We typically notice these more later in the summer when the gall changes color and texture to a thin, brown, papery shell. The round gall is actually a deformed or mutated leaf. You can see in the photo that the normal growth of this red oak leaf went haywire. A small wasp laid an egg at the base of the midvein of a leaf bud. The larva that hatched released chemicals that caused the tree to form a protective structure around the egg -- the gall.
Inside the center of the gall the larva eventually pupates and with time an adult wasp emerges. The adult wasp then drills its way out -- if you find the brown papery galls later in summer look for the tiny drill hole. The oak apple gall wasp -- the cause of these odd-looking "apples" on oak trees -- is apparently harmless to the tree.
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