In this discussion of the morphology and habits of mites and the damage that they cause on fruit trees in Belgium, a distinction is made between gall-forming eriophyids (of which the commonest are Eriophyes pyri (Pgst.) on pear, Aceria mali (Nal.) (E. mali) on apple and E. similis (Nal.) on plum) and free-living phytoptids on the buds and leaves (of which Aculus schlechtendali (Nal.) infests apple, A. fockeui (Nal. & Trt.) plum and cherry, Epitrimerus pyri (Nal.) pear and Diptacus gigantorhynchus (Nal.) peach and plum). More detailed studies on A. schlechtendali and E. pyri in 1967-82 showed that the timing of the different development stages varied greatly from year to year and needed to be monitored annually for accurate timing of insecticide applications. Damage tended to pass unnoticed at first, but leaf withering, defoliation and bud destruction on all the fruit-tree species and necrotic patches on leaves or witches' broom in addition on stone-fruit trees had serious consequences, and treatment needs to be begun before these symptoms appear. Investigations of different methods and products for treatment at the Gorsem Research Station in 1970-81 indicated that satisfactory control (over 90% in some cases) could be obtained with winter oil sprays, with spring applications to prevent migration or 2 post-floral applications against these mites and also Panonychus ulmi (Koch), using many compounds of which lists are given; some control was also afforded by fungicides if they were applied repeatedly from April onwards.