Never seen a sycamore tree without some of them, as long as the leaves stay green it will survive.
Don't know if these mites climb up the trunk, on some trees you put an oil soaked cloth round the trunk to stop the climbers (and cut back foliage touching other ground based objects - fences, other trees etc).
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates