HOST PLANT PREFERENCE AND NATURAL INFECTIVITY OF INSECT VECTORS ON COMMON WEEDS KNOWN TO HOST XYLELLA FASTIDIOSA Project Leader:

Common weed species can harbor Xylella fastidiosa (Xf) and its insect vectors. Should weed control be part of a Pierce’s disease control program? To address this question, we will survey weed species in agricultural areas known to host Xf, and determine the level of insect vector activity and the proportion of potential vectors that carry Xf. In the laboratory, we will compare three techniques to detect Xf in insects. Current methods produce mixed results; assessment of each method’s accuracy will improve comparison of research projects and field survey results. This project will provide information for control decisions by investigating the importance of vegetation management in reduction of insect populations and inoculum potential. INTRODUCTION The emergence of Pierce’s disease (PD) of grape in the General Beale Road area in Bakersfield in 2001 and 2002 exemplifies the threat posed by the glassy-winged sharpshooter (GWSS). While GWSS populations and PD are currently managed by an area-wide insecticide spray program and diseased vine removal, endemic GWSS populations are still present. This may be especially true in weedy fields, abandoned vineyards, and along roadsides and windbreaks where PD and GWSS are not managed (J. Hashim personal communication). Numerous common weeds and windbreak species are hosts of Xylella fastidiosa (Xf) in greenhouse studies (Purcell and Saunders 1999, Costa et al. 2004, Wistrom and Purcell 2005). More importantly, nine weed species common in the Central Valley were found to be naturally infected with Xf (Shapland et al. 2006). Since so many different plants can harbor Xf to some extent, more information on sharpshooter host plant use in the field is required before those studies can be translated into concrete recommendations to growers. While GWSS have been observed feeding on a wide range of ornamental and weedy species (CDFA host list at www.cdfa.ca.gov/phpps/pdcp/index), the quantitative data on the numbers and host plant preference of GWSS in agricultural settings focused mainly on presence on citrus, grapes and urban areas, with insect choice determined on plants provided in pots (Naranjo and Toscano 2003; Perring and Gispert 2004; Daane and Johnson 2004; Phillips et al. 2004). The first objective of this study is the identification of preferred feeding and oviposition hosts by GWSS, among plants already identified as hosts of Xf in agricultural areas. Year-round information on sharpshooter presence on host plants would provide information about the need for vegetation removal or modification in and around vineyards. For example, the identification of major breeding hosts of blue-green sharpshooters (BGSS) in northern California enabled the development of a riparian management plan. When the major breeding and feeding hosts of BGSS were removed and replanted with other plants less attractive to BGSS, large reductions in sharpshooter populations and Pierce’s disease in adjacent vineyards resulted (Purcell et al. 1999). Similarly, in Central Valley almond orchards affected by almond leaf scorch, the identification of common sharpshooter, treehopper, and spittlebug insect vectors has just been completed (Daane et al. – unpublished data), as well as concurrent assessment of Xf presence in sampled weeds and sharpshooters (Shapland et al. 2006). Field-based data is critical for practical application of treatment thresholds in development (Perring 2004), for GWSS control in areas with endemic sharpshooter populations. One important variable in the infectivity model is the proportion of vectors carrying the pathogen. This second objective of this study will determine the proportion of field-collected sharpshooters, in the San Joaquin Valley, that carry Xf in agricultural areas. With a functional treatment threshold, growers can predict the relationship between GWSS population and Pierce’s disease potential, and better plan insecticide applications for GWSS control. The natural infectivity of BGSS captured in riparian area was highly variable, ranging from 5 to >40% (A. Purcell -