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 apical, middle, and basal leaves. We found <7% of surviving mealybug crawlers, inoculated on vines in July and August trials, were able to acquire and transmit GLRaV-3. This is a lower acquisition rate than previously reported from greenhouse and laboratory studies, as would be expected. The seasonal variation of GLRaV titer in the plant, mealybug feeding performance and survival in the field, and trial conditions are possible factors, Future studies will determine seasonal changes in the acquisition and transmission rate in order to provide better guidelines for vector control decisions with respect to insecticide timing. A field trial was conducted in a newly established 20 acre vineyard, which is bordered by older blocks that contain both GLRaV-infected vines and mealybugs. In this five year trial, we are testing a ‘zero tolerance’ for mealybugs, established using selective pesticides applications, as a GLRaV control for new plantings. A June and August inspection of vines found no mealybugs in either control or treatment plots. However, data from 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 this first year trial, all vines were inspected for GLRaV symptoms and 1 vine tested positive for GLRaV. The trial will continue for four 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 began studies of grape phylloxera as a possible vector of GLRaV. Previous studies in New Zealand excluded this insect as a probably vector and we consider this to be the standard guideline. Nevertheless, we are conducting trials to alleviate grower concerns of phylloxera as a possible vector. In laboratory trials, grape phylloxera were placed on plants with either GLRaV-2 or GLRaV-3. After six months, no phylloxera tested positive for GLRaV-2, while 5% tested positive for GLRaV-3. 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.