Once the symptom or gall appears, the causative arthropod is protected within the structure. This means that remedial actions, other than pruning in some cases, are not effective. Preventive action is necessary to attempt to reduce the infestation the following season and these may be of limited value. Fortunately, most galls, especially those on leaves and leaf structures, do not harm the health of the tree. Bladder galls often turn bright red and are very conspicuous on the leaves.
Most galls occur on early spring foliage that develops near the trunk and on larger branches.
Additional galls are formed on newly developing leaves later in the summer but mites are most active early in the season. Galls usually form on a small proportion of the leaves and tree health is unaffected. Maple Bladder Galls Maple bladder galls occur on silver and red maples. They are irregular, spherical growths that are usually found on the upper surfaces of the leaves. These hollow, purple-green to red structures are attached to the leaves by short hollow stems.