Biology and Control of Insect Vectors

The glassy-winged sharpshooter is  significantly larger than the blue-green sharpshooter

Studies aimed at developing methods to lower populations of blue-green and glassy-winged sharpshooters, and reduce the incidence and spread of Pierce’s disease in grapes, are now under way at UC Riverside, UC Berkeley, and through the UC Integrated Pest Management (IPM) Project.  The red-headed and green sharpshooters are also being studied, although there is little evidence to show that these grass-feeding insects are a threat to grapes in the Central Valley. Since Pierce’s disease is spread by different insect vectors in Northern and Southern California grape-growing regions, the management approaches being investigated vary depending on the location.

Management of riparian areas.  On the North Coast, the incidence of Pierce’s disease is most severe in vineyards located adjacent to streams and irrigated ornamental landscapes on the valley floors and the edges of forest lands at higher elevations.  Spring infections are believed to be critical in establishing chronic Pierce’s disease in North Coast grapes and researchers are examining ways to discourage overwintering adult BGSS from entering vineyards.  The insecticide dimethoate can be applied to riparian vegetation in Napa and Sonoma counties to control adult BGSS during the early growing season, but research conducted over the past four years by UC Berkeley entomologist Alexander Purcell has focused on developing more effective, nonchemical methods for reducing early spring populations in environmentally sensitive riparian areas.

This involves replacement of plant hosts preferred by BGSS with non preferred plants. The objective is to disrupt BGSS breeding.  Early results are promising with activity reduced by 70% to 99% in the control sites as compared to untreated areas.  What remains to be determined is if there is a similar reduction in the spread of PD to adjacent vineyards. Final guidelines for implementing riparian vegetation best management practices are now in the approval process. On a related note, research is under way on the North Coast to study the incidence of ornamentals as reservoirs for X. fastidiosa.

Application of pesticides to control sharpshooters.  The use and effectiveness of pesticides in controlling sharpshooter populations and the spread of X. fastidiosa is being investigated in Northern and Southern California vineyards.  UC Berkeley scientists are focusing on BGSS, while scientists from UC Riverside and the UC IPM area advisor in Ventura County are examining GWSS.

Tests of a soil-applied formulation of the insecticide imidacloprid (AdmireŽ), registered in 1999 for use on grapes to control leafhoppers and mealybugs, are underway.  This material is systemic in grapes and kills sharpshooters at approved application rates, although mortality typically requires at least several days on treated vines.  In addition, a formulation of imidacloprid for foliar applications (ProvadoŽ) has been approved for use against grape leafhoppers and is effective against sharpshooters for two to three weeks after application.

More needs to be known about the effects of insecticides on vector movements and the transmission of X. fastidiosa. Still remaining are research trials to evaluate the effectiveness of these insecticides in reducing the incidence of PD. Large-scale test plots in vineyard conditions and at least three years of trials are needed.

Physical barriers.  Insecticide-treated barrier crops planted on vineyard margins are being evaluated by UC Berkeley scientists as a technique to prevent BGSS from entering vineyards from riparian areas.  Scientists at UC Riverside have plans to evaluate the use of large screen barriers (up to 8 m in height) in Temecula to disrupt the movement of GWSS from citrus orchards into vineyards.

Natural enemies. Although GWSS is a common insect in the southeast, it is much more abundant in citrus and some other plants in California.  This suggests that natural enemies may play a role in lowering GWSS populations in its home regions.  UC scientists have discovered several natural enemies of GWSS in Southern California and another in Mexico.  These are parasitoid wasps that attack the eggs of GWSS. 

Field studies in Riverside and Ventura counties have found that a tiny wasp,  Gonatocerus ashmeadi, is particularly active and accounts for at least 95% of the observed GWSS parasitism.  At least three other species, G. morilli, G. incomptu and G. novifasciatu account for the remaining 5% of egg parasitism. Most of the observed parasitism occurs during summer and early fall months on egg masses of second generation GWSS. No effective parasite has been recovered that preys on the first generation GWSS egg masses laid in late winter and spring.