Evaluation of the potential vector role for the alien planthopper Ricania speculum in Flavescence dorée phytoplasma epidemiology

Ricania speculum (Walker) is an exotic planthopper, accidentally introduced in Europe and officially reported in Italy (Liguria, Genoa) in 2014. It is now also spread in Tuscany (provinces of Massa Carrara, Lucca, Pisa). Ricania speculum is highly polyphagous on herbaceous and woody plants, either wild or cultivated, and can cause damages by sup suction, honeydew emission and egg-laying. This phloem sap-sucking insect often colonizes grapevines and Clematis vitalba plants, which are known hosts of Flavescence dorée phytoplasma (FDp), a quarantine plant pathogen causing severe damages to European viticulture. The purposes of this work were to determine if R. speculum could be a vector of FDp and assess its eventual impact on epidemiology of this grapevine disease. Nymphs of R. speculum were isolated on FDp-infected broad beans to evaluate acquisition efficiency and then transferred onto grapevines, broad beans and C. vitalba plants to determine transmission capability. Ricania speculum was able to acquire FDp, but not to transmit it to any of the inoculated plant species. The pathogen multiplication rate measured within R. speculum was significantly lower than that observed within the vector Euscelidius variegatus. Consistently with absence of transmission, the phytoplasma was almost never detected in salivary glands dissected from R. speculum individuals exposed to FDp. Under our conditions, the role of R. speculum in spreading FDp is unlikely to be of any concern. Nevertheless, population and spatial distribution of this alien planthopper are rapidly increasing, and therefore attention threshold in monitoring this pest across Europe should be maintained very high.