Little Black Stones, Early Black Stones and Early Brown Stones are very hit or miss, usually miss, as far as hatch matching action goes.
The best action I have had has been in lowland, northern MD freestones for the Early Brown Stones. The Gunpowder has a reputation for the Little Blacks and Early Blacks, but I have never hit those right. The Early Browns there I have hit once.
To hit these right, the hatch and egg laying needs to be concentrated. It usually has to be a very nice day and the water temperature ideally goes at least to 50F. This can be very difficult to find in mid-March to early April, and in my mind does more to explain the hit or miss aspect than anything else. We had a good stretch of nice weather the last few weeks where the odds of hitting these were about as good as you are likely to get.
That is not to say you can't catch trout when it is colder to the smaller black stones. Nor does it mean you can't catch trout by skittering an EHC or similar pattern in the absence of visible hatching or egg laying at this time of year. It's just that the odds are not as good.
There is sometimes confusion about sizes. I was puzzled about this until academics started publishing articles and lists on the the internet and some streams of interest had stream wader sampling data published. Not only are there different species within each of the three general categories, there are different genera and even families. So even though they may look basically alike except size, you could often be looking at completely different bugs. To see what I mean, check this:
Plecoptera of PA link
The rule of thumb for hatching is that they migrate to shore. I presume that is the case accross the board, but since there are so many different families, genera and species, I leave open the possibility that hatching may occur instream in some cases. Preston Jennings, as I recall, thought early brown stones were instream emergers.
On the day I had my best Early Brown stone action, I did not catch a single trout (all wild browns) on a dry dead drifted or skated. A swung soft hackle, however, was great. Backing up that notion was the fact that all of the rise forms were bulges, with no part of a trout breaking surface, and the only bugs I saw were the early brown stones. Correction, I did see a very splashy take of an adult stone that fell to the stream from the concrete wall of a bridge. I switched to a dry to try and duplicate that "off the wall" plop, to no avail.
The Susquehanna at Harrisburg one of the Early Black Stone ermergences/egg laying can be very heavy, almost like a blizzard. I think this happens in April. Clouser's shop had a dry Black Stone pattern for sale of the appropriate size, I think a # 14, but typically the river is too high to wade at the time. I never tried the lower Swatara, which is nearby. On another little black stone emergence, I noticed that many of the adults crawling up trees along Front Street were wingless--strange stuff. These were smaller, more like a # 18.
