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Out in The Woods With… Oak Galls

Oak apple galls on a browsed oak sapling.

By Kevin McKeon, Maine Master Naturalist

Often found under oak trees, and growing on oak tree leaves and twigs, oak galls are made by an oak tree, but the Oak Gall Wasp makes the tree do it! This is the story about the Oak Apple Gall, one of over seventy kinds of oak galls.

The wasp injects her egg into the oak. The egg contains certain chemicals and hormones that make the tree produce the gall, which also contains food for the developing larvae. As the larva develops, the forming gall is green with brown spots and feels rubbery…like big grapes. The gall increases in size as the larva grows causing the formation of (in the case of the relatively smooth oak apple gall wasp) a round gall. The mature gall’s fiber then degrades, turns brown, and falls to the ground. The larva transforms into a wasp, hatches through a very small hole in the degraded brown gall, mates, and burrows into the ground.

The oak apple gall wasps have 2 life cycles; one generation of females that reproduce by themselves, and a second generation of males and females. The first generation of females burrows into the ground, laying eggs on the oak tree’s roots. After developing for about 16 months, the fertile and wingless female emerges from underground, crawls up the tree, and injects one or more eggs into a developing leaf.

The galls usually do not harm the oak. However, the gall formation is a defensive measure by the oak tree and therefore contains strong natural astringent compounds such as tannic acid. As such, these galls have been used as medicine by many cultures around the world (as far back as the ancient Greeks), for dye and tanning material, and for ink. Leonardo da Vinci wrote his notes with gall ink, Bach composed with it, and Rembrandt and Van Gogh drew with it. The Magna Carta, the 1215 English document generally describing the foundations of modern constitutional law, was penned in gall ink, as were our own Declaration of Independence, The Constitution, and The Bill of Rights. An extract from oak gall will kill a type of mosquito larvae that spreads malaria. Oak galls contain unique and potent properties that are just beginning to be studied by mainstream scientists for a host of useful applications. According to, oak galls are the most astringent vegetable compound in the world. Tannin comes from the old German word tanna, meaning oak!

Watch a video; it’s a different species illustrating the process:

Photo Credit: Kevin McKeon

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