More info required on exactly how the fruits are affected, and what time of year, and any other leaf symptoms. Pear trees are subject to all kinds of infestation and disease, although they don't suffer from Peach Leaf curl. Did the fruits develop scabby patches, or have a tunneled area inside, or a caterpillar inside, or did they just develop a blackened or brown area at one end, etc. The 'mites' you describe - were they winged, just on the leaves, or on the fruits, and when did they appear, what size were they? Any signs of sooty mould anywhere, either on the fruits or the leaves?

Okay, Pauncho, you have told me that you get caterpillars burrowing into the fruits when they're still really small - it does sound like Pear Midge, and the maggots inside should be whitish orange up to 2mm long. Usually, affected fruits become blackened and then fall from the tree in late spring and early summer. If that fits the picture, then it is Pear Midge, Contarinia pyrivora, which is a small fly or gall midge - they go into the soil when the fruitlets drop, pupate and emerge as adults next spring, so clearing all and any fallen fruits immediately is a good idea. On small trees, pick off and destroy infested fruitlets before the maggots complete their feeding. Prevent the adult flies from laying eggs by spraying with pirimiphos-methyl or fenitrothion when the blossom is at the white bud stage - when the petal colour can be seen, but before the flowers open.
if the chemicals mentioned are no longer available (they're always being withdrawn, though I know fenitrothion is still available in certain insecticides) seek out replacement sprays which treat for Pear Midge.