FWIW, there are still mayfly hatches, but timing is the key.
June is a relative "gap" in mayfly hatches in PA. That said, every evening on many waters there will be a fair number of cahills flying about, and it's not unheard of to get a late sulpher hatch now and then either. They are hardly "mega" hatches but usually get the occasional rise, and those fish are catchable.
Much more spotty, but there are also Drunella variety BWO's in places, and they can be in large numbers, especially with a spinner fall right at dark. They generally inhabit northern tier freestoners, but Penns has em too.
There are also some blue quill (aka jenny) spinner falls, which typically happen late morning. They can get fish looking up, but it's kind of hit or miss as timing the hatch isn't easy, and even if you time it right, they're smallish bugs, typically modest in number, and at a time of day when the sun is high so fish don't always respond real well.
Many of the rivers have just blizzard hatches of hex's throughout June and July. But as a general rule, these aren't trouty places, but could be good for smallies and other warm/cool water species.
For a more reliable trout hatch, you could wait a month or so to when the trico's really get rolling. Tiny bugs, but massive in number, and very reliable (happens basically every morning from mid July to mid-September, and really, I've fished them through October to the first frost). Morning spinner fall, can happen anytime from right at dawn up to noon or so. The hotter and sunnier it is, the earlier, heavier, and shorter lasting it is. They come off on a very high number of limestoners. In central PA Spring Creek is well known for them, and the LJR has em as well. In SC PA Falling Springs has a very well known hatch, and I'm sure there are many others as well. In SE PA, the Tully is famous for them (albeit over stocked fish). That's just to name a few. They are found on a great number of streams. Without knowing for sure, I doubt Clarks is one of them, though.
After the trico's, the fall (Baetis variety) BWO's come back in, just like they were in the early spring. But then that's pretty much it till next spring. Late April-early June is certainly the prime mayfly season in PA.
