Napa County agricultural officials may pursue state quarantine violations against a Riverside County nursery after a shipment of landscaping trees received by a winery was found with viable Glassy-Winged sharpshooter (GWSS) eggs during an inspection.

This is the second time this year Napa County has found GWSS egg masses on imported plant materials.

According to Napa County Agricultural Commissioner Greg Clark, the shipment of trees came with a certificate of quarantine compliance from Riverside County. After testing the plant materials for pesticide residues, Clark said residue levels were well below the minimum necessary for GWSS control, suggesting the previous treatment was insufficient based on University of California research.

Along with the Napa County investigation, which will seek answers into why officials there were not notified ahead of time of the shipment as stipulated by state quarantine laws, Clark says Riverside County agricultural officials will also seek answers into why protocols there were allegedly not followed.

The reason agricultural officials remain concerned about the GWSS is its ability to effectively vector Pierce’s Disease (PD), a bacterial disease that can cause grape vine death. According to Clark, GWSS is not known to be in Napa County and his goal is to keep it that way.

Evidence of infestation

While inspecting 100 large trees delivered in late March from a Riverside County nursery, Napa plant inspections staff found over 50 older and apparently nonviable GWSS egg masses and one that appeared to be fresh and viable. The California Department of Food and Agriculture confirmed the viability of one of the egg masses.

That discovery led to the ordered treatment of the trees with an insecticide with immediate and residual control effective against the GWSS. Treatment cost for the application and the included supervision by county agricultural staff was borne by the winery.

Keeping GWSS out of Napa County is a high priority for the local agricultural department, Clark says. In 2014, grape growers produced over $700 million of high-quality grapes which eventually went into bottles of wine that sell for more than $100 each.

Clark encourages growers and landowners to source their nursery stock from local nurseries in an effort to avoid the import of invasive pests.

“The second find of a viable egg mass this year is of great concern and demonstrates the ongoing need and importance of our GWSS exclusion program,” Clark said. “It is imperative that individuals and businesses source landscape plants locally and which have ideally been grown in northern California where GWSS is not established.”

The recent GWSS find follows the discovery just a few weeks earlier of another egg mass in nursery stock coming out of Orange County. Those egg masses were viable at the time of inspection, according to Clark’s office.

Sonoma County inspectors also found an egg mass on the Orange County shipment. That plant material was later destroyed, Clark said.

According to university researchers, the GWSS is a prolific flyer and has the ability to vector PD over a wide area. Also troubling about the pest is its wide host range – at least 250 different known plant species. The region already has the native blue-green sharpshooter, which is also capable of vectoring PD, but is said by University of California experts to be less effective than GWSS at vectoring the disease.