Pine and oak processionary caterpillars

Analysis of alternative methods to chemical treatment of pine and oak processionary caterpillars in an urban context

Processionary caterpillars are insects that are found in many regions of France, including urban areas. They compromise the health of trees, and also have an impact on human health as their bristles can cause inflammatory skin reactions, as well as more serious allergic reactions of the respiratory mucosa. ANSES, asked to assess alternatives to chemical treatment for the control of these insects, recommends a combination of preventive and curative measures. 

Processionary caterpillars are insects that are found in many regions of France, including urban areas, and their proliferation can cause trees to weaken. They also have an effect on human and animal health, since their urticating bristles cause allergic reactions and are responsible for skin irritations, as well as more serious allergic reactions of the respiratory mucosa.  

The caterpillars are found in different regions: the pine processionary caterpillar is mostly found around the Mediterranean Basin and the Atlantic seaboard as far as the Pyrénées-Atlantiques département, while the oak processionary caterpillar is commonest in the Alsace, Bourgogne, Ile-de-France, Centre, Poitou-Charentes and Midi-Pyrénées regions.

The tree species on which the caterpillars feed are found in urban areas, where airborne insecticide treatments cannot be used due to their effect on public health and the environment.

The work of the Agency

In 2012 the Ministries of Health, Labour and the Environment requested ANSES to provide a comprehensive review of the alternative control methods to be considered (physical, chemical, etc.) for use in urban areas which would preserve the health of both the local population and the environment. In order to effectively carry out this work, the Agency instructed its Expert Committee on Biological risks for plant health to conduct an expert appraisal.

The appraisal found that there are currently no biocidal(1) insecticides authorised for use against pine or oak processionary caterpillars. However, certain plant protection products which can be applied from the ground are available for controlling these caterpillars. 

Oak processionary caterpillars

The current scientific and technical knowledge on the biology and epidemiology (population health) of oak processionary caterpillars appears to be too scarceto allow any recommendations for their control.  Considerable research efforts must therefore be undertaken in order to establish methods for the integrated alternative management of the risks associated with the oak processionary caterpillar. 

Pine processionary caterpillars

The Agency analysed two scenarios: a "zero tolerance" scenario in which local constraints (school playgrounds, exceptional or protected trees, high-traffic public parks, pedestrian tourist areas, etc.), require the eradication of processionary caterpillar populations and another scenario in which the presence of low numbers of caterpillars might be acceptable and understood by local residents (roadside trees, large urban public parks, etc.). 

For "zero tolerance" situations

The Agency considers that, as a preventive measure, it would be preferable to avoid planting tree species that are highly vulnerable or attractive to pine processionary caterpillars in these areas, or to plant them in copses along with deciduous species having a similar rate of growth (e.g. birch trees).

Checks should also be made to ensure that there are no chrysalises (the pupal stage between the caterpillar stage and the adult moth) in the soil of the containers in which the trees are brought from nurseries for planting; alternatively, the earth in the containers should be treated. 

In situations where the trees are already in place, measures should be taken in summer to detect whether pine processionary caterpillars are present through the use of pheromone traps (traps containing chemical substances similar to hormones which attract insects).

If males are found in the traps, two curative control measures can be combined: 

  • if the trees are not too numerous or tall and they are easily accessible,the nests can be destroyed physically and insecticide can be applied from the ground in compliance with plant protection regulations;
  • in all cases, caterpillar traps (sticky bands around the tree trunks) can be set before the start of the winter processionary season (at which time the caterpillars abandon the pine trees to bury themselves in the soil), to avoid the risk of urtication caused by caterpillars on the ground.

For situations where the aim is reduction of population levels

The Agency recommends monitoring population levels of the pine processionary caterpillar with the use of pheromone traps in summer and by counting the nests in winter.

ANSES also recommends favouring preventive measures, although these can be of limited efficacy:

  • the forest floor and the edges of pine woods should be covered with dense vegetation (shrubs);
  • avoid unmixed pine plantationsby designing parks and avenues with an assortment of conifers and deciduous species, particularly fast-growing species such as birches or willows, which are repulsive to the pine processionary moth and which also provide shelter for a wider range of wildlife, and consequently more effective control;
  • biological pest control can be reinforced by installing nest boxes for insect-eating birds (hoopoes or tits) and shelters for bats.

Lastly, any curative methods used in this context should be repeatable every year to ensure a long-term effect:

  • physical destruction of nests;
  • mass trapping;
  • sexual confusion techniques or the use of processionary moth repellents in the case of isolated trees or parks (pheromone treatment);
  • in areas with high levels of infestation, insecticide treatments may be applied from the ground. 

(1) Biocidal products are formulations of active substances intended for household or industrial use. These common, everyday products include household disinfectants, insecticides and other products designed to eliminate, destroy or repel harmful organisms (fungi, bacteria, viruses). The active substance in the biocidal product can be a chemical compound or derived from a micro-organism that exerts its biocidal action on or against harmful organisms.