Impact of the Glassy-Winged Sharpshooter on Pierce’s Disease Spread in

Our second year of studying the epidemic of Pierce’s disease (PD) in Temecula, CA, has revealed a pattern of diseased grapevines that is consistent with vine-to-vine spread. In 1998, the best description of disease incidence was “scattered” with local hot spots, and we documented a strong relationship between PD severity and distance from citrus. Surveys in 1999 revealed vineyards with between 5%and 96%PD-infected vines, and disease incidence was not strongly linked to proximity from citrus. Yellow sticky-card and sweep-net sampling over the past two years indicate that glassy-winged sharpshooters (GWSS) is overwhelmingly the most important PD vector in this area. We also observed low numbers of other PD vectors, including the smoke-tree sharpshooter and the willow sharpshooter, both of which are native to this area. GWSS population densities were lower this year than last year. Anecdotal observations suggest that egg parasitism made a substantial impact on GWSS population density. Consistent between the two years of sharpshooter monitoring was evidence of the importance of citrus to GWSS populations. Also consistent were peaks in GWSS activity in the mid-late summer and in January. In January of 2000, sharpshooters were observed feeding from dormant grapevines and excreting fluid. Our first year of monitoring in the southern Kern County, where the GWSS was observed for the first time in 1998, revealed a small, yet apparently stable population. Studies on GWSS dispersion show that they are strong flyers and disperse deep into vineyards from adjacent citrus groves. We showed no statistical difference between sharpshooters caught in traps placed from 10 to 40 meters from the edges of vineyards. Traps at the edge of vineyards (0 meters) had significantly fewer sharpshooters. Traps in vineyards next to citrus groves had over twice the number of GWSS as traps in vineyards next to native vegetation. Sticky cards set vertically from 1-7 meters showed that GWSS’s do not fly at high altitudes. In fact, a seasonal mean of 95.4%of all the GWSS’s we caught were at altitudes of 5 meters or less. As with last year’s investigation, this year we found insecticides that could be part of a GWSS/PD management program. Our comparison of five neonicotinoids, both foliar and soil-applied, documented GWSS mortality as a function of time of exposure to treated plants, and as a function of time after treatment. The best performers were Admire, a soil applied insecticide that is registered for grapevines, and Acetamiprid, a foliar applied insecticide that is not registered. Admire and Acetamiprid caused over 63%and 81%GWSS mortality, respectively, in 24 hours even 8 weeks after application. Further studies with Admire indicated that GWSS did not feed enough to generate excreta within 3 hours of given access to treated plants. This anti-feedant property may in fact be more important to reducing PD spread than GWSS mortality. In addition to insecticides, we experimented with kaolin, a formulation of aluminum silicate. Kaolin disrupts insect sensory cues which caused a greater than 50%reduction in GWSS in treated verses untreated vineyard plots.