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 berry yields up to 40%, delay fruit maturity, and impede fruit pigmentation. During the past decade, there has been an unexplained increase in disease incidence and damage. GLRaV can be spread from vine to vine by several species of mealybugs and soft scales. 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. Less than 2.7% of mealybug crawlers, acquired GLRaV-3 from vines in the field and transmitted leafroll virus to vines in the greenhouse, compared to 0.7% in 2009. This is a lower acquisition rate than previously reported from greenhouse and laboratory studies. This acquisition percentage is likely influenced by the seasonal variation of GLRaV titer in the plant, and the feeding and survival of mealybugs on field-grown grapevines. Future studies will determine seasonal changes in the acquisition and transmission rate to provide better guidelines for insecticide applications for vector control. We continued a five-year field trial testing the impacts of ‘zero tolerance’ for mealybugs on GLRaV infection establishment and spread. A newly-established 20 acre vineyard, bordered by older blocks that contain both GLRaV-infected vines and mealybugs, received selective pesticide treatments in 2009 and 2010. No mealybugs were found in visual inspections of control and treatment plots in June and August. However, pheromone traps showed the presence of male grape mealybugs in both treatments, indicating the possibility of an ephemeral mealybug population moving into the vineyard block, or a resident population that was too small to find using a visual search. In the first year of the trial, all vines were inspected for GLRaV symptoms and 1 vine tested positive for GLRaV-3, while in the second year, 2 vines tested positive for GLRaV-3. The trial will continue for three more years and the results will show whether blocks can be established free of GLRaV 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. In 2009, 5% of grape phylloxera tested positive for GLRaV-3 and none tested positive for GLRaV-2, after six months on plants with GLRaV-2 and -3. In 2010, additional plants with GLRaV-1,-2-3, and -5 were infested with phylloxera in the greenhouse, and none of the 125 insects tested positive for GLRaV after one generation on the leafroll-positive plants. We note that this does not show that phylloxera can transmit GLRaV, and pathogen acquisition is only the first step in transmission by a potential vector. We stress that at this point in time we the standard guideline remains in place and we do not consider phylloxera to be an important GLRaV vector.