Belowground population dynamics of grape phylloxera (Daktulosphaira vitifoliae Fitch, Phylloxeridae) in cool climate viticulture

By parasitizing the leaves and roots of Vitis species, grape phylloxera (Daktulosphaira vitifoliae Fitch, Phylloxeridae) is one of the most devastating pests in viticulture. D. vitifoliae is mainly controlled by grafting susceptible Vitis scions to tolerant Vitis rootstocks. But in most grafting combinations, D. vitifoliae populations still develop below ground. The successes of newly established control methods, such as the use of entomopathogenic fungi, rely heavily on the knowledge of the population dynamics of D. vitifoliae in vineyards. However, fundamental questions still remain regarding the belowground dynamics of D. vitifoliae populations under field conditions.  Here we present results from a two-year field study in which we describe densities and development of belowground D. vitifoliae populations as well as the dynamics of their habitat on Vitis berlandieri x Vitis riparia rootstocks (cv. 5C and cv. 125AA). We recorded the abundance of juvenile and adult wingless females, as well as nymphal larval stages per cm² root surface on a monthly basis. Furthermore, we assessed the occurrence and quality of root galls, root surface and diameter, and soil and environmental parameters. D. vitifoliae population densities increased from May to January, with highest peak in August and a second peak of juvenile wingless females in September. We detected significant differences in the development of population structure between the two rootstock variants. In winter, juvenile wingless females tend to crowd, and occur in significantly higher densities than in spring. Population densities were related to biotic factors, such as gall development and type of conspecific neighbours, whereas population structure was related to abiotic factors (GLM, log-link for Poisson distribution). Our results suggest that grape phylloxera population dynamics relies both on the ability to crowd and on the ability to develop root galls.