Norwegian Crop Research Institute, Plant Protection Centre, Fellesbygget, NO-1432 s, Norway

Background and objectives
The PRA document [1] focuses on the biology, pathways and options of risk management of the pine wood nematode (PWN), Bursaphelenchus xylophilus. It confirms the quarantine status of PWN, justifying the phytosanitary measures taken to exclude the nematode from Europe. The biological grounds of these actions are still questioned by wood-exporting countries. In connection with this, the extrapolation of data on PWN from areas with epidemic pine wilt disease (PWD) to Europe has caused much controversy . This presentation examines the PRA document, focusing on the biology of transmission and establishment of PWN in the absence of the principal vector insect. It also considers how the PWN biology might affect PWD expression in Europe, and the options of risk management in trade.

Results and conclusions
The biological plasticity of PWN allows it to form dispersal dauer larvae in wood chips in response to changing temperatures or to the presence of cerambycid beetles. This would allow European Monochamus specimens, accidentally trapped in chips, to become infected by dauer larvae and transmit PWN to forest trees. Even superficial contamination of other tree-living insects with PWN could cause transmission. Once established in forest trees, PWN will gain full benefit of the existing Monochamus-transmission system of the native nematode Bursaphelenchus mucronatus.

Considering the volumes imported, the level of risk associated with each commodity and its possible end use, the most important commodities from a phytosanitary point of view are sawn wood, packaging, dunnage and wood chips. The latter three categories are often made from low-grade material very likely to be infested with PWN. Uncontrolled end use of PWN-infested wood may lead to transmission of nematodes. PWN has been shown to migrate from infested wood through soil and infect host trees through both wounded and intact roots. Wood chips used as soil cover or mulch around trees represents a potential danger of transmission. The use of infested sawn wood for casting moulds in construction work in forest areas would offer opportunities for PWN to infect the damaged roots of nearby stressed pine trees, which also would be highly attractive as oviposition sites for the local Monochamus spp.

PWD has been recorded at mean temperatures higher than 20C. Hence the expression of the disease is supposed to be restricted to the southernmost member states of the EU. In a growth-chamber experiment simulating the daily temperature changes of an exceptionally warm Swedish summer, PWN inoculations of Scots pine resulted in mortality, although the calculated mean temperature was approximately 14C. Hence PWD could possibly be expressed even in Nothern Europe during hot spells. In addition to temperature, effects of water stress, light quality and air pollution, as well as mycorrhizal infection and co-existing tree species, may influence PWD development. Pathogenicity of PWN is dominantly inherited in crosses with the widely distributed and closely related species B. mucronatus. It is not known if such hybrids could damage European pine species at temperatures lower than those required for damage by PWN.

Selection of healthy trees for felling and an elevated quality control during processing [1] could eliminate the vector but not necessarily PWN, which can persist for at least 6 years in asymptomatic Scots pine trees. Heat treatment remains the safest method of eliminating both PWN and its vector.

PWN has many attributes of a good invader which are expressed in a high biological plasticity. High-trade-volume commodities represent a condiderable risk of PWN transmission even without the principal vector. Forest selection and quality checks would not detect latent PWN infections. Heat treatment, carried out correctly, is the safest means of control.

1. Evans HF, McNamara DG, Braasch H et al., 1996. EPPO Bulletin 26, 199-249.