By the way. The ladybugs or ladybirds as we call them are just the intermediate stage between larvae and ladybird so they help the tree by eating any aphids of the trees leaves.
Melampsora rust is probably the answer. It's common on Willow trees and is a fungus that eats the leaves. If you collect the leaves as soon as they fall and burn them you may help fight against the spread.
Failing that you could heavily prune or coppice the tree and burn or remove from site all the foliage. This may rejuvenate it's growth. If it is quite a stressed tree anyway this may not be your best option.