Pine wilt disease control in Portugal: synthesis of the scientific knowledge.

Abstract

Increased international transport of plants or plant-products have brought with them associated risks of dissemination of pests between countries and ecosystems. This was also the case in 1999 with the detection,for the first time in Portugal and in Europe,of the pine wood nematode (PWN) B.xylophilus in dead maritime pines (Pinus pinaster). This organism is the causal agent of Pine Wilt Disease (PWD)and a quarantine organism within the European Union, and therefore its introduction was received with great apprehension both in Portugal and in the rest of Europe.A key element of the nematode’s life history and the initial determinant of its ultimate impact on living potential host trees, is its transmission by insect vectors, namely longhorn beetles of the genus Monochamus (Coleoptera: Cerambycidae).ConsequentlyPWD is the result of an interaction of three distinct agents (the pine wood nematode, an insect vector and a tree host), although only the first is the fixed element whenever the disease is present, as the vector and the host can vary. Soon after the nematode’s introduction into Portugal, various studies on the local specificities and regional variations of this disease wereconducted, namely on the bioecology of the nematode B. xylophilus, specificities of the interactions between nematode-vector (efficiency of vectoring and spread) and the nematode-host (aggressiveness of the nematode as a mortality agent), of which a global synthesis is presented here.The sole vector of the PWN in Portugal is the pine sawyer Monochamus galloprovincialis. Before B. xylophilus was introduced, this insect was considered a secondary xylophagous and its biology and ecology had never been studied. Studies recently conducted have found that the pine sawyer has one generation per year in Portugal, with beetles emerging from May to August. Biological parameters such as longevity, sexual maturation, fecundity and oviposition rates were studied under laboratory conditions. Adult beetles live for about two months and begin laying eggs at about 20 days, mainly in the upper trunk and branches of adult P. pinaster trees. Under laboratory conditions female beetles layeggs on other hosts such as P. sylvestris, P. pinaster, P. halepensis, P. radiata, P. pinea, and Pseudotsuga menziesii, but the insects only complete their life-cycle on the first four pines.The PWN can be transmitted into new hosts by both maturation feeding and oviposition activity of the adult beetles, although transmission by feeding appears to be more efficient. Regarding the PWN, several studies have been conducted regarding its biology and taxonomy, using both classical, morphological features, as well as recent molecular approaches. One main issue concerns its origin, and our results have shown that the PWN has entered Portugal from East Asia, possibly China or Japan. Ongoing research includes detection methods using real-time PCR and specific rDNA probes, and without the need to isolate the nematode from wood. Histopathological studies are also providing insights into how the nematode progresses inside the tree and causes damage. The potential involvement of associated bacteria in the disease process is also being investigated. Nematode-infected trees develop wilt symptoms which are not specific to the infection by the PWN. Overall, our results suggest that the most important aspects of the B. xylophilus/M. galloprovincialis interaction are similar to other well-established nematode-vector associations from distinct geographic locations. Control of the disease is made mainlyby felling and destruction of symptomatic and dead pines in the winter (destroying the nematode, vector larvae and other decline agents such as bark beetles), and by the use of traps with lures to capture flying beetles during the spring/summer months. Other potential control methods include biological control and the use of essential oils (phytochemicals) as nematicidal substances, however the practical application of these methods still raises questions.