The processionary caterpillars have swept across the southern part of the Yvelines this late May 2026, prompting temporary closures of public parks in several municipalities and an emergency mobilization to curb health risks. In Saint-Arnoult-en-Yvelines, Arsonneau Park was shut on Wednesday, May 27, after nests were found on the town centre park’s only oak tree. In Jouars-Pontchartrain, it is the media library park that is under alert, with a request for the public not to approach the affected tree or its surroundings while awaiting a specialist intervention. The Aleu Park is also affected, though it cannot be completely closed.
In Saint-Arnoult-en-Yvelines, an insect-control operation by the local company "l'Abeille Noire" was scheduled for Thursday, May 28, to allow for a swift reopening of Arsonneau Park. The strongest outcry and mobilization are in Rambouillet, where authorities have rolled out unprecedented emergency measures: Vigie teams and a specialized contractor are carrying out caterpillar-burning defoliation operations in identified zones, with absolute priority given to areas around schools and other educational facilities. Several public parks are being temporarily closed as a precaution. Nocturnal biological treatments (from 9 p.m. to 6 a.m.) have also been deployed in 18 identified zones since March, a non-toxic method for humans, animals, and plants. Other southern communes in Yvelines find themselves in the same situation, as the issue affects the entire Yvelines area.
Each year, with the arrival of warmer days, the pine and oak processionary caterpillars reappear in the parks, forests and gardens of the Yvelines. Their early appearance in 2026, flagged as early as January in some municipalities, calls for heightened vigilance. Climate change and the scorching heat of this May have amplified the phenomenon this year, pushing the caterpillars to shed their cocoons earlier in search of cooler spots. The oak processionary caterpillars are particularly active from May to July in Île-de-France, a period when they move in colonies across the oaks and pines of parks and green spaces.
The caterpillar's hairs shed readily on contact or in gusts of wind and can trigger strong allergic reactions—redness and itching on the skin, eye irritation, or even breathing difficulties. For animals, the risk is even more serious: ingestion can cause tongue necrosis in a dog, or worse. Absolutely keep any pets away from infested areas unless supervised. These insects have been classified as a public-health nuisance by decree since April 2022.
The Île-de-France Regional Health Agency is clear about the precautions to take: do not touch the caterpillars, keep away from trees bearing nests, wear protective clothing on forest outings, and thoroughly wash home-grown fruits and vegetables. It also advises against drying laundry near infested trees. If you experience a skin, eye, or respiratory reaction, consult a doctor. To report a hotspot, the national platform signalement-chenilles-processionnaires.atlasante.fr helps pinpoint affected areas and alert the relevant services. Since closing forest areas nationwide is hard to enforce, individual caution remains the best line of defense against this stinging species.
This article is intended for anyone who visits the parks and forests of southern Yvelines with children or pets, particularly around Rambouillet, Jouars-Pontchartrain, and Saint-Arnoult-en-Yvelines. Exercise special vigilance through the end of June, the period of peak activity for the oak processionary moth caterpillars.
Also worth reading on Sortiraparis:
Invasion of processionary caterpillars in Paris and the Ile-de-France region: how to protect yourself?
Now that processionary caterpillars have been classified by decree as a species harmful to human health, these little insects with their stinging hairs continue to invade the country, particularly in the Paris region. So how can you recognize them and protect yourself and your pets, especially dogs? We tell you all about it! [Read more]
Invasion of processionary caterpillars in Paris and the Ile-de-France region: how to protect yourself?
