Vineyard mealybugs (Hemiptera:Pseudococcidae) are an increasing matter of concern for
grape growers, as the economic losses resulting from their infestations continue to increase.
Several species of mealybugs are present in California, but the vine mealybug (Planococcus ficus,
Signoret) is the one creating most problems. It is an invasive species introduced ~20 years ago,
and not yet under control. The control is complex in conventional viticulture but is even harder in
organic production where systemic insecticides (e.g. spirotetramate) are not available and only
contact products are allowed. These products cannot easily reach the insect population often
located under the bark. This project aimed to compare on a large vineyard trial the efficacy of most
common and best-suited products commercially available in organic production: pyrethrins, neem
oil, diatomaceous earth, Chromobacterium subtsugae. The project also controlled for side effects
on grapevine physiology and grape composition related to spraying dusts and oils on leaf and berry
surfaces. The experiment was set up as a randomized complete block design with 4 treatments plus
control and 4 replicates. Each replicate was 1 acre large for a total size of the trial of 20 acres. The
vineyard was planted with Pinot Noir and moderately infested with mealybugs.

None of the products were effective in controlling mealybugs on the trunk and performed
similarly. Pyrethrins were the least effective in controlling mealybugs on leaves, while
diatomaceous earth and neem oil were most effective, but not significantly different from the
control. An inverse relationship appeared between efficacy on the leaves and presence in the
cluster. The most effective products had larger amounts of mealybugs inside the clusters, and the
least effective products on leaves had fewer mealybugs in the clusters. Plants treated with neem
oil had significantly lower stem water potentials. Treatments did not have any significant effect on
gas exchange measurements, except for one date, when azaguard had cooler leaves and higher
photosynthesis and stomatal conductance. Diatomaceous earth never significantly reduced
photosynthesis or stomatal conductance, even though it well covered the leaves with dust.
Although not statistically significant, we observed a strong tendency in sugar content. Brix level
was lower in the control than in all other treatments and reached up to 1.9 °Bx of difference in the
second measurement date. This delay in ripening was significant at the pH level when the control
had a significantly lower pH than most of the other treatments.

This project provides new and unbiased information that will help organic growers to
choose the right products to apply for controlling mealybugs in vineyards.

This project continues to develop management strategies for grape growers with grapevine leafroll disease. We recently completed a mapping project spanning 4 grape growing seasons (2010-2013) to determine the relationship between grape mealybug (GMB) populations and the incidence of grapevine leafroll disease (GLD) in Napa County vineyards. We surveyed 10  vineyards and conducted a preliminary analysis to demonstrate the link between the vector, GMB, and disease incidence. In vineyards with medium to high levels of GLD, rates of disease spread are influenced mainly by (1) the initial number of vines in a vineyard with GLD (disease pressure) and (2) GMB populations in the previous growing season. In vineyards with low levels of GLD, rates of disease spread are influenced by the initial disease incidence as well as the number of GMB found in the current season. These observations are consistent with similar studies in other grape growing regions around the world, and have clear implications for management: in order to successfully manage GLD incidence and spread, growers must evaluate disease pressure and GMB populations. Our preliminary analysis suggests that managing the vector alone may not be sufficient to decrease rates of disease spread, particularly in vineyards with medium to high GLD incidence.

Studies of other pests and diseases have demonstrated that a regional approach to disease management has more potential for success than individual efforts. This is especially true in Napa County, where a single or few growers rarely control large swaths of contiguous vineyard acreage. In Napa County, where neighboring vineyards share the burden of disease management, growers may jointly launch regional responses to GLD. We are developing a template for these regional efforts, with a pioneering group of 20 grape growers farming 1900 contiguous acres. The grower group is committed to implementation of coordinated, regional GLD management strategies that include monitoring of GLD and GMB, as well as lowering disease and vector pressure. Most importantly, the group shares information, communications and develops goals and activities at regular meetings (4-6 meetings per year) focusing on the implementation of GLD management strategies at a regional level.

This project is also developing the use of hyperspectral imaging to map the incidence of GLD in commercial vineyards and nurseries. Imaging spectroscopy provides a potentially valuable alternative to lab testing and field scouting in that it is efficient, non-destructive and relatively inexpensive. From an aircraft thousands of acres may be imaged in a single three-hour flight. Additionally, remote sensing provides continuous measurements to construct a map of the target vineyard. Our studies in Cabernet Sauvignon vineyards have shown a 94.7%correlation between leafroll-diseased vines identified in ground surveys with those identified through aerial imaging, suggesting a high level of confidence in the accuracy of our measurements for this variety, and potential to develop the use of this technology for other grape varieties. Hyperspectral airborne imaging could revolutionize the the collection of disease incidence data by increasing ease, accuracy and efficiency of measurements.

The overarching goal of this research is to obtain information about transmission of Grapevine leafroll-associated virus 3 (GLRaV-3), the primary virus species associated with spread of the economically damaging Grapevine Leafroll Disease (GLD) in Napa Valley. Such information is necessary to inform control strategies, it is clear that knowledge-based management of vector-borne diseases requires a robust understanding of how the pathogen spreads in vineyards. Mealybugs are the vectors associated with spread of GLD, but nothing is known about differences in transmission efficiency among mealybug species inhabiting vines in California. Furthermore, genetically distinct variants of GLRaV-3 exist but nothing is known about differences among these variants in terms of their ability to spread, or what the relevance of that variation is to GLD epidemiology. Lastly, all previous GLRaV-3 transmission studies were done under greenhouse conditions, and it is not known how well the results of such studies predict transmission in vineyards. This research addresses these significant gaps in knowledge.

