History and Status of the Glassy-Winged Sharpshooter in California

by Matt Daugherty

 Fifteen years ago Southern California grape growers were in the midst of a severe outbreak of Pierce’s disease (PD), a fatal disease of grapevines caused by the bacterium Xylella fastidiosa (fig. 1). The disease itself wasn’t new, having been known to occur in the region since the late 1880s (table 1). What was notable about this outbreak was its especially rapid pace and that it was attributable to a novel insect vector, the glassy-winged sharpshooter Homalodisca vitripennis (GWSS; fig. 2).

Although California is home to several native sharpshooter species that are also vectors of X. fastidiosa, GWSS is an exotic species here, native to the Southeastern United States. As with several other recent exotic invasive insects in the state, its initial establishment and spread was tied to urban and suburban areas. H. vitripennis was first reported in Ventura and Orange counties, perhaps following introduction via infested nursery plants prior to 1990. But by the late 1990s it had spread throughout Southern California and into the southern Central Valley, including into vineyards in Riverside and Kern counties, promoting severe outbreaks of PD (i.e., > 90% prevalence at some sites) and other novel Xylella diseases (e.g., oleander leaf scorch).  

Xylella fastidiosa is a xylem-limited bacterium that is capable of infecting a broad range of host plants, though it causes severe disease in a minority of them. Notably, GWSS is not especially efficient at transmitting X. fastidiosa compared to some other sharpshooters. What is unique about GWSS compared to California native sharpshooters is its ability to achieve much higher population densities and a propensity to feed on a far broader range of host plants. This broad host range likely contributed to its role in the development of novel Xylella diseases and potential role in the resurgence of historically minor diseases (e.g., alfalfa dwarf, almond leaf scorch). It addition, it creates challenges for the nursery industry due to quarantine restrictions on the movement of the many plant species or varieties on which GWSS purportedly feeds or reproduces.

Table 1. Timeline of notable Xylella disease events.

Since the onset of the disease outbreaks in Temecula Valley 15 years ago, numerous measures have been put in place to mitigate the damage caused by H. vitripennis. A major element has been the area-wide control programs in Southern California and Central Valley grape-growing regions. These include GWSS monitoring, releases of biological control agents (i.e., the parasitoid Gonatocerus ashmeadi) and regular chemical control. Specifically, foliar insecticides and especially systemic imidacloprid applications target GWSS populations in citrus, to limit spread into nearby vineyards. Collectively, these measures have greatly reduced disease pressure in the region. A recent survey of Temecula vineyards found GWSS densities that were a fraction of the purported “hundreds” of sharpshooters per vine 15 years ago, and overall PD prevalence in the area of approximately 0.5%. The second major piece of sharpshooter management involves quarantine requirements for nursery stock shipping from GWSS-infested areas. A compliance agreement requires that nurseries monitor for sharpshooters, designate a controlled staging area for readying plant shipments, and inspect the source and potential receiving locations. More recently, an approved treatment program was established that relaxes the monitoring requirements in exchange for targeted insecticide applications (i.e., carbaryl or fenpropathrin) prior to shipping. Available data suggest that these measures greatly reduce the risk posed by nursery shipments.

 

Homalodisca vitripennis, though widely distributed in the southern part of the state, has not yet become prevalent in Northern California or much of the Central Valley. GWSS is endemic in most Southern California counties and portions of Kern, Tulare and Fresno counties. In addition, there have been localized infestations found in parts of Santa Clara, San Luis Obispo, Sacramento, Solano, Contra Costa, Butte and Imperial counties — most of which are believed to have been eradicated. The most recent was an apparently successful eradication of a residential infestation in San Luis Obispo. Yet, within the last couple of years there has also been cause for concern regarding GWSS control. Specifically, the General Beale region of Kern County has seen a resurgence in H. vitripennis and an accompanying localized outbreak of PD. One explanation may be poor management on the part of certain growers in the area (i.e., no removal of diseased vines or weedy hosts). Alternatively, the resurgence may be indicative of partial failure of area-wide chemical control due to poor uptake of systemic insecticides in some contexts, specifically some of the windbreak trees, or the development of insecticide resistance. The latter possibility has motivated UC Riverside researchers to initiate studies to determine whether there is evidence that the several years of insecticide applications in commercial citrus and nurseries have led to any resistance in H. vitripennis. Regardless of the explanation for this resurgence in GWSS, it illustrates the need to continue to remain vigilant about the threats posed by this invasive insect.

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Matt Daugherty is Assistant Cooperative Extension Specialist, Department of Entomology, UC Riverside.

References 

Almeida RPP, Blua MJ, Lopes JRS, Purcell AH. 2005. Vector transmission of Xylella fastidiosa: Applying fundamental knowledge to generate disease management strategies. Annals of the Entomological Society of America 98:775-786. 

Blua MJ, Phillips PA, Redak RA. 1999. A new sharpshooter threatens both crops and ornamentals. California Agriculture 53: 22-25. 

[CDFA] California Department of Food and Agriculture. 2014a. Pierce’s disease control program. CaliforniaPlant Quarantine Manual, Document 454.  http://pi.cdfa.ca.gov/pqm/manual/pdf/454.pdf.  (includes GWSS host list) 

[CDFA] California Department of Food and Agriculture. 2014b.Glassy-winged sharpshooter in California, Pierce’s disease control program maps, infested areas. http://www.cdfa.ca.gov/pdcp/Maps/GWSS_Distribution2013.jpg. 

Hopkins DL, Purcell AH. 2002. Xylella fastidiosa: Cause of Pierce's disease of grapevine and other emergent diseases. Plant Disease 86: 1056-1066. 

Perring TM, Farrar CA, Blua MJ. 2001. Proximity to citrus influences Pierce’s disease in Temecula Valley vineyards. California Agriculture 55:13-18.