Three species of stinging caterpillars found in mainland France and Corsica were the focus of the risk analysis carried out by ANSES: the oak processionary caterpillar, the pine processionary caterpillar and the browntail moth caterpillar. The venoms contained in their hair can affect the skin — causing urticaria comparable to nettle rash for example — as well as the respiratory tract and eyes. They can also cause skin allergies in the event of repeated contact with stinging hairs. Tongue and skin irritations have also been observed in animals.
“The oak processionary caterpillar has extended its range in France westwards, while the pine processionary caterpillar has moved northwards and westwards,” explains Emmanuel Gachet, Head of the Expert Assessment of Biological Risks (ERB) Unit at the ANSES Plant Health Laboratory. “Areas that were only recently colonised are now having to take action against the proliferation of these insects. The risk may be higher because people living in newly infested areas generally take fewer precautions than those used to the presence of these caterpillars.”
The expert appraisal carried out by ANSES assessed the health risk associated with exposure to these caterpillars and worked on the appropriate management and control measures to be implemented, taking account of an analysis of the effectiveness of the preventive and curative control methods currently available.
Mapping the risk level per municipality
Risk maps have been drawn up for the three species of stinging caterpillar, assigning one of five classes to each French municipality. This classification is determined by the hazard level and the level of exposure of vulnerable human populations to these caterpillars.
The hazard is linked to the probability of the caterpillars’ presence, as assessed using data supplied by the Forest Health Department of the Ministry of Agriculture. The vulnerability of exposed populations is estimated on the basis of the number of inhabitants, children under the age of five and the number of professionals likely to be exposed as a result of their work.
“These maps are tools designed to help local, departmental and regional authorities tailor prevention and control measures to the level of risk,” explains Emmanuel Gachet. “The classification of municipalities according to the level of risk linked to caterpillars with stinging hairs is not static, and may evolve with the caterpillars’ geographical range or the management measures put in place. It should also be stressed that risk levels are not comparable from one species to another. The highest risk class, level 1, does not reflect the same risk of exposure, for example, depending on whether it is related to the pine processionary caterpillar or the browntail moth caterpillar.”
Anticipating risk through monitoring and prevention
Possible management measures range from informing the public in the event of an occasional outbreak of caterpillars with stinging hairs to banning access to infested woodland areas and green spaces in the municipalities most at risk. These measures should be implemented gradually, starting with a monitoring phase to confirm the presence of caterpillars and the level of infestation in a given municipality.
Depending on the level of risk, the measures recommended may be preventive or curative. For example, planting trees that are not hosts of the pine processionary caterpillar is one way of preventing the colonisation of a new area. Once the caterpillars are established, experts recommend mechanical means of elimination, such as destroying the nests or setting traps in the case of the pine processionary caterpillar.
Workers can also develop an allergic reaction following repeated exposure to the venom toxins contained in the hairs, which can even lead to anaphylactic shock. ANSES initiated a survey in late 2022 to characterise exposure, health effects and associated risk factors among the most at-risk professionals. These are people working in woods, forests or green spaces, farmers and those in the equine sector. The Agency recommends conducting prevention campaigns aimed at such workers, and advises them to wear protective equipment when working in areas infested by processionary caterpillars.
Keeping an eye on domestic animals
Animals can also fall victim to stinging caterpillars: according to data from veterinary poison control centres, 91% of reported cases of exposure involve dogs, with cats accounting for around 7% of cases. Among dogs, the tongue and mouth are most frequently irritated, young dogs being more frequently affected. Among cats, the paws and digestive system are most at risk. Cases of exposure have also been reported in horses and ruminants. In infested areas, it is therefore important to keep an eye on domestic animals.
Lack of knowledge about stinging caterpillars in French overseas territories
Little is known about the presence in tropical areas of caterpillars with stinging hairs: the species are mainly known and described when adult (butterflies) with little information on caterpillars. One aspect of ANSES's expert appraisal therefore entailed identifying and describing species whose stinging caterpillars may be found in French Guiana, the French Caribbean, Mayotte and Réunion Island. Further work will be needed to assess the health risk they could represent in these overseas regions.
The economic impact of stinging caterpillars on the environment remains to be measured
Large-scale outbreaks of processionary caterpillars affect the growth and vitality of forests, and limit the use of wooded areas for leisure activities. These consequences in turn reduce the benefits (or ecosystem services) associated with woodlands. Although one part of the appraisal focused on assessing these impacts, with a focus on the associated economic loss, data are too sparse for a precise evaluation. Further work would therefore be needed to quantify the loss of services in terms of forestry production, carbon storage and the recreational potential of the forests affected.