Success stories related to agricultural pest and disease issues rarely include eradication, but that’s just what California could face later this year.

For Napa Valley and its world-class wine grape industry, the discovery of the European grapevine moth (EGVM) in massive numbers in 2009 threatened significant economic damage from something never before seen on the North American continent.

In 2010, officials trapped over 100,000 adult male moths on more than 3,800 traps in Napa County alone as officials began using a pheromone developed in Japan. The pest was also discovered in other grape regions of California, including the San Joaquin Valley, the North Coast, Central Coast and Nevada County.

Nobody was perhaps closer to the issue in Napa County, or more visible as the issue became public, than Ron Wicker, a grower of Chardonnay and Cabernet Sauvignon wine grapes in the heart of Napa Valley.

For Wicker, the clarity of hindsight suggests EGVM was well-established in the Napa Valley before his 2009 discovery in 11 acres of Chardonnay grapes.

“We always had a little bit of Botrytis in our vineyard and I just attributed that to the tight clusters and heavy crop,” Wicker said of indications he now knows were evidence of EGVM in his vineyard.

Prior to about 2005, Wicker said his block of Chardonnay grapes in the Oakville Region of Napa Valley produced large crops each year, peaking at about 88 tons for 11-acre spread on the valley floor.

Wicker admits that production declines starting in 2006 did not capture his attention at first, but then began to make him wonder as yields continued to fall.

Wicker now knows that Botrytis – a fungal disease in grapes that among its sources can be caused by EGVM feeding on ripening berries – was a harbinger of problems to come.

Wicker’s story

Wicker was inspecting the vineyard close to harvest in 2009 when he noticed Botrytis on the south end of his vineyard. His viticulturist had already been watching it on the north side of the vineyard, so Wicker asked the viticulturist to come down and walk the vineyard with him.

“We got curious after finding it in every cluster, so we broke open some clusters and found live larvae,” Wicker said. “We had no idea what it was, so we pulled a sample and took it to the Ag commissioner’s office.”

This led to an identification process that took several weeks. Eventually the U.S. Department of Agriculture made the EGVM determination.

“We went back through the vineyard a week after we pulled that original sample and there was not one cluster unaffected,” Wicker said. “So we went through the vineyard to see if we could do a clean pick and pick some undamaged fruit. It took us about an hour and a half to pick four boxes full of fruit. I ran it up to the winery we were selling to – I was a little leery of even running it up there at the time. We took one look at it and decided to run it back to the vineyard and dump the crop on the ground.”

The result: a total loss of about 60 tons of grapes that at the time could have sold for $2,200-$2,400 per ton.

Biology

Since then, Wicker learned much about the biology of the EGVM.

Wicker points to an opportunity to visit one-on-one with a researcher from Italy in his Oakville vineyard on how bad the situation was early in the outbreak.

One evening while Wicker and the researcher were standing in his Oakville vineyard the mating flight of EGVM was in full swing.

“He was absolutely stunned at what he saw,” Wicker said. “He said the research community had never seen this before.”

The researcher told Wicker that it was paramount to get complete control of the adult population if he was to have success in ridding his vineyard of the pest.

Wicker later learned that evening is when the mating flights of EGVM take place, making adults more visible.

Invasive pest

The European grapevine moth is not native to North America. Regulators and university researchers still do not know how it arrived in California, though studies by Kent Daane with the University of California found biological similarities between those found in Napa with populations discovered in Chile and Europe.

EGVM has been found in Europe, North Africa and in Chile, according to Monica Cooper, viticulture and pest management specialist with the University of California Cooperative Extension in Napa County.

The Chileans discovered EGVM two years ahead of Napa, which was fortuitous for Napa County growers as Chile’s response helped shape Napa’s response.

According to Cooper, the EGVM may complete two to five annual generations, depending on latitude. Observations in California suggest they can complete three generations before going into diapause and overwintering under the bark of vines.

During the first flight, researchers believe the female glues a single egg on or near the flower cluster, and will continue laying solitary eggs in separate locations. Larvae from that generation feed on the flower before bloom, causing yield losses.

Second and third generations of EGVM lay eggs on grape berries, where larvae then tunnel into the berry, consuming it from the inside. They later exit through the same hole bored into the fruit.

