What is the gall of a tree?

Galls are abnormal growths that occur on leaves, twigs, roots, or flowers of many plants. Most galls are caused by irritation and/or stimulation of plant cells due to feeding or egg-laying by insects such as aphids, midges, wasps, or mites. ... Each type of gall-producer is specific to a particular kind of plant.

Are galls bad for trees?

In most cases, galls are unsightly but not damaging to the tree. Small plants may be stunted because the water and nutrient circulatory system of the plant may be damaged.