Grape rust mite: a reoccurring viticultural pest

Abstract: The grape rust mite (GRM) Calepitrimerus vitis (Nalepa) is a well known eriophyoidin European vineyards. High densities of hibernating GRMs can significantly retard the growth ofshoots in early spring and they can also cause leaf and shoot distortions. Retarded shoot growth isan increasing problem in the Valais, the hottest and driest viticultural region of Switzerland. Toavoid damages, winegrowers rely on pre-bud burst treatments mostly based on the basis of thedamage in the previous season. Occasionally, an empirical threshold level of 20 GRMs/bud isapplied. For a better estimation of this threshold, the dynamics of early spring GRM populationswere studied in five vineyards of the cultivar “Amigne” over the last three years. With oneexception, GRM densities below the current threshold had no effect on shoot growth and clusternumber. However, values near or above this level, necessitated chemical interventions. Thetrapping of GRMs at the basis of shoots with double-faced adhesive tape helped to identify thestart of GRM activity in the spring and highlighted a massive migration of individuals from theold wood towards developing shoots. Although the number of overwintering GRMs captured atthe basis of shoots was highly correlated with populations found in the buds, it explained someshortcomings of the latter. The use of double-faced adhesive tape may therefore provide areliable method to estimate pest pressure and to monitor the period of bud colonisation.These two information may be very helpful to winegrowers in order to assess the necessityof a chemical intervention against GRM and to determine its timing in early spring.