What Gall! The Crazy Cribs of Parasitic Wasps

Plenty of animals build their homes in oak trees. But some very teeny, tricky wasps make the tree do all the work. “What nerve!” you might say. What… gall! And you’d be right. The wasps are called gall-inducers. And each miniature mansion that the trees build for the wasps' larvae is weirder and more flamboyant than the next.

If you’ve ever spent a Summer or Fall around oak trees - such as the stalwart Valley Oak - Quercus lobata, or the stately Blue Oak, Quercus douglasii - you may be familiar with the large, vaguely fruity-looking objects clinging to the branches and leaves. Commonly called oak apples, these growths are the last thing you’d want to put in your mouth. They are intensely bitter, loaded with tannin compounds - the same compounds that in modest amounts give red wine its pleasant dryness, and tea its refreshing earthy tang.That said, the oak apple’s powerful astringency has been prized for millennia. Tanning leather, making ink or dye, and cleaning wounds have been but a few of the gall’s historical uses.