Lutte biologique et écologie spatiale                                                  Biological control and Spatial Ecology

The oak processionary caterpillar Thaumetopoea processionea (Lepidoptera : Notodontidae) is one of the major defoliators of european oaks. Its damage are mainly visible from june to mid-july, when typical 'processions' of numerous caterpillars walking head-to-tail are noticed.  Sudden outbreaks of oak processionary moths have been observed in the last decades in Western and Central Europe. Urticating hairs released by the caterpillars are significant problem for human health in rural areas (Flanders, south of the Netherlands, Bayern, Lorraine), as well as in the surrounding of large cities (Antwerpen, Eindhoven, Nancy, Paris, Vienna) or in urban green areas (London). 

Life cycle

Img1Caterpillars mainly feed at night, and congregate during the day under a silk webbing spun up between branches or on the trunk. The communal "nests" only appear from the fith larval instar onwards and protect caterpillars duting the pupation. These are bag-shaped and made of an assemblage of silk, hairs and old caterpillar skins. Adult moths typically appear during the warm nights of August and only survive a few days. Then, females lay eggs on one or two-year oak twigs, and eggs remain in place till next spring (young caterpillars hatch in April).

Host plants

The oak processionary caterpillar is specific to the Quercus genus. Its favored species in Europe are the pedunculate oak Quercus robur, the sessile oak Q. petraea as well as the Turkey oak Q. cerris. In Belgium, the caterpillar is therefore mostly found associated with pedunculate oaks. In case of heavy outbreaks, the caterpillar may also migrate to other species, such as red oak Quercus rubra, hornbeam Carpinus betulus, hazel Corylus avellana, beech Fagus sylvatica, chestnut Castanea sativa or birch Betula pubescens. Except a small number of species in the genus Quercus, such host switches do not the complete development of the caterpillar.

Forest health

The oak processionary caterpillar preferably establishes on urban trees, along forest edges and in amenity woodlands. There, numerous caterpillars are found on the sunny sides of the trunks. The species is less common  in forest stands, where a biological equilibrium rapidly establishes with Imgnatural enemies. However, colonizations of  mature forest stands have been already reported, notably in Elzas and in Lorraine, provoking defoliations and oak growth reduce in variable size sectors. In all cases, the death of the tree may never be imputed to the caterpillar, but may be observed if the processionary caterpillar is followed  by secondary pests (wood-boring and bark beetles,…) or by pathogens such as oidium. In most cases, oak processionary populations are characterized by different phases of development, culminating to a one or two years plague stage, then a decline. Between two outbreaks, sometimes speced by several years, the caterpillars remain quite discrete. While defoliations are observed in the presence of a variety of defoliators, is it sometimes uneasy to impute the damage to the oak processionary moth.

Impact on human health

ImgBecause of the skin irritation and the allergic reaction its caterpillars cause to the man and the domestic and wild animals, T. processionea is a particularly threatening insect. In addition to the long and flexible hair that cover the whole larva's body, the last instar caterpllars also produce hundred of thousands of very small urticating hairs, or setae (2-3 mm). These setae contain a specific toxic, structurally related to the pine processionary thaumetopoeine.When the caterpillar is disturbed, these setae are released in particularly huge amounts and disperse in the ambiant air streams. The contact with these setae, or their inhalation, may occasion pseudo-allergic reactions characterized by skin rashes,  conjonctivitis and respiratory problems such as pharyngitis or asthma. The abandoned nests, containing shed skins, pupal cases as well as a large number of setae, may also conserve their urticating potential for several years. 

In Belgium , height species of potentiall urticating/ allergizing caterpillars  are present : the oak processionary Thaumetopoea processionea, the browntail Euproctis chrysorrhea , the oak eggar Lasiocampa quercus, and in a lesser extent the yellowtail Euproctis similis, the first larval instar of the gypsy moth Lymantria dispar, the garden tiger moth Arctia caja, the vapourer Orgyia antiqua and Orgyia antiquoides. The oak processionary caterpillar is by far the most threatening species, provoking a great number of discomforts or even harmful damage  dunring the years of outbreaks. In example, 21 000 consultations to the generalists related to a contact with the oak processionary caterpillar have been recorded between the months May and August 1996 in the South of the Netherlands. In total, for the well-documented 1994-1996 outbreak, the total number of hit persons in the area was estimated to 60 000 – 100 000  (approximately 15% of the considered populations). The great majority of complaints concerned rashes, in a few cases related to other troubles (ocular and respiratory problems, fever, malaise, insomnia). Similar inquiries performed in the Netherlands and in Belgium have demonstrated comparable impacts during later outbreak events. Long term impacts, such as the development of an allergic sensibility have also been described. More generally, a growing number of the nuisance related to the oak processionary is observed from one or two decades in Central and Western Europe . In Austria, in 2003, a study has shown that 5 to 6 % of the surveyed persons have developed different characteristic symptoms of the allergic reaction.

Control methods

There is no direct way to completely eradicate the oak processionary. Various control measures are though practiced  in  public or private green urban areas (roads, parks, schools, etc.). They readily permit a significant reduction of populations in the upcoming season. However, all of these techniques also present some disadvantages. The first decision of the the manager is therefore to determine if a striking process has to be set up. Then, the choice of the method has to be made reasonably, according to the results of the risk inventory and the monitoring. If the risk is considered low, decision is made to do nothing, or to limit the management process to a simple warning, or to close the risky areas. In case of heavier infestation, the  colonies and the nests may be eliminated using industrial vacuums or fire treatments. These two methods present the advantage of destroying the urticating setae but are limited for trees less than eight meters. The chemical strike is only used as an adjunct in case of outbreaks but is not sustainable in controlling the caterpillar populations. Moreover, it has a considerable impact on the entomofauna and on natural enemies. The biological strike is also widely used, spraying Bacillus thuringiensis (Bt) in aqueous solution. The action of this biopesticide is based on the ingestion of a toxin that causes punching of the digestive tract of the caterpillar. Its cost is weak and its application modes are various (ground or aerial spraying, local or on larger areas…). Bt is the most widely used forest treatment used against lepidopteran defoliators. Its direct action is limited to the caterpillars that feed when the treatment arises (but also some protected species), and has indirect effects on populations of natural enemies (parasitoïdes). The Bt application has to be performed in a limited window of time, after oak bImgudburst and while the caterpillars are actively feeding (prerequisite to the product ingestion). Its application is usually recommened against second and third instar caterpillars , considered as the more sensitive to the biopesticde application. The Bt thus necessit a careful monitoring and the regular survey of the larval development in the field. The low persistance of this product on the leaves (UV rays degradation, rain leaching), also makes favorable meteorological  necessary for its application.

Pictures (up to down):
  • Thaumetopoea processionea,emerging imago (N. Meurisse)
  • Pedunculate oak in open oak, favorable site for T. processionea establishment (N. Meurisse)
  • Pedunculate oak defoliated by T. processionea (N. Meurisse)
  • Thaumetopoea processionea, last instar caterpillars construct their pupation "nests" (N. Meurisse)
  • Control of T. processionea, spraying with Bacillus thuringiens (Commune Ternat, Belgium)