In two separate instances, once in July and once in August, I flew to a trees mystery island and clear cut all but 3 trees. I have caught 4+ of every beetle in the game and have gotten models of the supposed most rare bugs in the game, and saw literally zero cicada shells so far.
NH July 12th 1:00pm - Got it! I was trying to find one for so long my eyeballs hurt! Bathers was not impressed...
I've engineered a mystery island and been trying for several hours for 3 days straight and still nothing. I've caught all the rare beetles really easily, but this cicada shell just refuses to spawn.
I used to find these with ease in new leaf. They really shouldn't be so hard to catch with how little they sell for. I've been trying for 2 days at this point.
After trying for like 4 hours, I've noticed they appear more between 5pm and 7pm because less types of other bugs appear. Also I caught mine July the 1st.