Oak processionary caterpillars: why the stinging hairs are still dangerous in September 2026

There is one call we get every year at the end of August, and it almost always sounds the same: "It isn't an insect bite, I never saw anything sting me. I spent the afternoon pruning under an oak tree and ever since, I've had red patches on my forearms and around my neck. It burns."

Nine times out of ten, the explanation comes down to three words: oak processionary caterpillar — Thaumetopoea processionea. Or more precisely, what it left behind.

That is the whole peculiarity of this pest, and the reason it catches so many people out at the start of the school year: the caterpillar is gone, but the danger has stayed put. The processions end in July, the moths take flight in August, and yet the silken nests clinging to the trunks keep releasing hundreds of thousands of stinging hairs for months. September — the return to gardening, autumn pruning, the new school year in tree-lined playgrounds — is precisely when we come back into contact.

Brown rat with grey fur standing in green grass, snout raised and whiskers visible

Two processionary species, two calendars: don't mix them up

The most common confusion is lumping two different species under a single label, even though their cycles barely overlap.

In other words: in September, a white cocoon hanging from a pine is still young and barely irritant, whereas a greyish pouch stuck to an oak trunk is a concentrated store of stinging hairs built up over the whole spring.

That pouch is what causes trouble at the start of the school year, and it explains the majority of "unexplained" September dermatology consultations in the Île-de-France region, where oaks dominate the forest massifs of Fontainebleau, Rambouillet, Sénart, Notre-Dame and Meudon, as well as virtually all urban parks and street plantings.

What actually stings: the setae, not the caterpillar

600,000 hairs per caterpillar

From its third larval instar onwards (usually late May), the caterpillar develops areas on its back called mirrors, covered in microscopic stinging setae. A mature individual is estimated to carry 500,000 to 700,000 of them. These setae measure 0.1 to 0.2 mm, are barbed like a harpoon, and detach at the slightest draught.

They do not contain injected venom like a wasp's. Their aggressiveness comes from two combined mechanisms:

• a mechanical action: the barbs anchor themselves in the stratum corneum of the skin or in the mucous membranes and stay there;
• a chemical action: they release a protein, thaumetopoein, which triggers a massive release of histamine.

The result is therefore not an allergy in the strict sense — anyone will react on first exposure — but a violent irritant dermatitis, sometimes made worse by sensitisation in repeatedly exposed people (loggers, parks and gardens staff, tree surgeons).

Why they stay dangerous for months

The setae are made of chitin. They do not break down quickly, are not washed away by mere rain, and retain their irritant power for several years in an abandoned nest, according to the work relayed by ANSES in its assessment of stinging caterpillars (referral published in 2021, which led to pine and oak processionary caterpillars being added to the list of species harmful to human health by the decree of 26 April 2022).

A September nest therefore contains:

• the caterpillars' successive moults, saturated with setae;
• the pupal exuviae;
• droppings;
• and a silk web that acts as a sponge for hairs.

Wind, a chainsaw, a leaf blower or a simple sweep of a broom is enough to put all of that back into the air.

Recognising an exposure: the symptoms that should raise the alarm

The clinical picture described by Ameli and Vidal is fairly characteristic, and above all delayed: symptoms often appear 2 to 12 hours after contact, which completely obscures the trail in the patient's mind.

On the skin — the most common case

Red rash, in patches or linear streaks (where clothing has rubbed), intensely itchy, with a burning sensation. Uncovered areas are affected first: forearms, nape, neck, ankles, waist. The itching lasts a week on average, sometimes two.

On the eyes

Conjunctivitis, watery eyes, swollen eyelids. The serious risk, though rare, is a seta migrating into the cornea or the anterior chamber: any eye involvement warrants an ophthalmological opinion, without rubbing and without delay.

On the airways

Dry cough, sneezing, throat discomfort, more rarely an asthma attack in predisposed individuals. This is the classic scenario of the leaf blower or the mower run under an infested oak.

Severe cases

Generalised reaction with widespread urticaria, faintness, breathing difficulty: call 15 or 112. Such cases remain rare but do occur, particularly in children and after massive exposures.

In dogs, the situation is different and far more urgent. The animal explores with its nose and tongue: contact causes lingual necrosis that can progress very quickly. If a dog is drooling heavily, rubbing its muzzle or has a swollen tongue after a walk under oak trees, rinse thoroughly with clean water (without rubbing) and head straight to the vet. This is an emergency, not something to see about tomorrow.

Brown rat with lowered snout foraging for food on ground strewn with seeds and debris

The right steps after contact

The order matters, and mistakes are common.

1. Don't rub, don't scratch. Rubbing drives the setae in deeper and breaks up the barbs.
2. Remove clothing carefully, rolling it outwards, and wash it at at least 60 °C, separately from the rest of the laundry.
3. Rinse the skin thoroughly with lukewarm water, then take a full shower and shampoo your hair (hair traps enormous quantities of setae).
4. Lift off the remaining setae with wide adhesive tape, pressed onto the exposed areas then pulled off sharply — a roll of cloth duct tape does the job very well and is preferable to plucking with tweezers.
5. Rinse the eyes with single-dose saline vials if needed, holding the eyelids apart, for several minutes.
6. Calm the inflammation: over-the-counter oral antihistamines relieve the itching effectively; a topical corticosteroid may be prescribed by a doctor for extensive cases. If in doubt, the pharmacist is your first port of call.

