Health warning with France in peak period for dangerous caterpillars

‘Processionary’ caterpillars can cause severe skin and throat reactions, blisters, conjunctivitis, and asthma attacks

The caterpillars may be seen in single-file lines or in larger ‘clumped’ lines like this
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A warning has been issued over the possible health risks caused by ‘processionary’ caterpillars in France, as the seasons for both oak and pine caterpillars overlap at this time of year.

Anses, France’s national agency for food, environmental and occupational health safety, said the caterpillars’ hairs or bristles can detach and sting your skin, passing on “a very irritating and inflammatory toxic protein: thaumetopoein”.

The hairs can cause severe skin and throat reactions, blisters, conjunctivitis, and asthma attacks, including among adults.

The caterpillars are called ‘chenilles processionnaires’ in French.

Last year they were officially recognised as a harmful species in France

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

They can be recognised by their ‘fluffy’ bodies and tendency to move in long lines or ‘processions’. They make nests in trees and travel on the ground during their development. They can be seen on the ground or on tree bark. Their nests look like cotton wool in the branches.

There are two kinds of the ‘processionary’ caterpillar, and their seasons of ‘peak’ appearance can overlap in March-April.


  • Chenilles processionnaires de pin

  • Found near pine trees

  • Brown-orange in colour

  • Found across most of France, except some parts of the north-east (left/blue on the map below)

  • Seen from January to May, with a peak in March.


  • Chenilles processionnaires de chêne

  • Found near oak trees.

  • Greyish in colour

  • Found in the northern half of France, and increasing numbers of southern departments (right/green on the map below)

  • Begin to appear in March-April, with a peak in June, until August.

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

Climate change has helped the caterpillars to spread across the country, and they appear earlier and earlier each year.

Departments with at least one report of pine caterpillars (left/blue); oak caterpillars (right/green).

MAP: Anses / Map of studies 2007-2021

Sources: INRAE, DSF, ARS et Réseau FREDON


Hikers or people who spend a lot of time near trees or forests are likely to have seen the caterpillars.

They do not bite or sting. It is their hairs which are the problem and can be very irritating and inflammatory.

They can cause severe problems in children and pets, especially as they are more likely to pick them up or eat them.

They can be highly irritating even if you do not touch them. Even out of season, there is a risk, as their empty nests may still contain their stinging hairs. The hairs can also be spread by the wind.

Analysis of data from poison control centres showed that in 50% of reported cases, the people affected had not seen any caterpillars but had instead been exposed to hairs transported by wind, animal or pet fur, or gardening equipment.

The riskiest period is when the caterpillars migrate from their tree to the ground, to bury themselves in the ground before metamorphosing. Up to 300 caterpillars can then be seen in a long line coming down from the tree and continuing along the ground.

Problem symptoms

Coming into contact with the caterpillars can cause a reaction similar to that of nettle stings, including:

  • Red blister-like pimples

  • Red itchy patches, appear on the skin.

Airborne hairs can also reach the respiratory system. This causes:

  • Coughing

  • Runny nose

  • Sneezing

  • Asthma attacks (more rarely).

Finally, mouthing or touching a caterpillar with the mouth can occur in toddlers with hand-to-mouth behaviour, or pets that sniff or lick the floor when exploring their environment.

This can cause:

  • Swelling of the face and throat

  • Difficulty in breathing

  • Increased saliva production

  • Necrosis of the tongue

With heavy or prolonged exposure, the immune system can go into overdrive. This can lead to:

  • A sudden drop in blood pressure

  • Dizziness or fainting

  • Loss of consciousness

A study by Anses showed that less than 5% of the cases were severe, or concerned hairs in the eyes. However, eye exposure can lead to pain, tearing, redness, and swelling of the eyelids.

Severe cases can damage the cornea (keratitis) and cause persistent visual impairment.

Avoidance advice

People are warned:

  • Do not touch a caterpillar or go near the processions

  • Do not touch the ‘line’ where they have gone

  • Do not touch the nests. Instead, contact professionals with the equipment to intervene

  • Be careful with children

  • Avoid walking in parks and gardens where trees are affected

  • Keep pets away

  • If infested trees are near, avoid drying clothes outside. Be careful when mowing the lawn. Wash fruit and vegetables picked in the area.

  • When walking or spending time in infested tree areas (such as a forest), wear long, covering clothes. Avoid rubbing your eyes during and after a walk.

If you suspect contact, take a shower and rinse off any hairs as soon as possible. Change your clothes, and try to avoid scratching any irritations, which may make it worse.

In case of any suspected or worsening issues, consult a doctor or a vet as soon as possible. In case of urgent signs of distress, including trouble breathing, swelling in the mouth or throat, or loss of consciousness, call 15 or 112 (or 114 for people with hearing issues).

You can also consult an anti-poison centre or an eye doctor in case of eye irritation. Take a photo of the offending caterpillars if you can to help the medical professional identify the species.

Anti-caterpillar measures

Some areas have introduced measures designed to combat the caterpillars.

These might include a ‘collar’ around a tree trunk, which holds a bag that traps the caterpillars. They can also be controlled with biological insecticide sprayed on trees (the bacteria Bacillus thuringiensis kills the larvae).

Male moths can also be attracted and trapped with pheromone boxes hung in the trees, to prevent reproduction and thus limit the outbreak. Professionals may also destroy the nests by burning them.

Their predators, including bats and certain birds, may also be encouraged, as can tree varieties that do not attract them (such as ash, birch, or cedar).

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