These innocuous-looking fluff balls are actually the nests of Pine processionary caterpillars Thaumatopoea pityocampa [Processionnaire du pin /chenilles processionaires in french.]
It still sounds quite harmless until you see that the trees containing the nests are less healthy looking with missing and yellowing pine needles.
The caterpillars are voracious eaters and weaken the trees by stripping the branches and reducing its growth cycle ands this in turn makes the tree more susceptible to plant disesases.
In fact, its easy to see the yellow needles and bare twigs in this photo.
As the caterpillars grow, the nest gets filled with rubbish. There is no entrance or exit to the nest, the caterpillars just push their way through the fibres. You can see one of the mature caterpillars on the outside here.
This is the final stage of caterpillar developement
I did not take any extreme close ups because these caterpillars are very dangerous.
The caterpillar hairs are extemely toxic and easily released as a defence against predators , causing terrible rashes , swelling and breathing problems [especially in the children, asthmatics and the elderly] and eye problems such as conjuntivitis and in extreme cases, galucoma and cataracts.
Even empty nests contain a lot of hairs which continue to be dangerous - some folk have reported problems from wearing clothes that are being aired in gardens next to the empty nests.
This was another ball of caterpillars perched on the edge of a kerb.
The caterpillars are also a huge problems for pet and horse owners -I've read that eating one caterpillar can kill a cat and 3 will kill a dog.
Dogs, horses [ and seemingly to a lesser degree, cats] are attracted to the pheromone trail and pick up the irritant hairs on their tongues and this can lead to necrosis and gangrene of the tongue or death if the animal isn't treated quickly enough.
Many animals every year require partial ampuataton of the tongue or have to be put to sleep after licking or eating these caterpillars.
This is straggler trying to catch up with the rest of the caterpillars on top of the kerb. The caterpillars follow a pheromone trail and are often found walking in long lines nose to tail, which is why they were given the name processionary caterpillar.
In experiments, these caterpillars can be made to wander round and round in circles endlessly following the pheromone trail.
After all the havoc of the processional stage , the caterpillars burrow into the soil and make a cocoon/crysalis. Over time they eventually hatch out as nondescript moths , who lay eggs on the pine needles and start the cycle over again.
The caterpillars have a few natural predators - cuckoos and great crested tits will eat the caterpillars untill they are covered in the irritant hairs and the hoopoes eats the cocoons when they are buried in the ground.
Humans have a variety of ways of dealing with the caterpillars - from releasing bacteria [Bacillus thuringiensis kurstaki ] and chemical insecticides - to removing the beginning nests by crudely blasting them with shotguns [the spread of hairs is still possiblity though] or carefully cutting off the branches whilst wearing protective clothing, face masks and gloves and then incinerating the nest , branch and all.