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leafy oak gall

Q:  What is causing this unusual growth on my oak tree?

A:  Your oak tree is producing galls.  This particular gall, called the leafy oak gall, is actually tree tissue forming around the egg of the wasp, Andricus quercusfoliatus.  Oak trees are notorious for forming all types of galls as a result of insects (usually wasps or flies) ovipositing (egg-laying) into the tissue of the leaf, bud or twig of the tree.  Galls generally cause no harm to the tree and provide an important role in protecting the predatory insect through adulthood.  Some galls are very attractive and mimic fruit, cones or nuts.  Galls are best left alone.  If they are unsightly, some light pruning to remove the growth will give the best control.