Stylet Penetration by Adult Homalodisca coagulata on Grape: Electrical Penetration Graph Waveform Characterization, Tissue Correlation, and Possible Implications for Transmission of Xylella fastidiosa

Abstract
The sharpshooter Homalodisca coagulata (Say) (Hemiptera: Cicadellidae: Cicadellinae) is an exotic vector of the Pierce’s disease (PD) bacterium, Xylella fastidiosa (Wells et al.), that was first observed in California in 1989. H. coagulata has since greatly increased the threat of PD to the grape industry as well as stone fruit, nursery, and ornamental industries in California. This is the first in a series of articles that together describe how sharpshooter stylet penetration behaviors (especially intricate stylet activities, salivation, and ingestion) control transmission (i.e., acquisition and inoculation) of X. fastidiosa. Herein, we categorized and characterized alternating current electrical penetration graph (EPG) waveforms from glass-winged sharpshooter stylet penetration on petiole of susceptible grape (‘Cabernet Sauvignon’), paying special attention to waveform fine structures that are likely to be the key to detecting the instant of inoculation. We also correlated waveforms with salivary sheath termini in grape tissues. For the first time in any EPG study of leafhopper or planthopper feeding, we demonstrate through case studies of individual probes how to follow the process of stylet penetration step by step as it is occurring, including salivary sheath branching and when the stylets first puncture a xylem cell. Finally, we discuss the implications of our findings for understanding the transmission mechanism of X. fastidiosa, in comparison with hypothesized mechanisms in the literature.