Question
Hello.  I have two red maple trees in my backyard that have small, hard orange or red bumps on the top of their leaves.  Can you tell me what this is and how I can take care of it?  Thank you.

Answer
These are called bladder galls. The upper surface of silver and red maple leaves often become covered with small, red, round wart-like structures about 1/16 to 1/8-inch in diameter. These are maple bladder galls caused by a small mite, Vasates quadripedes (Shimer). The structures are generally noticed first in May, about the time the leaves have become fully expanded. At first the galls are green but they quickly turn pink to red and eventually black. Leaves frequently become so covered with the galls that they completely twist out of shape and may even drop early.


Homeowners often become quite alarmed when they discover that their maple tree leaves are infested with leaf bladder, spindle or gouty vein galls. They fear that the tree is going to die unless something is done quickly. This is not the case. Maple leaf galls seldom, if ever, cause permanent injury to a tree, but they do detract from the beauty of the leaves.

The maple bladder gall and maple spindle gall mites overwinter as free living mites under loosened bark and around the callous growth of wounds, scars and pruned branches. These overwintered forms produce the gall forming stage in early spring. When the maple leaves first appear, the mites migrate to expanding buds and begin to feed on the undersurface of leaf buds. This causes the formation of a blister which expands into a hollow bladder or spindle as the leaf expands. The mites enter the cavity and continue to feed within its protective walls. This stage reproduces asexually within the galls and the new mites mature by late June to mid-July. At this time the galls dry out and the tiny entrance hole opens up to allow escape of the mites. These mites then seek out overwintering sites.


Since these leaf galls of maple do not cause any real harm to the trees, control measures are not generally needed. Tree owners and tree managers are encouraged to learn about the life cycles of these pests and learn that no lasting damage will result. Since there is no real health issue with the bladder galls I would nor recommend doing anything. The presence of galls, even in considerable numbers, does not usually harm trees. Rarely does a severe infestation cause serious damage. The problem is largely one of unsightliness. There are registered products available should the problem be severe enough to warrant control. They should be applied in spring before leaf buds open. The names of currently registered pesticides can be obtained from your local county Extension agent.