Practical Winery
58-D Paul Drive, San Rafael, CA 94903-2054
phone:415/479-5819 · fax:415/492-9325

September/October 2002

Temecula Valley was the first viticultural area in California to be seriously impacted by the glassy-winged sharpshooter (GWSS). The pest is responsible for the spread of a bacterium, Xylella fastidiosa, the causative agent for Pierce’s disease (PD). Although PD problems were first identified in 1996, it took until 1999 for the wine industry to realize the severity of the situation.

In response to the crisis, an ongoing cooperative demonstration project was initiated in 2000 to examine the impact of area-wide management strategies on GWSS populations and PD incidences in Temecula Valley. The Temecula Advisory Committee consists of representatives from winegrape growers, citrus growers, University of California-Riverside, USDA, CDFA, and the Riverside County Agricultural Commissioner’s office.

Based on grower surveys, there were approximately 2,030 acres of wine grapes in production in 2000 in the Temecula viticultural area, along with 1,600 acres of citrus. About 850 acres of grape vines were removed because of PD by May 2002.

Key strategies of the project are to reduce the GWSS population and remove sources of Xylella fastidiosa. This includes removal of abandoned citrus trees and vineyards in Temecula.

GWSS populations were monitored weekly in citrus and grapes from March 2000 to the present by trapping with yellow panel (about 500 traps), visual counts (adults, nymphs, egg masses), beat sampling, and sampling for GWSS with a hand-held vacuum device.

Although winegrapes are the most vulnerable crop in Temecula Valley to the diseases caused by X. fastidiosa, other crops were scrutinized for contributions to GWSS population growth. Citrus was the most prominent year-long reproductive host of GWSS in Temecula. GWSS seemed to concentrate in citrus over the winter months when grapes and most ornamental hosts were dormant.

Clear objectives
Objectives of the cooperative demonstration project are:

  1. Determine impact of the 2000 area-wide management program on GWSS populations in citrus, grapes, and other plant hosts in the ecosystem for their applicability to the 2001, 2002, and 2003 seasons.
  2. Determine impact of the area-wide program on GWSS adult oviposition and nymphal development.
  3. Determine impact of the GWSS program on beneficial citrus insects, pest upsets, and GWSS parasitism.
  4. Evaluate the biological and economic effectiveness of an area-wide insecticide program against GWSS.
In the 2000 season, most of the commercial citrus in the Temecula Valley viticultural area was treated in April and May in an effort to destroy a substantial portion of the GWSS population. The treatment of 1,300 acres of citrus in Temecula with the soil-applied insecticide Admire (imidacloprid) represented a pivotal shift toward area-wide management of GWSS.

Although effective in most cases, Admire was not 100% successful on citrus in 2000. Improper application of Admire or poor uptake by weak trees sometimes prevented it from reaching the foliage. Lorsban was used in 2000 in areas where Admire failed to provide adequate control of GWSS in citrus.

GWSS populations tended to be clumped, with high numbers often found on weak trees. Results from the 2000 project indicated that every tree or acre does not need to be treated. GWSS numbers remained low in citrus treated with Admire during 2001.

Troublesome areas or “hot spots” were identified from the 2000 project, and by early monitoring from January to April 2001. This led to the treatment of 269 acres of citrus with Admire in March and April 2001. An additional 319 acres of citrus were treated “as needed” with foliar applications of Baythroid between May 21 and August15. A helicopter applied Baythroid to all but 45 of those acres, with the remainder done by an airblast sprayer.

An additional 106 acres of organic lemons received treatments of horticultural oil Gavacide C (certain horticultural oils such as Gavacide C are allowed by California Certified Organic Farmers and OMRI). Two small abandoned vineyards with absentee owners and high incidences of PD and GWSS were treated with Danitol by helicopter.

Organic citrus groves were problematic areas. GWSS populations remained high in these groves until August 2001 treatments of a 440 oil by airblast sprayer. In April 2001, Surround (kaolin clay), 1.2% Gavacide C (a 440 oil) and 1.4% Gavacide C were applied to certified organic lemons (four replicates of three acres each).

Results from Gavacide C were promising, and two more organic lemon groves (30 and 52 acres) were treated with 1.25% Gavacide C on August 4, 2001 (750 gallons per acre). Follow-up oil treatments were applied to organic lemons and to 24 acres of organic grapefruit in August 2002. This was the first time that GWSS populations were impacted in organic situations.

The oil resulted in 62% and 71% reductions in adult GWSS populations in these groves. About 50% of the egg masses contacted by the oil were killed, and subsequent oviposition was reduced. About 99% of parasitic wasps from oil-treated parasitized egg masses emerged, compared to only 2% for Baythroid-treated egg masses.

In April 2002, 140 acres of citrus were treated with Admire. An additional 89 acres were treated with foliar applications of Assail (acetamiprid, another nicitinoid recently registered in California) for citrus on an “as needed” basis in July 2002. Many grape growers treated grapevines with Admire or made foliar applications of Provado (imidacloprid), or Danitol (another pyrethroid). Recommendations were made to destroy infected vines in order to remove bacterial reservoirs.

The current situation in Temecula is serious, but cautious optimism prevails. First, GWSS populations in Temecula citrus groves are currently lower than on any other citrus in the Riverside area. For example, a conventional citrus grove in Redlands had weekly peak catches of 66.0 adult GWSS per trap, and an organic lemon grove in Temecula had 26.4 GWSS per trap in July 2001.

In 2002, the July peak for all non-organic Temecula citrus was 0.25 GWSS per trap, while the peaks for the grove in Redlands and organic Temecula grove were 217 and 4.6 GWSS per trap respectively. The Temecula organic lemon grove was treated with Gavacide C August 6 to 9 in 2002.

Second, there was some replanting in 2002 (2% or less), especially in high visibility areas, for both aesthetic reasons and to explore the feasibility of reestablishing lost vineyards. Based on a survey of five Temecula wineries, 2001 wine grape production ranged from 47% to 77% of the 1995 harvest with production ranging from 3.7 to 7.8 tons per acre in 1995 to 4.3 to 6.0 tons per acre in 2001.

However, Temecula is providing few wine grapes to other areas due to economic forces. As a result, the 2000 and 2001 harvests were more than adequate to sustain Temecula wineries with substantial surplus. In the future, Temecula will likely concentrate on producing quality wine grapes for Temecula’s wineries.