Eradication programs against non-native pests and pathogens of woody plants in Europe: which factors influence their success or failure?

When a non-native species succeeds in establishing in a new habitat, one of the possible responses is to attempt its eradication. In the present study, we analysed European eradication programmes against non-native pests and pathogens of woody plants (PPWP) from 1945 to date. Our main goal was to identify which factors affect the success of an eradication programme, reinforcing guidelines for future eradication of PPWP. Data on eradication campaigns were obtained from online databases, scientific and grey literature, and Plant Protection Organizations’ reports. Factors influencing eradication success for both arthropods and pathogens were analysed with LASSO regression and decision tree learning. A total of 848 cases officially declared as eradication attempts were documented in our database (8-fold higher than previous reports). Both the number of programmes and their rate of success increased sharply over the last two decades. Only less than 10% of the non-native organisms affecting woody plants were targeted for attempted eradication despite the high economic and ecological impacts caused by some species for which no efforts were undertaken. Almost one-third of the officially declared cases of eradication concerned organisms that were still restricted to the material with which they were introduced. For these cases the success rate was 100%. The success rate of established species was only 50% for arthropods and 61% for pathogens. The spatial extent of the outbreak was the factor that most affected the outcome of eradication campaigns. The eradication success decreased abruptly above 100 ha for arthropods and 10 ha for pathogens. Additionally, other variables were shown to influence the outcome of eradication programmes, in particular the type of environment, with the highest eradication success rate found in nurseries and glasshouses, with successful outcomes increasing if quarantine measures were applied and when monitoring included asymptomatic plants. Particular species traits may reduce eradication success: parthenogenetic arthropods, saprotrophic pathogens, wind dispersal, the possibility to remain asymptomatic indefinitely, and the existence of resting spores or stages. In conclusion, small affected areas, quick response, and efficient implementation of quarantine restrictions, together with particular species traits, may allow a high probability of eradication success. Preparedness at the country and European level would allow a larger number of target species to be included in future eradication programmes.