The caterpillars have nests in the trees, which can often look like cotton wool.
They will often crawl down the trunks in a “processionary” line (hence their name), and reach the ground, often to find a sunnier place to rest.
One line can include hundreds of caterpillars.
It is here that they pose a danger to humans and pets, as the little “hairs” on the outside of their bodies can be an extreme irritant.
Their hairs can be picked up and spread by the wind, and can cause intense itching, rashes, and sharp skin flare-ups. They can also irritate eyes.
They can also damage the trees, as the caterpillars devour pine needles.
The caterpillars are usually seen in early spring, and are especially prevalent where the weather has been warm. This year they have been spotted especially early - particularly in Brittany, the Haute-Garonne, and the Charente-Maritime.
Jérôme Rousselet, researcher at agricultural research institute INRA, speaking to newspaper Le Telegramme, said: “It is a little animal that loves the sun,” but also explained that too-high temperatures can also be too strong for the insects.
He added: “They like it when it is mild in winter and warm in summer.”
The insects were first seen in southern areas of France, but in recent decades have moved further north as average temperatures have risen.
The caterpillars are notoriously difficult to get rid of or avoid. Methods include traps, spraying the caterpillars and trees with special pheromones, or using natural insecticide as a last resort.
You can also request a specialised pest control through your local Mairie, among other natural methods.
Researchers even say that local authorities should help to stop their spread by planting fewer pine trees and choosing more mixed vegetation.
But Mr Rousselet advises that for individual inhabitants, the best policy is to try to avoid them.
He said: “In Mediterranean areas, which are more used to the caterpillars’ presence, there are fewer accidents: people take precautions."
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