Can differences in feeding behaviour between Scaphoideus titanus males and females be related to phytoplasma transmission efficiency?

Abstract: The leafhopper Scaphoideus titanus (Hemiptera: Cicadellidae) is a vector of the phytoplasma causing the Flavescence dorée of grapevine which is one of the most economically threatening disease of European vineyards. In plant disease solely transmitted by insects, pathogen transmission occurs during the feeding behaviour, therefore quantifying the different feeding activities is a key to understand the disease transmission process. Feeding behaviour of piercing-sucking insects can be studied using the electrical penetration graph technique (EPG). This technique has been popularized by Tjallingi in the 1980s and further works especially on aphids. By connecting the insect and the plant in an electrical circuit it is possible to monitor electrical resistance fluctuations during probing. These voltage fluctuations occur in a number of distinctive patterns called waveforms that have been correlated to different behaviours according to the stylets position into the leaf and to feeding activity (salivation, puncture, ingestion, etc…). We have adapted this technique to S. titanus in order to investigate if differences in the feeding behaviour between non infected males and females could explain different ability in phytoplasma transmission. EPG waveforms representing probing activities were obtained from adult S. titanus probing in Cabernet Sauvignon cultivar. Three waveforms: salivation, phloem and xylem ingestion were characterized in both sexes by comparing them to previously published ones for other Hemipteran species. The first interesting result is that xylem ingestion occurred in both sexes, while S. titanus is always described as a phloem feeder. Interestingly, males exhibited more frequent and longer activity in phloem. The number, mean and total durations of each type of waveform differed significantly depending on the sex. Feeding behaviour differences affect the ability to acquire and then to inoculate phytoplasma and may partly explain the higher rates of transmission of Flavescence dorée phytoplasma that were obtained in the laboratory with males.