The Precessionary moth (Thaumetopoea pityocampa) is a little bug but a rather major pest to pine forests in southern Europe. This little critter builds nests (called "tents") in pine trees. When the larvae are ready that march out of the "tents" in single file to feed on the pine needles. If there are several tents in a single tree the caterpillars can literally eat very last green needle right off the tree (see images below). They are called Processionary due to their single file "procession" that can often be found along the ground under the trees. One curious thing about these processions is that if you injure any of the caterpillars in the line they will instantly all disconnect and go in different directions as if in a panic. They do this even if you only pick on the very last on in the line and out of sight of those at the front of the line. If you give them a few minutes however they will get in a new line again. Whatever you do don´t touch these with your hands as they can cause irritation to the skin.
A "tent" ready for the caterpillars to march out (below). Sometimes these get so heavy that they fall out of the trees.
The image below is of an emptied tent where they have already gone out and cleaned the green needles of f of the branches.
The image below is of a newly built tent nest.
The next image shows a young pine tree that has been totally cleaned of all green needles. This can kill the tree but most often just stunts the growth severally.
Here is a close up of how the branches look after the critters have eaten their full.
This is a picture of hope. A new growth the next year after a tree has been affected.
Some forests are so afflicted by the Thaumetopoea pityocampa moth that they whole forest looses its green color and turns a grew-brown color.