Mealybug Pests and an Emerging Viral Disease: Vector Ecology and Their Role in Grape Leafroll Associated Virus Epidemiology

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 evaluated grape mealybug acquisition and transmission of GLRaV-3 from trunks, spurs, canes, and leaves. Two point three percent of mealybug crawlers acquired GLRaV-3 from vines in the field and transmitted leafroll virus to vines in the greenhouse. This is a lower transmission rate than previously reported from greenhouse studies. Surveys of grape mealybugs from five Napa County vineyards were taken monthly (May through October) and found 56%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. 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 older blocks that contained both GLRaV-infected vines and mealybugs, received pesticide treatments for mealybugs in 2009, 2010 and 2011. No mealybugs were found in visual inspections; 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 each year, while in 2011, six new vines tested positive for GLRaV-3. 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 vines. One hypothesis is that virus-carrying grape mealybug crawlers were ?blown? into the plot, and infecting previously-healthy vines during feeding. This trial will continue for two more years and to see if disease-free blocks can be established though the use of annual insecticide treatments to eliminate mealybug vectors. We investigated grape phylloxera as a possible vector of GLRaV. Previous studies in New Zealand excluded this insect as a vector and we consider this to be the standard guideline. Nevertheless, we are conducting trials to alleviate grower concerns and eliminate the possibility that phylloxera play a role in GLRaV transmission in California. After feeding on virus-positive vines in the greenhouse for six months, between 1 and 5%of phylloxera acquired GLRaV-3 or -5, while none acquired GLRaV-1 or -2. None of the 44 healthy vines planted in the same pots as virus-and phylloxera-infected plants developed GLRaV. Additionally, none of the 939 phylloxera collected from GLRaV-2 infected vineyards contained leafroll virus, leading to a conclusion that phylloxera is unlikely to be a vector of GLRaV.