Technical Abstract: The incidence of Xylella fastidiosa (Xf) in GWSS populations was monitored between August 2004 and February 2005 using various analytical techniques as indirect measures and transmission to grapevines as a direct measure of GWSS inoculativity. Field collections of GWSS made between August 2004 and February 2005 showed an increasing proportion of the population positive for Xf. The mean titer of Xf the field samples also increased through the fall months, but then diminished from peak levels during 3 collections made in the winter months of December and February. Differences among collection locations were observed in the proportion of the respective populations positive for Xf. Results from a transmission experiment conducted 6 February 2005 demonstrated that field-collected adults not only test positive for Xf by ELISA, but also transmit Xf to grapevine test plants (var. Chardonnay). An initial evaluation of xylem fluid collected from multiple branches per test plant revealed 11 plants Of 15 exposed to individual field-collected STSS adults and 5 out of 8 plants exposed to GWSS adults became infected with Xf. Analysis of the STSS and GWSS adults used in the 6 February 2005 transmission tests yielded absorbance readings in the lower positive range similar to levels observed in the Xf monitoring of the general population study. Further tests of the titers of Xf in these test insects and future test insects will be conducted once a real-time PCR test has been successfully developed.