Grape downy mildew spread and mite seasonal abundance in vineyards: effects on Tydeus caudatus and its predators

The tydeid mite Tydeus caudatus can prey upon grape eriophyoids but little is known about its alternative foods. Observations in a commercial vineyard located in northeastern Italy showed that densities of T. caudatus were often correlated to downy mildew spread on the vegetation. T. caudatus numbers increased in late summer when downy mildew symptoms occurred on a high number of leaves. The predatory mite Paraseiulus talbii also increased in late season following the increase of tydeids. T. caudatus populations were frequently higher on leaves with downy mildew symptoms than on leaves without symptoms. In late summer tydeid densities were positively correlated to the extent of symptoms. These relationships were sometimes confirmed for P. talbii. In two vineyards colonised by T. caudatus, untreated plots or fungicide treated plots were compared. Downy mildew infections rapidly spread in the control plots while they were low on fungicide-treated plots. T. caudatus densities reached significantly higher densities in the control than on fungicide-treated plots. Most of these fungicides did not reduce tydeid populations when downy mildew was absent. Isoelectric-focusing electrophoresis (IEF) was used to detect downy mildew in mites and glucose phosphate isomerase (GPI) was selected from enzymes. A high proportion of T. caudatus females, collected from infected leaves, showed a GPI isozyme allele corresponding to the downy mildew isozyme in addition to the tydeid intrinsic alleles. This phenomenon was also observed for P. talbii. All T. caudatus females confined on symptomatic leaves became positive by IEF after few hours. P. talbii females became positive after preying on tydeids collected from symptomatic leaves or reared on pollen and then confined on symptomatic leaves.