of the flies; large trout become ever watchful for them, even when they aren't emerging.
All the species of Isonychia are similar in appearance and behavior.
Welcome to the site! Many find that the Delaware is also a great place to use the slate drake as a searching fly. The rainbows of the Big D seem to be especially fond of a big Iso imitation fished anytime from late spring into the fall. Personally, I'm a big fan of imitating the nymphs.