Potential vectors of xylella fastidiosa in Europe

Xylella fastidiosa is a xylem-limited bacterium that is exclusively transmitted by xylem-sap feeding insects belonging to the order Hemiptera, sub-order Cicadomorpha. Vectors acquire the bacterium by feeding in the xylem of an infected plant and can inoculate the pathogen to healthy plants immediately after acquisition. Bacteria are restricted to the foregut and do not systemically infect the insect body, therefore vectors loose the infectivity after moulting. However, once infected, adults transmit persistently for life, because the bacterium multiplies and persists in the vector foregut. As for transmission specificity, although X. fastidiosa transmission is restricted to xylem-sap feeding insects, there is no species-specificity and all xylem-sap feeding insects are considered potential vectors. Following the recent introduction of X. fastidiosa in the Salento area of Italy, a thorough analysis of the potential European vector species was undertaken. The results underline a striking difference in the fauna of xylem-sap feeding insects between the New World and Europe. In particular, while in the Americas there are numerous sharpshooters species (family Cicadellidae, subfamily Cicadellinae) and almost sixty have been identified as X. fastidiosa vectors, very few sharpshooter species are present in Europe. Actually, out of nine species of this subfamily recorded in the Fauna Europaea database only one species, Cicadella viridis, is widespread and common, though mostly restricted to hygrophilous environments. On the contrary, thirty six spittlebug species (families Aphrophoridae and Cercopidae), are present in Europe and some of them are very common and widespread. Among these, the “meadow spittlebug” Philaenus spumarius, already identified as a vector of the CoDiRO strain in Salento, is very common and abundant in diverse ecosystems, and feeds on mono- and dicotyledonous grasses, on trees and shrubs. Among the European xylem-sap feeding insects, cicadas (families Cicadidae and Tibicinidae) are represented by tens of species, often with high population level, namely in the Mediterranean area. Some species, like Cicada orni can also be very abundant on olive trees. It can be suggested that, while in Northern and Southern America sharpshooter vectors have been primarily associated with X. fastidiosa epidemics, in Europe xylem-sap feeders other than sharpshooters might play a more important role in the spread of this bacterium