I have not seen red spiders in the front garden where holly is, but they are rampant in the back garden. Do they reside on the back of leaves in a sort of blister? Holly had a large Melianthus major in front of it that had really overgrown its spot, so the holly was suffering (but not now as that has died back completely with freezing weather). It is looking sad, so please advise on what to use. The blisters can be squashed, but it's a lot of prickly leaves!
Variegated Holly Has Small Red Things on Back of Leaves
Certified GKH Gardening Expert
Those are most likely spider mites, which can be treated with neem oil. These articles will help: https://www.gardeningknowhow.com/plant-problems/pests/insects/spider-mite-treatment.htm