The pinewood nematode: a threat to French conifers
The pinewood nematode (Bursaphelenchus xylophilus) is a microscopic worm that affects conifers, especially maritime pine. It is a particularly destructive parasite responsible for severe dieback in pine forests. Transmitted by an insect vector, its spread is mainly due to the transport of wood or plants. If this parasite were introduced into France, the Landes area would be at risk due to the widespread presence of maritime pine. As the National and European Reference Laboratory for identification of the pinewood nematode, ANSES has conducted several expert appraisals to assess the risks of introduction and establishment of the organism in France and to recommend management measures for pine wood and bark.
What is the pinewood nematode?
The pinewood nematode is a microscopic worm that attacks certain conifers, mainly pines. When affected, the trees turn red, lose their needles and die within a few weeks. Originally from North America, the pinewood nematode spread to Asia before entering Europe. It is currently found in Portugal and in localised areas of Spain. The nematode is spread by an insect vector, which in Europe is the beetle Monochamus galloprovincialis. This insect becomes a carrier of the nematode when it develops in an infected tree. It then transports the nematode larvae from one tree to another.
A high risk of contamination in France
The spread of the pest is mainly due to the transport of wood or plants. The risk of contamination has increased in recent years for several reasons:
- greater demand for wood chips and bark, used for mulching or composting in agroecology;
- the circulation of very large volumes of wood and bark on the European market following the 2017 fires in the Iberian Peninsula, an area partly contaminated by the pinewood nematode;
- the discovery in France of contaminated bark and wood packaging in 2018.
The Landes area, in particular, would be at risk if the nematode were introduced because of several factors favouring the spread of the parasite: climatic conditions, the proven presence of the insect vector, and a forest area consisting of maritime pine, a species susceptible to the nematode.
In the event of a pinewood nematode outbreak, the risk of disease spread would be associated with the insect vector’s ability to transmit the nematode from tree to tree. This risk depends on:
- the nature of the products and by-products resulting from forestry exploitation and in particular their dimensions;
- the fate of these materials: storage for varying lengths of time and transport;
- the period considered: during or outside the insect vector's flying period.
ANSES, the National and European Reference Laboratory
The Nematology Unit of ANSES's Plant Health Laboratory in Rennes is the National and European Reference Laboratory for identification of the pinewood nematode. In this capacity, it has been working for more than a decade to develop innovative detection methods to ensure enhanced surveillance in France and Europe. Every week, the laboratory supervises or carries out tests on wood samples taken from ports, forests or factories, as well as insect samples. Following a request by the Ministry of Agriculture, ANSES also conducted several expert appraisals to assess the risk of introduction and establishment of the parasite and to recommend wood management measures to be applied in France.
Three expert appraisals to pre-empt the threat of pinewood nematodes
Management of pine bark potentially contaminated by the pinewood nematode
In 2018, following the discovery of bark from Portugal contaminated by the pinewood nematode and intended for sale in France, ANSES's expert appraisal focused on managing the risk posed by pine bark susceptible to this parasite.
The Agency concluded that planting young seedlings of susceptible pine – especially maritime pine – in forests increases the risk of the nematode becoming established in France. In addition, the presence of wounds on the roots and their close contact with contaminated compost increase the risk of direct transmission of the nematode.
ANSES's main recommendations are:
- pay close attention to compost made from contaminated bark and other woody material and used for growing such plants, even if the risk of direct transmission is considered to be low or very low;
- removing soil between contaminated wood and the roots of existing trees is unnecessary due to the nematode's limited ability to survive and move in bare soil. To limit the risk of direct transmission of the nematode, one solution to consider is the destruction of contaminated seedlings in the nursery.
In addition, in consignments of bark without woody residues, the risk of transmission of the nematode by its insect vector is considered negligible. It is high if the wood chips are larger than 3 x 3 x 3 cm because L4 larvae and pupae can survive long enough in these woody residues to produce immature adults in the spring capable of transmitting the nematode to nearby host trees.
Species likely to facilitate the multiplication of the pinewood nematode
The Agency also assessed the susceptibility of plant species to the pinewood nematode and its insect vector. The main host plant species for the pinewood nematode belong to the genus Pinus, but the list of susceptible plant species includes other conifers of the genera Abies, Cedrus, Larix, etc. At present, there are no species of pine established in Europe that are resistant to the pinewood nematode, although some are less favourable to its multiplication.
If a nematode outbreak is detected:
- the species on which the nematode can multiply should be eliminated as a priority: maritime pine, Scots pine, black pine, Monterey pine, and probably Aleppo pine and loblolly pine;
- susceptible species should be used to produce wood chips, whose dimensions are all less than 3 cm to prevent them from harbouring larvae of the insect vector.
Concerning the stone pine (Pinus pinea), according to the data available in the Iberian Peninsula, the risk of dieback in the field is low. However, uncertainties remain in the scientific literature, with regard to the multiplication of the nematode, the expression of dieback symptoms and the insect’s ability to feed and reproduce on this species. In particular, further studies need to be conducted to verify whether it may be a "healthy carrier" of the disease.
Measures for the transport, storage and treatment of wood or bark susceptible to pinewood nematode if an outbreak of this parasite is declared
Following the devastating forest fires of 2017, under the European regulations, Portugal asked to be able to authorise the removal from the country of symptomatic wood – susceptible trees that had died, were dying, were the casualties of storms or fires, or were contaminated by the pinewood nematode – in the form of chips. This led ANSES to assess the risk of the pinewood nematode and its vector spreading from all materials resulting from wood processing in the forest, from their storage in the forest through to their final processing. In addition, an EU Decision stipulates that in the event of detection of pinewood nematode, clear-cutting measures are to be taken within 500 m of the infested tree. The wood from this clear-cutting should then be managed in such a way as to limit the risk of spreading the pinewood nematode and its insect vector.
Based on current scientific knowledge, the Agency has determined phytosanitary measures for control or prevention to be applied to the various forest products and by-products according to their fate and the time of year:
- during the flying period of the adult insect vector M. galloprovincialis, from 1 April to 31 October, only shreds and/or chips smaller than 3 x 3 x 3 cm can be left in place without any risk of insect development. Only the chips, being a commercial product, are then transported. If they are stored for less than 48 hours, they do not require treatment in the forest but must then be transported securely to avoid attracting and transporting adult insect vectors. Other wood products, not subject to chipping or shredding in the forest, must be protected in the same way as wood chips, taking into account the storage conditions in the forest, whether short (<48h) or long. All wood products stored outside the forest in dedicated sites or industrial processing sites must continue to be protected to prevent the emergence of young adults or infestations by mature adults of M. galloprovincialis;
- outside the flying period of the adult insects, from 1 November to 31 March, no treatment is necessary in the forest, as the wood products and by-products cannot, in principle, attract these insects. Treatments are therefore only to be applied in storage areas or sites if storage lasts beyond the winter period.
Measures are in place to prevent the key stages in the development of the insect vector that increase the risk of spreading the nematode, i.e. attraction and oviposition or flight. However, these measures remain experimental or are not authorised in France. Scientific research and/or technological advances are therefore needed to conceive realistic technical solutions at the scale of forest exploitation. These will have to ensure risk-free storage and/or transport of material from tree species susceptible to the pinewood nematode and its insect vector. These could include, for example, the use of insecticide-impregnated protective netting or heat treatment of wood products and by-products.