The pine processionary caterpillar (Thaumetopoea pityocampa) is a real, well-documented hazard to dogs across most of France, not a stereotype worth downplaying. The caterpillars live in silk nests in pine and cedar trees and move across the ground in long, visible lines, usually as they descend from their nests. Their body hairs are urticating — they sting on contact — and a dog that sniffs, licks or mouths one risks a severe reaction.
The danger is specific and serious: contact with the mouth, tongue or nose can cause rapid swelling and, in bad cases, tissue death (necrosis) of the tongue, described by French veterinary sources as a genuine veterinary emergency. Some severely affected dogs have required emergency surgery or, in the worst cases, have had to be euthanised. Puppies are disproportionately at risk simply because curiosity leads them to investigate the caterpillars with their nose or mouth.
The caterpillar's range has spread northward over recent years and now covers most of France outside the coldest high-altitude areas and the far north, affecting pine and cedar trees in both towns and countryside. If your dog makes contact, treat it as an emergency and get to a vet immediately rather than waiting to see how bad it looks — don't attempt to handle the caterpillars or their nests yourself, and don't try to remove hairs without veterinary guidance.