Mealybug Vectors and Grapevine Leafroll Disease: Temporal and Spatial Studies to Understand the Relationship Between Vector Populations and Disease Incidence

Grapevine leafroll disease is a threat to vineyard health and sustainability worldwide. Mealybugs are the main vector of consequence. This project continues to provide critical information on the relationship between mealybug populations and incidence of leafroll disease in commercial vineyards. In 2011, we completed our 2nd year of data collection in fourteen, 10 acre study areas in Napa County. Near harvest time in each of the previous two years, we have collected data on (1) mealybug populations in grape clusters and (2) symptoms of leafroll disease incidence. Over the course of the study period, this work will accurately map chronological changes in vector density and location with reference to leafroll disease incidence. This study is the first of its kind; ultimately, differences in leafroll disease incidence among sampled vineyards can be used to model leafroll disease epidemiology, as related to vector density and location, virus species, and vineyard cultural practices. A similar study with a collaborator in Oregon is modeled on this work. These data, in combination with information on virus transmission and mealybug control strategies, will facilitate the development of management strategies for grapevine leafroll disease. We are also developing the use of sticky traps as sampling tools. Baited with pheromone lures, these traps attract and capture male grape mealybugs, and are used to follow the flight patterns of the male. Consistent with previous field observations on sessile grape mealybug populations, we have verified that there are two male grape mealybug flights per year in Napa County. Peak flights occur in June and September/October, respectively. Sticky traps have the potential to be more sensitive than field surveys for detecting small populations of grape mealybug. As historical trap catch data are collected, they may also be used to track changes in mealybug populations over time. Additionally, trap catch data may be used to predict events in the life cycle of the grape mealybug. The ability to accurately predict these events is essential for appropriate timing of insecticide applications. In 2012 and 2013, we propose to work with Dr. Daane to develop a predictive model for grape mealybug development, similar to work that was completed for the invasive vine mealybug. Once laboratory studies of grape mealybug development are complete, we will use the trap catch data generated in this study to verify the development model under field conditions. Once developed, the model has the potential to allow growers to predict events in the life cycle of the grape mealybug, eventually leading to optimal timing of insecticide applications on a field-scale.