Developing an Integrated Pest Management Solution for Pierce’s Disease Spread by the Glassy-Winged Sharpshooters in Temecula
Monitoring of non-agricultural areas demonstrated that glassy-winged sharpshooters occur at significant densities within the landscaped, riparian, and natural areas surrounding and interspersed within the agricultural areas of the Temecula valley. As it will be both politically and biologically impossible to eradicate sharpshooters from these non-agricultural areas associated with the Temecula vineyards, these areas will have to be considered to be permanent and constant sources of glassy-winged sharpshooters.
Insecticide treatments of citrus had a dramatic and significant effect on parasitoids of glassy-winged sharpshooters in the Temecula region. With insecticide treatments few sharpshooter eggs were found in treated areas and only a minute proportion of these were parasitized. At the beginning of April, 2000, glassy-winged sharpshooter parasitoids were recorded in 30%of all sharpshooter eggs collected. By June the parasitism rate had risen to 90%. Parasitism rates continued in excess of 90%for the remainder of the year. The primary parasitoid encountered was Gonatocerus ashmeadi (Hymenoptera: Mymaridae).
Results from the first six months of sampling have yielded positive detection of X. fastidiosa in grapes at many sites throughout the Temecula valley. Xylella fastidiosa was also consistently detected in almond and oleander in the Temecula region. Three other plant species (coyote brush, mustard, and elderberry) showed weak positive results for the presence of X. fastidiosa when tested by ELISA; however, neither culturing or PCR techniques supported these results. We expect that more plants will yield positive results as we begin to routinely use the more sensitive immunocapture PCR method. Thus far, no obvious source of inoculum of PD, other than grape and almond, was detected in the valley using these methods.
To date, five parasitoid species have been investigated as potential biological control agents of GWSS. Two were found to lack the host plant range required of an effective biological control agent and the remaining three were found suitable for further research. Rearing methodologies for sharpshooters and parasitoids have been significantly improved. We are now providing sharpshooter egg masses to two laboratories for biocontrol research, one laboratory for behavior research and three laboratories involved in insecticide research. The work determining host plant preference of sharpshooters was largely abandoned as it was deemed no longer necessary with the successful development of rearing methodologies. The construction of the phenological degree-day model is continuing. To date we have taken the insect through a single generation at several temperatures; this research will require additional funding and at least another 18 months of effort before any significant conclusions are made.
Field-trials conducted at U.C. Riverside in 2000 showed that two soil-applied and one foliar-applied neonicotinoid induced high sharpshooter mortality through an 8 week period. Most striking is our observation that imidacloprid applied to grapevines in September 1999 had a substantial and disruptive impact on sharpshooter feeding almost a year later. This may, in fact, be more important to protecting plants from Xylella fastidiosa-carrying sharpshooters then inducing mortality as disease acquisition and transmission will likely be disrupted as well.
Plant microelements such as zinc, copper, manganese and iron, as well as three antibiotics were tested for inhibition against Xylella fastidiosa in vitro. Tetracycline was the most effective antibiotic and zinc was the most toxic microelement. Prophylactic materials being evaluated included 3 inducers of systemic acquired resistance (SAR) and 4 microelement formulations. Therapeutic materials include several formulations of microelements and 2 antibiotics. All prophylactic field plots were mapped for PD each fall, however no new infections were found in either the treated or control vines in 2000. Bactericides were applied as foliar sprays, as materials packed into hollow nylon (DP) screws or in drilled holes packed with bactericides that were suspended in agarose and the ends of the holes were sealed with DP screws. Several of the drilled through/DP screw treated vines did not show any PD symptoms following treatment, however these vines were also severely pruned following treatment. A final assessment of the efficacy of these bactericide treated vines will be made in the summer of 2001. An injection machine that is widely used for injecting avocado trees was found to work well for injecting vines in the spring but less effective in the fall. Several potted plant experiments were performed using soil drenches of microelements as therapeutic or prophylactic bactericides. Although significant phytotoxicity occurred with some of the materials, managanese and zinc treatments may have some potential. A custom-made pressure bomb was purchased and used to express xylem sap from 1 meter long grapevine canes that were treated with various microelements. Surprisingly high concentrations of zinc and managanese were found in the xylem sap of grapevines treated with amino acid chelates of these elements. Additional experiments are now being done to determine whether the xylem sap is actually toxic to Xylella fastidiosa or if the ions are too tightly bound.
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