Our expert, David Williams, has responded with the following answer:
If you are seeing mottling on leaves but no webbing or cast skins (they will be white and shrivelled) then check the wood on nearby shoots and twigs. Bryobia mites often move off the leaves in the brighter parts of the day (mid-morning to mid-afternoon) so you may find them on the wood.
If the leaves have a rusty appearance as well as mottling and webbing is present, the culprit is probably Two-spotted mite. The motiles (adults and nymphs) may have vacated the leaves because they no longer provide quality food. They may be establishing on other leaves that are not yet showing symptoms. If that is the case then check for presence of predators before deciding to spray.