These mysterious objects that look like desiccated golf balls were originally destined to be oak leaves but ended up housing wasp larvae. They are apple galls, a gall being any abnormal growth on a tree and the apple part because young ones resemble green apples.
In early spring, an “oak apple wasp” injects an egg into a leaf bud as well as chemical instructions that co-opt the bud into forming a cocoon for wasp larvae. In summer, a mature wasp emerges through a tiny escape hole.
Like many of nature’s most creative inventions, galls have a bad rap. However, oak galls do not harm the oaks. They provide food for birds and when pulverized, can be used to make an ink that was the medium of medieval scribes. The Constitution and the Declaration of Independence were written in oak gall ink!