It’s the feeding damage from second- and third-generation larvae that causes Botrytis and other secondary fungi, researchers say.

EGVM lays eggs in different locations and not in clusters, Wicker learned. Because of this he said the pest is efficient in quickly expanding its population, which is how he suspects it gained a hold in his Chardonnay vineyard.

Industry response

In April 2010, a group of researchers from the U.S., Europe and South America visited Napa vineyards to investigate the situation and help determine a course of action. First, they needed to see just how extensive the infestation was.

Cooper and Lucia Varela, integrated pest management advisor for the North Coast, were part of the Technical Working Group (TWG).

The group traveled to Chile shortly after the Napa discovery and studied literature to come up with a two-pronged approach that looked successful in eradicating the EGVM from California, though that formal declaration will not be made until later this year, pending the outcome of trapping through the rest of the growing season.

“We were very lucky we had a lot of sources to study from,” Varela said.

Also beneficial to the effort, Varela said California and the USDA allowed scientists to conduct research simultaneous to eradication efforts.

From the Chile trip, researchers learned the importance of first-generation treatment efforts, a practice not employed in Europe.

Because mating disruption efforts required a pheromone not labeled for used in the United States, Varela said that process had to be fast-tracked by the Environmental Protection Agency and California Department of Pesticide Regulations. That process took about three months to complete.

Just as the pheromone was approved in the United States, Wicker said the local agriculture department would not let growers employ mating disruption efforts until delimitation trapping could be completed. Because the same pheromone was being used in the traps, it was thought mating disruption efforts could compete with the traps and not give regulators a true count of EGVM.

In the meantime, growers like Wicker were treating vineyards with approved insecticides. Wicker said he used a broad-spectrum insecticide to provide a rapid kill.

“We made a really difficult decision because this vineyard had not had an insecticide put on it in eight or nine years, which was one of the problems because we hadn’t inadvertently attacked this pest,” Wicker said.

Fortunately for Wicker, his one-time use of the broad-spectrum insecticide did not have a long-lasting impact on beneficial insects.

Steven Moulds grows Cabernet Sauvignon and Cabernet Franc wine grapes in the Oak Knoll District of Napa Valley. He is also the board president for Napa Valley Grapegrowers (NVG), a trade association with over 700 members that represents more than 60 percent of the wine grape acreage in Napa Valley.

“EGVM was a big wake-up call for us,” Moulds said.

He did not experience the heavy population of EGVM in his area seen elsewhere, though he still participated in eradication efforts recommended by the TWG, and was part of the overall industry response as NVG board president.

NVG Executive Director Jennifer Putnam said her organization took charge of regular communications with members and growers in the Napa Valley as EGVM eradication efforts continued.

Putnam praised Whitmer for his attitude towards eradication.

“He always said this was not a prevention program but an eradication program,” she said.

Collaborative effort

Current Napa County Agricultural Commissioner Greg Clark worked as a deputy under Whitmer when the EGVM outbreak occurred. Clark credited the collaboration between local agricultural officials, the public,  state and federal government agencies and the research community with what is shaping up to be a significant success story.

Since his appointment as agricultural commissioner after Whitmer retired, Clark has used social media and other means to inform the public about pest exclusion efforts, including the glassy-winged sharpshooter.

Clark says formal EGVM eradication could be declared this fall if no further EGVM finds are made. Trap counts for the moth were at the highest point in 2010 – the first year of the program – at more than 100,000. Of that, 99 percent were discovered in the first flight.

The following year trap counts plummeted to 113 on the entire year, sinking to 77 in 2012 and 40 in 2013. There have been no moths trapped since the second generation flight of 2013.

The county currently has 11,500 pheromone traps at a density of 100 per square mile in the vineyards and 25 per square mile in the urban areas and will continue its regular inspections of those traps.

Napa’s quarantine zone has also shrunk considerably since 2013. Quarantines in the counties of Mendocino, Fresno, Merced, San Joaquin, Santa Cruz, Santa Clara and Nevada have since been removed. Only portions of Napa, Sonoma and Solano counties remain under quarantine for EGVM.

Clark said about $50 million was spent by the Napa wine grape industry for treatments, mating disruption efforts and quarantine requirements. Much more than that was spent by government agencies on eradication.