Chenilles processionnaires

Beware of stinging hairs from processionary caterpillars!

In forests or gardens, on the ground or on tree branches, certain caterpillars may be seen following each other in single file. These "processionary" caterpillars are insects with stinging hairs. The inflammatory reactions they can cause in humans and animals are sometimes serious.

What are processionary caterpillars?

Pine processionary caterpillars (Thaumetopoea pityocampa) and oak processionary caterpillars (Thaumetopoea processionea) are insects that live in colonies. The former colonise coniferous trees from the pine family (black pine, Corsican pine, Scots pine, Aleppo pine, maritime pine, Atlas cedar, etc.) while the latter prefer deciduous trees from the oak family (sessile oak, pedunculate oak, etc.). They get their name from the fact that they follow each other in nose-to-tail processions.

Pine processionary caterpillars are orange-brown in colour, while oak processionary caterpillars are silvery-grey. They measure up to four centimetres long when fully grown. However, it can be difficult to tell them apart outside their normal habitat.

Where and when can they be found?

The pine processionary caterpillar is found over a very large part of France, primarily in the south, centre and west. Since the 1960s, it has extended its range to the north and west of the country. It is mainly seen between January and May, but also between October and December in ocean regions.

The oak processionary caterpillar is commonest in the north-east, the Paris region and north-western France, but may also be found in southern regions. Its range is extending westwards. It is mainly seen between April and July.

What are the symptoms?

The sting of processionary caterpillars comes from their hairs, which are actually detachable bristles. When these stinging bristles penetrate the skin or mucous membranes, they release a venom containing different compounds responsible for toxic reactions. These can cause inflammatory reactions, mainly affecting the skin (redness, itching, skin pain, localised oedema, hives and sometimes small blisters), eyes (conjunctivitis, eye watering, eye pain) and respiratory tract (coughing, respiratory discomfort).

In the event of repeated exposure, the venom in these stinging bristles can cause allergies, leading to a sudden drop in blood pressure, faintness or loss of consciousness.

You can experience symptoms even without touching the caterpillars: their stinging bristles, which are shed following mechanical contact or when the caterpillar feels threatened, are easily carried by the wind.

Together with French poison control centres, ANSES analysed the different cases of adverse effects associated with stinging hairs from processionary caterpillars, recorded between January 2012 and July 2019. The results are available in the November 2019 issue of Vigil'Anses (PDF)

How can you protect yourself from processionary caterpillars?

Here is some advice on how to limit the risk from toxic processionary caterpillars:

  • Do not approach or touch the caterpillars or their nests: this applies especially to children;
  • Keep away from trees containing their nests;
  • Wear long clothing when walking in the forest or near infested trees;
  • Avoid rubbing your eyes during or after a walk;
  • Wash fruit and vegetables from your garden thoroughly if there is any infestation nearby;
  • Avoid drying laundry next to infested trees;
  • If you suspect you have been exposed to the caterpillars, take a shower and change your clothes;
  • In the event of any signs of a life-threatening emergency (respiratory distress, etc.), dial 15 (in France) or go to the hospital emergency department;
  • If any symptoms of poisoning occur, seek advice from a doctor or call a poison control centre;
  • If you come into contact with a caterpillar, take photos of it for easier identification;
  • If pets are affected, seek advice from a veterinarian or call one of the veterinary poison control centres (the Western France Animal and Environmental Poison Control Centre [CAPAE-Ouest] or the National Information Centre for Veterinary Toxicology [CNITV]).

> Read our leaflet on processionary caterpillars (PDF, in French).

What are the impacts on animals?

Oak and pine processionary caterpillars have caused injuries in various animal species. The reported cases of exposure almost exclusively concerned the pine processionary caterpillar: dogs accounted for 92% of cases and cats for around 7%. Unlike human cases, injuries in animals are mainly located in the oral cavity. This is due to the way in which they are exposed: dogs licking or eating caterpillars, horses and ruminants ingesting contaminated plants. A major complication is tongue necrosis of varying degrees, which can be prevented or limited by very prompt medical treatment of the animal. Lesions of the legs and digestive system may also occur.

What is the impact on trees and forestry?

Processionary caterpillars are phytophagous insects and can therefore cause significant leaf loss. Their impact on forestry production remains marginal. However, the pine processionary caterpillar can stunt the growth of infested pines. Both caterpillar species can also indirectly cause dieback in certain forest stands in the event of severe defoliation: the trees are weakened and may then be exposed to secondary pests such as wood-boring insects.

What means are used to control these caterpillars?

Preventive control of pine processionary caterpillars involves diversifying the species of trees. This approach is based on planting non-host plants that are not a food source for the caterpillars.

There are also curative control measures, mainly mechanical:

  • Destruction of nests by specially-equipped professionals, if only a few trees are infested and they are easy to access and not too tall;
  • Installing traps against the pine processionary caterpillar: collars can be placed around the tree trunks before winter, when the caterpillars descend from the pines to bury themselves in the ground.

Insecticides based on extracts of Bacillus thuringiensis subsp. kurstaki or spinosad are available to combat processionary caterpillars. However, they are only authorised in France as plant protection products. Before they can be used for human or animal health purposes, these products must obtain marketing authorisation as biocides.

In January 2023, ANSES conducted an expert appraisal of the health risks associated with exposure to these caterpillars and worked on the appropriate management and control measures to be implemented, taking into account an analysis of the effectiveness of the preventive and curative control methods currently available.