Government agencies are warning that two types of caterpillar which hatch at this time of year can cause serious health problems.
The oak and pine processionary caterpillars grow thousands of pointed hairs on their backs and can cause severe allergic reactions.
They are prevalent in pine and oak forests across France. They hatch at the end of April, and continue to live in caterpillar form until mid-June.
The pine variety is more common in the south west of the country, but the oak caterpillars are spreading north as Europe gets warmer.
There have been severe outbreaks as far north as Belgium, where the army has been called out to destroy nests.
Touching the caterpillars, or even venturing near them, can cause severe skin reactions and acute conjunctivitis.
Inhaling the microscopic hairs can lead to asthma attacks.
The caterpillars are not just a human health problem, but are seen as a pest, especially to oak forests where colonies can strip trees of all their leaves.
A spokesman for the Département de la Santé des Forêts – French forestry agency said: “If you come into contact with these caterpillars you should immediately get medical help in the form of appropriate anti-histamines.
“Pets are also especially sensitive to the hairs, and should be taken to a vet immediately if they come into contact.”
The agency also warned that the microscopic hairs can infect areas around the trees for many months after the caterpillars have cocooned and turned into moths.
Photo: Louis-Michel Nageleisen