Pine Wood Nematode, a New Threat to French Pine Forests

The species, Bursaphelenchus xylophilus, is native to North America, where it is not pathogenic to native pines but is lethal to most European pine species. It is the causal agent of pine wilt disease.
Present since the early 20th century in East Asia (first Japan, then Korea and China), where it has caused the decline of millions of pine trees, and later in Europe (Portugal in 1999 and Spain in 2008), this nematode represents a major challenge for the protection of French and European forests in the coming years.

This microscopic worm (less than 1 mm long) is transmitted to trees by an insect vector, the beetle Monochamus galloprovincialis in Europe. The insect becomes a carrier of nematodes when its healthy larva develops inside an already infected and weakened tree, which provides a favorable environment for Monochamus egg-laying. The nematodes are attracted to Monochamus larvae and enter their tracheae.
After metamorphosis, newly emerged adult insects are attracted to healthy trees for their maturation feeding. During this feeding phase, nematodes enter the xylem sap channels through wounds caused by the insect. Once inside the tree, depending on climatic conditions, they can multiply rapidly and block the circulation of raw sap, leading to disease symptoms: needle reddening, drying and shedding, wilting, and eventual tree decline due to embolism. Mortality may occur within only a few weeks. Latency (symptoms appearing in year n+1) and asymptomatic trees further complicate this interaction.

The pine wood nematode cannot move on its own from one tree to another. However, its insect vector, Monochamus, can travel on average up to 2 kilometers per day within a homogeneous pine forest, and several tens of kilometers over its lifetime. The cause of the pine wood nematode’s introduction into France remains unknown. Its spread, however, necessarily involves the movement of the insect vector. The outbreak site, located near a commercial transport route, was therefore more exposed.

In Aquitaine, research is being conducted within the EMERGREEN platform (https://ror.org/020dmyn61), a level 2+ containment facility.

These studies aim to better characterize the distribution of nematodes within the tree in order to improve early detection, to investigate the evolutionary dynamics of this invasive pathogen, to better understand the interactions between pine trees, the nematode, and associated organisms (fungi, local nematofauna) in a context of multiple risks (drought, rising temperatures, pathogenic fungi), and to identify genetic resources of maritime pine that are resistant or tolerant to the nematode.
In France, the INRAE “InterNématode” network brings together scientists specializing in the pine wood nematode, its insect vector, and its host trees, as well as stakeholders from the forest-wood sector. This network aims to study these interactions to better understand the dynamics of invasion.
In addition, the plant health epidemiological surveillance platform (Pine Wood Nematode Surveillance Working Group) seeks to assess and strengthen the monitoring system through enhanced risk analysis.