Sharpshooter X Wave: Correlation of an Electrical Penetration Graph Waveform with Xylem Penetration Supports a Hypothesized Mechanism forXylella fastidiosaInoculation

Abstract

Electrical penetration graph (EPG) monitoring is the most rigorous means of observation and quantification of feeding by piercing-sucking arthropods. Previous EPG studies with aphids and leafhoppers have demonstrated that the X wave identifies when the stylets of these phloem fluid-ingesting insects make contact with their preferred plant vascular cell, phloem sieve elements. This article presents the first direct evidence of an X wave identifying ingestion from a xylem tracheary element by a xylem fluid-ingesting type of leafhopper Homalodisca liturata Ball (Hemiptera: Cicadellidae: Cicadellinae), whose waveforms are nearly identical to those of the glassy-winged sharpshooter, Homalodisca vitripennis (Germar). We document consistent association of the sharpshooter X wave with salivary sheath termini in xylem, especially lignified secondary xylem cells, and absence of the X wave in the rare instance of ingestion from a nonxylem cell. The sharpshooter X wave is a complex, multicomponent waveform, composed of X wave-specific variants of waveform subtypes B1w (representing salivation), B1s (representing precibarial valve movement for tasting), types C1 (a new waveform type that may represent egestion) and C2 (a new designation for the waveform type representing ingestion/cibarial pumping). It is proposed that the sharpshooter X wave represents a blended suite of behaviors that function to 1) physically seal stylet tips into the cell via sheath salivation, 2) repeatedly taste then eject (egest) chemical constituents of the cell to determine acceptability, and 3) mechanically test the strength of the stylet seal via trial cibarial pumping (ingestion). It is further hypothesized that the X wave represents vector behaviors that control inoculation of the Pierce's disease bacterium, Xylella fastidiosa. The ingestion-(salivation and egestion) hypothesis is stated for the mechanism of transmission of X. fastidiosa.

Stylet Morphometrics and Citrus Leaf Vein Structure in Relation to Feeding Behavior of the Asian Citrus Psyllid Diaphorina citri, Vector of Citrus Huanglongbing Bacterium

“…when a hemipteran’s stylets are inserted in a plant [14], [16], [18]. Yet, it is reasonable to assume that plant sites where ACP adults or nymphs settle and attain their typical feeding postures also represent putative feeding sites, especially that we verified this later by microscopy of extended stylets and histology of citrus leaves as mentioned in the Methods section.…”

“…In Hemiptera, feeding structures (stylets and other mouthparts) and feeding behavior are very important in the processes of acquisition of various disease agents from diseased plants and their subsequent inoculation into healthy plants [13], [14], [15]. The piercing-sucking mouth parts of ACP adults and their salivary sheaths have been studied [16], [17], but those of nymphs have not been studied so far.…”

Pattern of Stylet Penetration Activity byHomalodisca vitripennis(Hemiptera: Cicadellidae) Adults in Relation to Environmental Temperature and Light Conditions

“…In particular, information on the feeding activity of an insect vector in relation to environmental factors is crucial for understanding X. fastidiosa epidemiology (Backus et al 2005) because a vectorÕs ability to trans-mit (i.e., acquire and inoculate) the bacterium is directly proportional to the vectorÕs probing (stylet penetration) behaviors Purcell 2003, Fritschi et al 2007). When the insect subsequently salivates and egests bacteria, X. fastidiosa is acquired via the insectÕs ingestion of infected xylem ßuid and likely inoculated into uninfected xylem vessels (Backus et al 2009). Electrical penetration graph (EPG) monitoring, the major technique used in this study, is the most rigorous method to elucidate direct, detailed information on hemipteran stylet penetration (Walker 2000, Doležal et al 2004.…”

Comparative Behavioral and Protein Study of Salivary Secretions in Homalodisca spp. Sharpshooters (Hemiptera: Cicadellidae: Cicadellinae)

“…The recently published salivation-egestion hypothesis for inoculation of X. fastidiosa (Backus et al 2009) implicates sharpshooter saliva (secreted into the plant and then taken up into the anterior foregut) as the critical carrier of bacteria subsequently egested from their foregut colonization sites. However, almost nothing is known about sharpshooter saliva.…”