Likelihood of establishment of tree pests and diseases based on their worldwide occurrence as determined by hierarchical cluster analysis

Abstract

The trade in woody plants for planting (WP4P) is a major pathway for the introduction of alien tree pests and diseases into Europe. Pest risk analyses are important tools to prevent such introductions, but they target recognised pests, their hosts and commodities that are likely to vector them. Moreover, the perception of risk partially depends on expert judgement. Our aim was to provide an objective assessment of the likelihood of establishment and potential sources of invasive species. We analysed distribution data for 1009 invertebrate pests and pathogens of woody hosts in 344 regions. Countries and regions with similar pest assemblages were identified for each organism group using hierarchical cluster analysis and the likelihood of establishment of those species was calculated as the frequency of each species in countries within the cluster containing EU and EFTA countries (“EU countries”). Meaningful European clusters were formed for all organism groups, except oomycetes, and species were ranked by risk to each EU country. The non-EU regions with the most similar pest assemblages to the EU are the Balkan countries, Japan, Kazakhstan, New Zealand, Turkey, Ukraine, parts of China, Russia and the USA, and EPPO countries, indicating that these may be the origin of new pests and pathogens. The quality of the assessment of the likelihood of establishment depends on the quality of distribution data for pests and pathogens, but data quality varies between countries and organism groups and records are often historical. For example, the recent increase in trade and the similar climate in which WP4P for the EU market are grown in China indicates that this may be an important source of new invaders, but this was not suggested by our analysis. Moreover, most pest species in our database are recorded in one or more EU countries and the risk of these species primarily comes from within the EU. Future studies should therefore take the potential spread and establishment of these species in Europe, as well as recent changes in world trade patterns into account.