According to SFGate, deer love to eat hydrangeas, preferring the smooth hydrangea and bigleaf hydrangea over other varieties. The best way to protect hydrangeas from deer is to surround them with wire cages or a fence.
Besides deer, hydrangeas are also plagued by slugs and snails. Bait can be used to deal with these pests, or they can be removed by picking them off the plant when they are most prevalent in the early morning hours. The moist leaves of the hydrangea are a favorite source of food for thrips and spittlebugs. Applying insecticide approved for use on hydrangeas can remove these damaging pests.