Why there are more processionary caterpillars in France this year

There are two main types of the insect and they can now be found in most of the country

A close up of a ‘procession’ line of processionary caterpillar
The caterpillars are named after their tendency to ‘process’ in lines
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[Update: April 17, 2024. This article was updated to remove references to ways to kill the caterpillars. Residents are warned not to try and kill the insects - as this could be more dangerous and environmentally damaging - and instead to avoid them as much as possible.] 

Processionary caterpillars are more prevalent this year in France, the national health and environment agency has warned.

The caterpillars - chenilles processionnaires in French - are so-named due to their tendency to ‘process’ in lines across the ground between trees and were officially classified as a harmful pest in April 2022.

Read also: Processionary caterpillars now officially a harmful species in France 

There are two main species in France, pine and oak (pin and chêne). They each live in trees of their respective types.

Read also: Two types of processionary caterpillars gaining ground in France 

Why are they particularly bad this year?

There are now more caterpillars in most of the country, experts say, due to warm weather. "Global warming is causing these caterpillars of exotic origin to move northwards,” said entomologist (insect expert) Benoît Gilles.

Emmanuel Gachet, coordinator of scientific expertise at the plant health laboratory at the official health body Agence nationale de sécurité sanitaire de l’alimentation, de l’environnement et du travail (Anses), confirmed that warmer weather changes the “geographic distributions” of the caterpillars and they have spread further north over recent years.

This is because warmer temperatures in both winter and summer “favour the development of this species, as there is a minimum temperature below which caterpillars can die”, he said. This means that there are more caterpillars in the warmer seasons.

Risky to humans and can be fatal for pets

Exposure to the caterpillars - which have tiny, irritating hairs all over their bodies - can cause irritation, inflammation, and rashes, as well as breathing difficulties for some. 

Their tiny hairs can easily come off when touched, or can simply blow into the air, and be spread by wind.

Severe health cases are isolated, but symptoms can become worse if people are repeatedly exposed to them - for example, if they work in forested areas regularly - said Mr Gachet.

“Forestry professionals, for example, can develop an allergy and more powerful symptoms as a result of a runaway immune system following repeated exposure to the venom,” he said.

A view of a processionary caterpillar nest in a pine tree

The caterpillars can also be dangerous to pets - for example if a dog eats one from the ground - and have been known to cause necrosis of the tongue or mouth, as well as breathing difficulties. These can be fatal if not treated. Any pets thought to have been exposed to the caterpillars should be taken to a vet immediately.

Children are also advised to stay away from the caterpillars, not to touch them, and certainly never to put them near their face or mouth.

Read more: What to do if your pet is hurt by processionary caterpillars in France
Read more: Health warning with France in peak period for dangerous caterpillars 

What are the symptoms of exposure to the caterpillars?

Symptoms of irritation (including in pets) are:

  • Painful, severely itchy rash within eight hours of skin contact

  • Conjunctivitis (red, painful, watery eyes) within one to four hours in case of eye contact

  • Sneezing, sore throat, difficulty swallowing and possibly breathing difficulties if inhaled

  • Hypersalivation, vomiting and abdominal pain if ingested.

The ARS (Agence régionale de santé in Île-de-France) has recently advised people “not to approach or touch these caterpillars or their nests”.

It also advises:

  • Wear long clothes when walking in the forest

  • Avoid rubbing your eyes during or after a walk

  • Wash fruit and vegetables from your garden

  • Avoid drying clothes next to infested trees

  • Take a shower and change your clothes if you suspect you have been exposed to the caterpillars.

Some birds do eat the caterpillars - especially blue tits and cuckoos. Putting up bird boxes to attract these birds may help reduce your processionary caterpillar and moth populations.