First of all congrats to MO for his 50 continuous iris months in the wild. My COD is that I'm unable to leave eggs/larvae in situ; I have to bring them home for protection, and then release the adult back into the wood of origin.
Concerning looking for eggs and larvae at this time of the year. The woods I go to vary enormously with respect to the amount of Sallow. The one where I saw, with a friend, 30+ adults in 2.5 hours on 17th July has, literally, wall to wall Sallow: most of the rides are lined with Salix, shoulder to shoulder, in all its hybrid forms. Where do you start to look for the early stages? Although the number of females in this wood are probably the highest for the woods around here [mid-Bucks], their eggs are distributed among hundreds of Sallows, and are therefore not easy to find at all......classic needle in a haystack.
On the other hand, I've been to another wood close by, twice in the last few days, and have found 3 eggs and 6 larvae, on just three Sallows. In fact, these are three of the four [only!] suitable Sallows I've found along two very long rides. Clearly, if a female is looking to oviposit, she will lay eggs on the few available and suitable Sallows. Moral of the story: choose a wood with not too many Sallows.