The behaviour and control of the blackcurrant gall mite Phytoptus ribis (Nal.)

SUMMARY By placing groups of potted blackcurrant bushes at weekly intervals in a plantation infested with Phytoptus ribis during 1959, 1960 and 1961 it was shown that the maximum rate of mite emergence and infestation coincided with the blossom period and that approximately half of the total infestation took place at this growth stage. Effective migration continued for 12–14 weeks and during the early part of this period the rates of emergence and infestation were directly affected by temperatures below 50° F. Temperatures above 50° F., particularly towards the end of the migration period increased the rate of desiccation of the old buds and forced mites to emerge. When lime sulphur was placed, by hand, in all new leaf axils at weekly intervals, on bushes where there were large mite infestations, complete control was achieved, but this was not found where all the old wood or all infested buds were covered with lime sulphur. These investigations indicate that, for commercial control, repeated treatments must be made especially during the blossom period. Single applications of 1% lime sulphur with a wetting agent were made at various developmental stages on blackcurrants at Long Ashton in 1961 and the percentage reduction of infestations of the blackcurrant gall mite estimated. The most effective were those during the blossom period and particularly where they coincided with the periods of greatest mite emergence and penetration of new axillary buds. Applications were made from the first open flower stage until early June; some control, although progressively decreasing, was given by applications made up to 6 June. More than one spray was necessary for adequate commercial control. Two commercial wettable sulphur powders, applied at the first open flower stage and again 2 weeks later, were found to be approximately as effective as lime sulphur. A wetting agent was used in all the sprays but at half the concentration with the wettable powders. Thiodan did not appear to provide a better control of the gall mite than lime sulphur when two applications of each were made with a wetting agent. Three applications gave better results than two and the addition of a wetting agent was of great importance: where none was used an increase of infestation resulted after three applications, whilst a 94.1% reduction was achieved by three applications of 1% lime sulphur with 0.025 dinonyl sodium sulphosuccinate (0.0125% active ingredient). Hand lances using high volumes of liquid were used throughout.