Luckily I haven't suffered from this, but can pass on some information.
Your "tigre" is almost certainly the sycamore lace bug, Corythucha ciliata. It's become a quite serious pest in central and southern France, and northern Italy, particularly in hot dry summers. In itself it does disfigure the trees in the way you describe, and by leading to premature leaf fall can weaken them. More seriously, the bugs can infect plane trees with two fungal infections which can actually kill the trees.
Control or cure is difficult, chiefly because it is so hard to deal with such a large "patient". (In Cahors, where the plane-shaded boulevards are badly hit, the municipality has adopted a no-treatment policy.) Chemical controls include contact insecticides based on bifenthrin or deltamethrin. Alternatively, people have used "green" insecticides (the UK Central Science Laboratory suggests repeated use of petroleum oil, potassium phosphate or products such as Majestik or Eradicoat which contain natural plant extracts that physically coat insects). A powerful jet of water may knock down the young nymphs when they emerge in spring.
