Insect vectors of Xylella: where do they hide?

Xylella fastidiosa is a bacterium which develops in the xylem of its host plants, that is the conducting tissue which transports xylem sap. It spreads from one plant to another via piercing-sucking insects, such as leafhoppers, which feed on sap and pierce several plants. This bacterium is particularly feared because it causes serious diseases in grapevines, olive trees, and almond trees. Native to the Americas, it has spread to Europe, particularly Italy, where it has devastated olive groves in the Apulia region. Its spread to other regions remains a major concern for Mediterranean agriculture. The challenge for scientists is to understand where these insects are found, what plants they feed on, and what the risk of transmission of the bacterium is.

A study conducted on 700 sites in vineyards in the south of France shows that insect vectors of Xylella fastidiosa are mainly found in meadows and alfalfa plots. These vectors are less common in olive or grapevine foliage but herbaceous borders and inter-rows can host large populations. The risk of numerous insect vectors passing through vineyards or olive groves and potentially transmitting Xylella is high when herbaceous vegetation dries out or is cut.

Part of the research also focused on what potential vector species feed on. The meadow spittlebug, Philaenus spumarius, already known as the main vector of the bacterium, has been confirmed as generalist in mainland France, whereas it is more specialist in Corsica: it feeds on a wide variety of plants, particularly in meadows. Other vectors are proving to be more generalist than expected, while some vectors, such as species of the genus Neophilaenus, specialize in Poaceae.

None of the analyses carried out by the scientists in this study detected the presence of Xylella fastidiosa on insects captured in Nouvelle-Aquitaine which indicates that the bacterium is absent or still rare in this region. However, the risk remains. The current climate conditions are (and will remain) suitable for the establishment of the bacterium. As there are many vectors present in various habitats, all the conditions are in place for Xylella fastidiosa to spread and cause damage if it were to be introduced into the region.

In their work published in the journal Plos One, the researchers recommend reinforcing surveillance, in particular using sentinel insects, in order to detect any new introduction as early as possible. The idea is simple: by feeding on different plants, the vector insects reflect the health status of their environment. Their infection status provides an overall picture of the possible presence of the bacterium in the area where they have been collected.