See also Pine processionary moth and Napoleon 3rd of France
Photos above and below:
Caterpillars on the ground going to pupate.
The caterpillars form long head to tail chains on the ground when they leave the trees.
The moth of Thaumetopoea pityocampa, is fairly small and a non descript drab greyish brown, generally nocturnal and unlikely to be noticed, normally it only lives for one night so it needs to go about its business quickly.
It lays its eggs in summer, generally fairly high up in most species of pine trees and although there is evidence that in some cases types of fir are used, all other conifers are avoided.
The young caterpillars make their initial nest and then in autumn they make the larger ones which are clearly visible, this is where they spend the winter, often in colonies of several hundreds. These webs are amazingly dense, virtually impenetrable and provide not only a safe place for the caterpillars but also an environment that is often several degrees warmer than the surroundings throughout the winter period, all clever stuff! Like the adult moth, the caterpillars are nocturnal, forcing their way out of the nest at night to forage on the pine needles where they can cause a fair amount of damage, this will obviously weaken the tree but is unlikely to actually cause its death, they then return to the nest before daylight arrives.
Pine processionary caterpillars web / nest in a pine tree in France. A tree with a severe infestation looks as though it's full of cotton wool.
Pine processionary moth
Processionnaire du pin