Whilst hiking on the Greek island Skopelos (Aegean) we heard a noise (not unlike rustling), which on investigation turned out to be thousands of the attached caterpillars on about 6-7 trees. Some of the trees had very few leaves which had not been consumed either in full, or in part, and we wondered which moth or butterfly the caterpillars would become, and if they would cause permanent damage to these trees.
I concur - gypsy moth, Lymantria dispar.
Thanks Weevil87 and Mike. I believe you are correct.