We have completed proposed single and simultaneous mixed GLRaV-3 variant inoculations in greenhouse trials, using grape and vine mealybugs. Though two GLRaV-3 variants from singly infected source plants did not differ in transmission efficiency, the transmission efficiency of one variant was substantially lower when acquisition occurred from a co-infected source plant, indicating inhibition of transmission by the other variant. This may mean that one variant can be transmitted more efficiently than another and increase its incidence in the landscape (e.g. Napa Valley). It is not known whether some GLRaV-3 variants are more pathogenic than others.

We also set up an experiment in Napa Valley in summer 2011, inoculating 60 mature grapevines with GLRaV-3 using grape mealybugs as vectors. Each vine was inoculated using 10 first instar mealybugs, and then treated with insecticide two days later. Three months after inoculation, 20 of 60 plants tested positive for GLRaV-3 from our inoculations. No symptoms appeared in 2011. During the following growing season, GLD symptoms first began to appear in our experimental vines in June. By July, symptoms appeared in 29 of 60 experimental vines, and no other vines became symptomatic in 2012. Berry quality was affected in symptomatic vines compared to asymptomatic vines in the experiment.

Transmission in a parallel greenhouse experiment was higher than in the vineyard. In 2012 we set up a second field inoculation experiment in Napa to compare transmission of two different GLRaV-3 variants, by grape and vine mealybugs, in both Chardonnay and Pinot Noir. Inoculations were completed in July. Results from the 2012 inoculations are pending. This is the first time it has been shown that GLD symptoms due to mealybug inoculation of GLRaV-3 into established mature vines (~15 years old) in commercial vineyards are expressed in the following growing season. Results also showed that the entire vines were symptomatic in 2012, instead of just the inoculation site. Lastly, transmission success in the field was about 6%per individual mealybug.

Grapevine leafroll-associated viruses (GLRaV) are a complex of viruses that cause leaf chlorosis and leaf margins to “roll” downward.  GLRaVs can reduce yields, delay fruit maturity, and impede fruit pigmentation.  Our work concerns GLRaV field epidemiology with respect to its insect vectors. In field studies, we continued evaluating seasonal densities of grape mealybugs and leafroll virus in five vineyards with varying levels of leafroll infections and found 43%of grape mealybugs collected on leafroll-infected vines tested positive for GLRaV-3. The proportion of leafroll-infective mealybugs was related to the number of leafroll-positive vines in the vineyards surveyed. Vineyards were mapped for leafroll virus symptoms in fall 2012 but the presence of Red Blotch Virus at two of the sites made accurate mapping difficult.

We continued a five-year field trial testing the impacts of “zero tolerance” for mealybugs on GLRaV infection establishment and spread. A 20 acre vineyard planted in 2008 from certified virus-free scion, and bordered by blocks with GLRaV-infected vines and mealybugs, received pesticide treatments in 2009, 2010, 2011 and 2012. No mealybugs were found in harvest counts; however, pheromone traps showed the presence of male grape mealybugs. All vines were inspected annually for GLRaV symptoms. In 2009 and 2010, there was one new infected vine, with strong leafroll symptoms and positive PCR test, each year in 2009 and 2010, six new vines in 2011, and 2 in 2012. GLRaV-3 variants in the spray trial block (-3a, -3c, and -3d) were different than the GLRaV-3 variants in adjacent virus-infected blocks to the north and west (-3b). The leafroll-infected vines were randomly distributed inside the mapped plot, with similar numbers in insecticide and untreated rows. Our hypothesis is that virus-carrying grape mealybug crawlers were “blown” into the plot, and infecting previously-healthy vines during feeding.

We conducted mealybug behavior studies in the greenhouse to determine if mealybugs prefer vines without GLRaV; this would result in movement of vectors away from infected vines to healthy vines. In non-choice tests, mealybugs survived equally well on infected and uninfected vines, while in choice tests, equal numbers of mealybugs were chose infected and healthy vines, indicating no preference for one over another.

In side-by-side comparisons, the five major GLRaV vector species found in California, EFLS, OMB, LTMB, GMB, VMB and GiMB all transmitted GLRaV-3 but at different rates. We showed that GLRaV-3 can be detected 3-4 weeks after first inoculation in any part of the inoculated vines using crude extraction of RNA and RT-PCR. The latency period studies (collected thus far) further signify the fact that VMB crawlers can acquire leafroll virus (GLRaV­3) from newly inoculated vines as soon as two weeks after first inoculation and then successfully transmit it to the healthy grapevines.  Ongoing studies are investigating the effectiveness of different insecticides to not simply kill mealybugs but reduce transmission rates.

Grapevine leafroll disease is a threat to vineyard health and sustainability worldwide. Mealybugs are the main vector of consequence. This project is providing critical information on the relationship between mealybug populations and incidence of leafroll disease in Napa County vineyards. In the first of a three-year project, we identified thirteen vineyard study sites in Napa County. We collected data on mealybug damage to the fruit and disease incidence at each site, just prior to harvest. We will continue to do this each year during the next two growing seasons. We will then analyze the data to correlate vector incidence to disease. We are also developing the use of pheromone-baited sticky traps as sampling tools. Growers have traditionally relied on field sampling to provide information on mealybug life cycle and density. Sticky traps have the potential to be more sensitive than field surveys for detecting small populations of grape mealybug. We have continuously deployed pheromone-baited sticky traps at the thirteen vineyard study sites in Napa County. We detected grape mealybug males at all field sites. We gathered preliminary information on male flight patterns from June to December, 2010. One peak in the male flight was detected near harvest, corresponding to published information on the grape mealybug life cycle.