Monday, December 10, 2007

The sharpshooter X-wave: Correlation of an electrical penetration graph (EPG) waveform with xylem penetration during sharpshooter feeding

Elaine Backus, ebackus@fresno.ars.usda.gov1, Wendy Holmes2, Brendon Reardon3, Fred Schreiber2, and Greg Walker, gregory.walker@ucr.edu3. (1) USDA ARS San Joaquin Valley Agricultural Sciences Center, 9611 S. Riverbend Ave, Parlier, CA, (2) California State University-Fresno, Dept of Biology, Fresno, CA, (3) University of California-Riverside, Dept. of Entomology, Riverside, CA



Electrical penetration graph (EPG) monitoring is the most rigorous means of observing and quantifying the feeding of any piercing-sucking insect.† Previous studies with aphid and leafhopper pests of agricultural crops have demonstrated the unique value of what is termed the X wave, i.e. the waveform that represents first penetration of the insectís piercing-sucking mouth parts, the stylets, into a preferred vascular tissue for ingestion, usually a phloem sieve element or xylem tracheary element.† This paper presents the first direct evidence of the sharpshooter X wave, as studied in the smoke tree sharpshooter, Homalodisca liturata (Hemiptera:† Cicadellidae).† We describe the consistent association of the X wave with salivary sheath termini in a xylem cell, especially mature (lignified), secondary xylem cells.† The X wave is a complex, multi-component waveform family that is composed of waveform types B1s (previously shown to represent precibarial valve movement and possible egestion), B1w (salivation), proto-C (possible micro-ingestion) and C (macro-ingestion/ cibarial pumping).† It is proposed that the X wave represents first penetration of xylem, and functions to:† 1) physically seal the stylet tips into the cell via sheath salivation, 2) taste the chemical constituents of the cell to determine acceptability, and 3) mechanically test the strength of the stylet seal via trial cibarial pumping.† Evidence for these functions will be presented.† We hypothesize that the X wave represents the behavior that controls inoculation of Xylella fastidiosa.


Species 1: Hemiptera Cicadellidae Homalodisca liturata (smoke tree sharpshooter)
Species 2: Hemiptera Cicadellidae Homalodisca vitripennis (glassy-winged sharpshooter)