Advancements in the study of Xylella fastidiosa vectors: biology and biometrics of Philaenus italosignus juveniles.

In this study, we explored the biology and biometrics of the spittlebug Philaenus italosignus (PI) (Hemiptera: Aphrophoridae), one of the confirmed vectors of the quarantine phytopathogenic bacterium Xylella fastidiosa (XF). PI juveniles are known to have only one host plant, the lily Asphodelus ramosus, while the nymphs of Philaenus spumarius (PS), the most studied XF vector, are notoriously polyphagous. Indeed, a better understanding of these spittlebugs is key for early detection of XF. In 2019 we first reported PI in Tuscany (Italy), nearby the outbreak area of Monte Argentario in which XF subsp. multiplex, was found mainly associated with Mediterranean scrub. Since then, we kept studying PI’s biology intending to verify its overwintering stage and the duration of the preimaginal development. Lastly, we investigated the possibility of using biometric traits of PI juveniles as an diagnostic identification tool. Insect samples, both target (PI) and non-target (PS), included nymphs collected in 2022-2023. For all five instars the head’s width was measured (500 specimens for each species). Finally, DNA tests were carried out using the PI-specific Sybr GREEN qPCR assay to verify the identity of a subsample of 80 nymphs. Our observations showed that, in Tuscany, PI first instar nymphs are found on the lily already in December and, unlike PS, they overwinter. Thus, the duration of the preimaginal development covers a total of four months, with newly emerged adults appearing in April. Our observations on the nymphs extended the current knowledge on PS biometrics, while providing original data on PI juveniles. Moreover, by comparing measurements we showed how each instar’s size differs between the two species, PI nymphs being significantly larger. Finally, molecular tests validated our claim on PI biometrics: in no case, morphological and molecular analyses gave conflicting results. In conclusion, the size of Philaenus nymphs may be used to discriminate both spittlebug species. The reliability of this trait and the new data on PI’s biology will allow a better monitoring of the ongoing XF emergency, both in agriculture and natural environments, also considering the prolonged timeframe during which the bacterium may be transmitted to trees by the polyphagous PI adults.