Delimiting of biological invasions – an overview of strategies and tools with pine wood nematode as a case study
In case of a detection of an invasive pest species in Sweden the first step will be a delimiting survey defining the infested area. It is important that the quality of the delimiting survey is high because it will form the basis for decisions that can have large economic and ecological consequences. It can concern e.g. decisions about eradication or containment and it is crucial that they are dimensioned correctly. The report gives an overview of strategies and survey tools used in delimiting surveys. Strategy denotes the principle after which the spatial and temporal allocation of sampling resources is decided (i.e. where and when to search for the species) while survey tools denotes the methods used for documenting presence of the species in a locality chosen for survey (e.g. traps or visual inspections). Special focus is on the pine wood nematode Bursaphelenchus xylophilus (PWN hereafter) as a case study. A detailed description of the biology of PWN and its vectors, in which ways PWN may be introduced, establish and spread in Sweden, and how a delimiting survey could be conducted is given in the report. PWN is native in North America where it mainly develops in newly dead conifers (saprophytic life cycle) and thus not is an important pest. It is totally dependent on pine sawyers (beetles of the genus Monochamus), which also breed in newly dead conifers, for its spread between trees. PWN has established in Japan, China, South Korea and Portugal where it also kills large numbers of native pine trees. The most probable way of introduction into Sweden is by wood package material infested by PWN and its vector. In Sweden the most likely scenario is that PWN would not be an important tree killer, because of the rather cool summers. The most probable vector in Sweden would be M. sutor, which is common in large parts of the country. The suggested strategy for delimiting a PWN invasion in Sweden is by survey of sequential zones around the detection point until the species is not recorded anymore (i.e. delimiting by documented absence in a zone surrounding the infested area). This strategy also offers the possibility to adapt the number of samples to a chosen level of statistical reliability and detection level. Because the vector beetles disperse large distances and PWN is not expected to kill large number of trees, it is most likely that PWN is already established over a considerable area before being detected. Thus, a radius of 25 km is suggested forthe first zone around the point of detection. Because PWN is not expected to generally kill trees in Sweden the sampling should instead be directed towards logging residues (tops and branches) on clear-cuts with signs of vector colonization. Wood samples from such objects are analyzed in laboratories for the presence of PWN. Trapping of vector beetles in traps baited with attractants is used as a complement. Also the beetles are analyzed at laboratory for PWN.