The production of plant galls is fascinating, complex, a bit bizarre and not fully understood. Galls are mutated plant tissue, many of which are formed by the chemical secretions of insects. In the case of the apple oak gall, it is the Large Empty Oak Apple Wasp (Amphibolips quercusinanis) that alters the genes of the oak and persuades it to produce a growth (which resembles an apple) that is used as a nursery for the developing wasp.
A female wasp lays an egg in the leaf bud of certain oaks. She also injects gene altering chemicals at the same time that affects the plant and stimulates this strange growth that provides all the food and shelter that the wasp needs to mature. The eggs and larva also ooze these chemicals.
When occupied the gall is green with dark purple-red spots. In the fall the wasp hatches through a tiny hole. Then the gall turns brown and papery and falls to the ground. Even though the gall can be up to 2” in diameter, it does not harm the tree.
There are over 700 gall wasp species in North America each producing a characteristic gall, although some live commensally within the gall of another species. It seems that one can’t help but to admire these tiny creatures that so dramatically alter the mighty oak for its own purpose.