Date Published: July 17, 2018
Publisher: Public Library of Science
Author(s): Daniele Cornara, Elisa Garzo, Marina Morente, Aranzazu Moreno, Javier Alba-Tercedor, Alberto Fereres, Arash Rashed.
The meadow spittlebug Philaenus spumarius plays a key role in the transmission of the bacterium Xylella fastidiosa to olive in Apulia (South Italy). Currently, available data on P. spumarius feeding behavior is limited, and a real-time observation of the different steps involved in stylet insertion, exploratory probes, and ingestion, has never been carried out. Therefore, we performed an EPG-assisted characterization of P. spumarius female feeding behavior on olive, in order to detect and analyze the main EPG waveforms describing their amplitude, frequency, voltage level, and electrical origin of the traces during stylet penetration in plant tissues. Thereafter, each of the main waveforms was correlated with specific biological activities, through video recording and analysis of excretion by adults and excretion/secretion by nymphs. Furthermore, the specific stylet tips position within the plant tissues during each of the waveforms observed was assessed by microcomputer tomography (micro-CT). Additional EPG-recordings were carried out with males of P. spumarius on olive, in order to assess possible sex-related differences. P. spumarius feeding behavior can be described by five main distinct waveforms: C (pathway), Xc (xylem contact/pre-ingestion), Xi (xylem sap ingestion), R (resting), N (interruption within xylem phase). Compared to males, females require shorter time to begin the first probe, and their Xi phase is significantly longer. Furthermore, considering the single waveform events, males on olive exhibit longer np and R compared to females.
One of the primary concerns of the global fruit industry is a group of systemic plant pathogens, i.e. virus, viroids, phytoplasmas and bacteria, for which there are not remedies once the plant is infected. The spread of these pathogens after the introduction into a new location generally relies on native or introduced vectors [1, 2, 3, 4]. Vector abundance, activity, and behavior, are key factors for the transmission of insect-borne plant pathogens . Furthermore, considering pathosystems where the pathogen is transmitted without strain specificity among vectors, as in the case of the plant-pathogenic bacterium Xylella fastidiosa Wells (1987) , a detailed knowledge of vector feeding behavior is of paramount importance. Only xylem-sap feeding insects are able to transmit X. fastidiosa . Xylem sap feeding is a characteristic shared by three taxa belonging to the order Hemiptera: Cicadellidae Cicadellinae (aka sharpshooters), Cercopoidea (froghoppers or spittlebugs), and Cicadoidea (cicadas) . Although occasional xylem sap feeding by insects that generally prefer other tissues may occur [9, 10, 11], the “non-xylem specialists” are unable to transmit the bacterium . To date, in Europe, the meadow spittlebug Philaenus spumarius L. (1758) (Hemiptera: Aphrophoridae) has been proven to be the main vector of X. fastidiosa to olive and likely other host plants [13, 14]. P. spumarius is a xylem feeder, either as nymph or adult. The spittlebug ingests considerable amount of sap from the main transpiration stream without causing vessel cavitation, overcoming dramatically high tension reaching -10 bars or more, and showing a mean excretion rate of 280 times its body weight in 24 hours [15, 16, 17, 18, 19, 20]. The association with symbionts potentially relaxes the severe dietary limitations related to xylem sap feeding, due to the poor nutritional value of the xylem sap and the cost associated with the process [21, 22].
The waveforms observed and correlated with feeding phases for P. spumarius were very similar to those described for B. xanthophis by Miranda et al.  using the same type of EPG device (DC-Giga amplifier).