Investigating the Spread of Pierce’s Disease in the Temecula Valley of Southern California
In 1997, Pierce’s disease (PD) was detected in the winegrape-growing area of Temecula for the first time. PD is now scattered throughout Temecula with a few vineyards showing a substantial proportion of infected plants. These outbreaks are concurrent with the appearance of the Glassy-winged Sharpshooter (GWSS), Homalodisca coagulata, a PD vector new to California. PD is the principle factor limiting grape production in the southeastern Unites States where it is spread by the GWSS. Since it was first discovered in Orange and Ventura counties in 1990, numbers of the GWSS have continued to increase as it spreads throughout southern California. In the summer of 1998 the GWSS was observed for the first time in southern Kern County. Our trapping studies indicate that the main PD vector in Temecula is the GWSS, which is generated in many acres of citrus there. Early in the season (Apr-June), greater numbers of GWSS were trapped in vineyards near citrus than in vineyards near other vegetation types. At the peak of trap catches in the season (July-Sept), similar numbers of GWSS were trapped near all vegetation types demonstrating the GWSS’s ability to disperse widely from citrus. The Smoke Tree and Willow sharpshooters were also found but are relatively uncommon native insects; we do not consider them important to the PD epidemic in Temecula. Trapping studies and sweep net sampling did not indicate the presence of other PD vectors including the blue-green, the Green, and the Red-headed sharpshooters. Field studies in Temecula did not reveal any plant hosts that are likely point sources from which the PD bacterium is acquired by the GWSS. For one vineyard we documented a strong correlation between PD symptoms and the proximity of grapevines to citrus, even though we could not detect the PD bacterium in samples collected from this orchard. We are currently conducting greenhouse experiments to examine the potential of citrus to support the PD bacterium. Field experiments with grapevines showed that Admire (Bayer Corp.), a soil-applied formulation of imidacloprid, has a potential to reduce spread of the PD bacterium by killing GWSS and by inhibiting feeding. Admire showed efficacy at least 8 weeks after application. Due in part to our support, Admire was granted a section 24C registration by the California Department of Pesticide Regulation for use in grapevines against leafhoppers, including sharpshooters. In experiments that are underway we will ascertain the potential of Admire to reduce transmission of the PD bacterium by the GWSS. One of the attractions of Admire as part of a management program for PD is that it does not have a negative impact on natural enemies as with most foliar-applied insecticides. For that reason, we examined the effect of Admire in citrus to control GWSS. Citrus management relies on natural enemies to keep pest populations in check, therefore foliar-applied insecticides are frequently avoided. Unfortunately, Admire did not control GWSS in citrus to the degree desired. We suspect that its efficacy may be increased by improving application methods and timing.