It is always difficult to judge something like that unseen. As you talk about skeletonising of the leaves, it could be insects or insect larvae. Leaf fungus or cicadas could basically do the same. But as intense as you described it, I guess it would rather be insects and larvae. Against insect (larvae) grub bacillus thuringiensis products are well proven. Harmless to humans and pets, just spray it on the affected plants, so it will be eaten with the leaves and it will kill the insects and insect larvae. Start with the application as soon as you discover any harm on the plant. And follow the manufacturer instructions of the insecticide.