And above all: vacuum the car and any textiles if you have transported contaminated equipment, then throw the bag out immediately, outside.

The big back-to-school trap: autumn work

September and October concentrate the highest-risk activities, precisely because nobody is thinking about caterpillars any more.

• Pruning and felling. An oak carrying nests releases a cloud of setae from the first cut of the saw. This is the leading cause of occupational accidents in this category, regularly documented by INRS in relation to biological risks in landscaping trades.
• Leaf clearing. Fallen setae accumulate in the leaf litter at the foot of the tree. A petrol leaf blower puts everything there back into suspension: it is probably the worst possible tool to use under a suspect oak.
• Firewood. Bringing in logs from an oak felled the previous summer and storing them in an attached garage amounts to installing a source of hairs inside the home.
• School playgrounds and play areas. Many schools in the Île-de-France region are planted with oaks. Local authorities normally carry out a summer survey, but a nest 8 metres up is easily missed.
• The start of the sports club season. Pitches lined with oaks, ground work in training, warm-ups on the grass: contact is direct.

If you have to work yourself in an area where nests have been reported, the bare minimum is to wear a disposable hooded type 5/6 coverall, sealed safety goggles and an FFP3 mask — not a cloth mask, which filters nothing at this scale. The equipment is thrown away after use.

Identifying a nest before the accident

In September, look on the trunk and the main scaffold branches, between 2 and 15 metres up, for:

• a flattened pouch, anywhere from fist-sized to the size of a rugby ball;
• a dirty grey to brownish colour, never the bright white of a fresh pine cocoon;
• a felted, grubby appearance, often streaked with run marks;
• on the ground, at the foot of the tree, an accumulation of small cylindrical droppings and silky debris.

A pair of compact binoculars makes this survey infinitely easier and safer than walking round the trunk with your nose in the air. Note the position, take photos, and touch nothing.

Brown rat with grey fur perched on a pale stone ledge, seen in profile in close-up

What a professional does — and why this is no job for an amateur

Removing an oak processionary nest follows a precise protocol:

• Mechanical suction using an industrial vacuum fitted with a HEPA filter and a telescopic pole, which captures the setae instead of scattering them;
• Pre-wetting the nest to weigh down the airborne hairs;
• Sealed bagging followed by disposal through a hazardous waste channel;
• Work carried out with no members of the public present, with the ground area cordoned off.

Methods to be avoided absolutely: burning the nest (the setae become airborne and inhalation is massive), hosing it down (run-off and soil contamination), cutting it off with secateurs from a ladder, or leaving it in place "since the caterpillars have gone".

On the preventive side, the battle is fought upstream, out of season:

• Pheromone trapping of male moths in July and August, to assess and reduce population pressure;
• Biological treatment with Bacillus thuringiensis var. kurstaki in spring, at the early larval stage only, applied by a Certibiocide-certified operator;
• Encouraging predators: tits (one family eats thousands of caterpillars), bats, hoopoes. Putting up tit nest boxes in autumn, 3 to 4 metres up and facing east/south-east, is a simple measure, free to run and genuinely effective over several seasons.

Co-ownerships, schools, businesses: who is responsible?

Since decree no. 2022-686 of 25 April 2022, pine and oak processionary caterpillars have been included in the list of species harmful to human health (article D. 1338-1 of the French Public Health Code). In practical terms:

• the mayor may order measures across the municipality, including on private land, under his or her public health policing powers;
• the owner or manager of an infested tree is liable for damage caused to third parties;
• a co-ownership managing agent informed of a nest in planted common areas incurs liability if nothing is done;
• an employer must include the risk in the single occupational risk assessment document when employees work in green spaces.

In short, reporting in writing matters. A dated letter or email to the managing agent, the landlord or the town hall, with a photo, is the foundation for everything else.

Key takeaways

• The caterpillars have been gone since July, the stinging hairs are still there — and will remain so all autumn and winter.
• Symptoms are delayed by several hours, which is why the diagnosis gets missed.
• The steps that make the difference: don't rub, shower, wash clothes at 60 °C, tape on the skin, saline in the eyes.
• In dogs, it is an immediate veterinary emergency.
• Never burn or pull down a nest yourself: professional suction with HEPA filtration.

If you have spotted a grey pouch on an oak in your garden, your residence or your children's school, have it treated before the big autumn storms, which will scatter the nest's contents across the whole plot. Our teams work throughout the Île-de-France region for diagnosis, safe removal and setting up a prevention plan for the following season.

Sources: ANSES (assessment of stinging caterpillars); Ameli.fr, "Chenilles processionnaires" factsheet; Vidal, caterpillar-related dermatoses; INRS, biological risks in green spaces; decree no. 2022-686 of 25 April 2022.