Development of a pathway model to assess the exposure of European pine trees to pine wood nematode via the trade of wood

Abstract

Pine wood nematode (PWN), Bursaphelenchus xylophilus, is a threat for pine species (Pinus spp.) throughout the world. The nematode is native to North America, and invaded Japan, China, Korea, and Taiwan, and more recently Portugal and Spain. PWN enters new areas through trade in wood products. Once established, eradication is not practically feasible. Therefore, preventing entry of PWN into new areas is crucial. Entry risk analysis can assist in targeting management to reduce the probability of entry. Assessing the entry of PWN is challenging due to the complexity of the wood trade and the wood processing chain. In this paper, we develop a pathway model that describes the wood trade and wood processing chain to determine the structure of the entry process. We consider entry of PWN through imported coniferous wood from China, a possible origin of Portuguese populations, to Europe. We show that exposure increased over years due to an increase in imports of sawn wood. From 2000 to 2012, Europe received an estimated 84 PWN propagules from China, 88% of which arose from imported sawn wood and 12% from round wood. The region in Portugal where the PWN was first reported is among those with the highest PWN transfer per unit of imported wood due to a high host cover and vector activity. An estimated 62% of PWN is expected to enter in countries where PWN is not expected to cause the wilt of pine trees because of low summer temperatures (e.g., Belgium, Sweden, Norway). In these countries, PWN is not easily detected, and such countries can thus serve as potential reservoirs of PWN. The model identifies ports and regions with high exposure, which helps targeting monitoring and surveillance, even in areas where wilt disease is not expected to occur. In addition, we show that exposure is most efficiently reduced by additional treatments in the country of origin, and/or import wood from PWN-free zones. Pathway modelling assists plant health managers in analyzing risks along the pathway and planning measures for enhancing biosecurity.

Nematodes in the Pine Forests of Northern and Central Greece

“…beetles, PWN insect vector, cover relatively short distances [3,49]. This fact, however, does not significantly reduce the risk of PWN invading Greece, as international trade and transport of wood products is considered to be the main pathway of B. xylophilus invasion and expansion [50,51], especially when B. xylophilus and its vector are introduced together [51]. In spite of the attempts to ensure proper treatment or monitoring of wood products, materials infested with B. xylophilus and/or its insect vector are being regularly recorded worldwide at points of entry, such as ports [52], even from countries known to be PWN-free [4,53].…”

“…However, the developmental rate seems to decrease above 30 • C for both PWN and pine sawyer beetles [56,59], although areas with climatic conditions that do not favour the expression of the disease could possibly act as reservoirs. [6,50].…”

Modelling for risk and biosecurity related to forest health

“…Different types of pathway models including epidemic networks and gravity models have been developed so far [13]. They track the PnP from their source area to their destination area where transfer to a suitable host may take place [13][14][15]. Such models help to identify locations where high propagule pressure (i.e., a set of PnP individuals) is expected (e.g., ports, trucks, ships, or factories) and to explore the effects of phytosanitary measures to prevent entry.…”

“…To generate predictions, invasion and emergence models need to be parameterized. Typically, three ways of parameterization are used or combined thereof: 1) collecting information on a parameter by setting up experiments or doing field measurements (e.g., by rearing the pest in the lab and running flight-mill experiments to determine the average flight distance [53-54]; 2) using existing information from literature, sometimes on related species or vectors (e.g., [15,28,32,55]); and 3) the use of Expert Knowledge Elicitation on plausible parameter values. Different EKE methods can be used to estimate the most likely value for the parameters, such as fixed and variable interval, and the roulette method [16].…”

PM 9/1 (6) Bursaphelenchus xylophilus and its vectors: procedures for official control

“…Survey effort should concentrate on larger specimens, with emphasis on detection of maturation feeding in the crowns and signs of breeding on the stems. As the purpose of the surveys is to optimize the prospects of detecting B. xylophilus as soon as possible after arrival in an area previously free of B. xylophilus, emphasis should be on the pathways as they enter the area, or on sampling in woodlands close to the end points of the pathways [see, e.g., Douma et al (2017) for an illustration of a pathway model]. Emphasis should therefore be on areas 1 Sites suitable for oviposition include weakened or freshly dead or felled trees, including those trees situated in storm-or fire-affected areas, as well as logging residues such as tops, branches and any trunk wood with bark remaining after felling.…”