Pine processionary caterpillars: what an owner of pines must know and do

The maritime pine makes the landscape of Arcachon Bay. It also produces, every winter, the most problematic caterpillar population on the Aquitaine coast.

Nests appear in December, white and woolly, hanging from branch ends. They are visible from a distance, which is misleading: you assume you would be bound to see them. You do not, because in December you are not there.

I. What the regulation says

The decree of 25 April 2022 added the pine and oak processionary caterpillars to the list of species whose proliferation is harmful to human health, under the public health code.

That text does not create a uniform obligation across the country. It gives prefects power to define, by departmental order, the surveillance, prevention and destruction measures to be applied. Such orders may prioritise the surroundings of sensitive sites — schools, nurseries, sports grounds, playgrounds — and set deadlines for action.

Separately, mayors hold general policing powers and may, on public health grounds, serve notice on an owner to deal with an infestation.

II. Why this is a genuine health issue

A processionary caterpillar carries roughly six hundred thousand microscopic urticating hairs, each barbed and loaded with a protein, thaumetopoein.

Those hairs detach at the slightest disturbance and stay airborne. They cause skin reactions, conjunctivitis, respiratory irritation and sometimes severe allergic reactions. They remain active for months in the environment, including in an empty nest fallen to the ground or in a barrow of grass cuttings.

For dogs the risk is graver: an animal that licks a procession can suffer necrosis of the tongue. It is a veterinary emergency, and a common spring scenario around the Bay.

Never destroy a nest yourself without equipment. A nest knocked down from the ground with a pole, unprotected, releases a cloud of hairs that falls straight back onto the operator. It is the classic mistake, and it costs several days of itching.

III. The biological calendar, which governs everything

Each control method has a window of effectiveness, and they do not overlap.

Bacillus thuringiensis (Bt kurstaki), the biological treatment, works only on young larvae, in autumn. Applied in January to developed caterpillars, it achieves nothing. That is the most common disappointment: people treat at the wrong moment and conclude that "it doesn't work".

Nest removal — cutting down and destroying the nests — is done in winter, in cold weather, when the caterpillars are gathered inside. It requires a tree surgeon equipped for work at height, a full suit and immediate incineration. It is not an owner's job.

The collar trap, the sleeve fitted around the trunk to intercept caterpillars descending to the ground, must be fitted before the processions start, so in December or early January. Fitted in February, it lets the first wave through.

Pheromone traps, set in June, capture male moths and reduce the following year's egg-laying. It is a background measure, better suited to a whole neighbourhood than to an isolated plot.

IV. What this means when you are not there

Look at the table again. Every intervention window but one falls between September and April.

That is precisely the period when a second home on the Bay is closed.

An owner arriving at Easter finds empty nests, caterpillars already buried in the soil, and a garden where a child or a dog cannot safely run for several weeks. They have done nothing wrong; they simply were not there at any of the four moments when something was possible.

It is a textbook case of what we call displaced seasonal maintenance: tasks that must be done outside the season of presence. There are only two answers — a mandated local contractor, or accepting the problem.

V. The neighbour question

Isolated treatment on a plot surrounded by untreated pines has limited effect: moths return each summer from neighbouring plots and from the forest.

In the wooded neighbourhoods of Cap Ferret, Pyla or Le Moulleau, control is genuinely effective when it is coordinated between neighbours. Several residents' associations around the Bay organise collective campaigns — street-scale pheromone trapping, joint nest removal — with a cost-effectiveness far beyond anything an individual can achieve.

If your neighbourhood does not organise one, it is often because nobody has suggested it.