Today I saw something that I saw last year and did not know what it was. This is an Apple Gall that an insect creates with its life cycle. This is green now but when they fall off the trees they will turn dark brown and hollow inside. This is the story of the Apple Gall: "The round growths you see on oak trees are apple galls, which is another name for cynipid wasp nurseries. These growths are part of a fascinating arrangement between insects and oaks. The tiny wasps lay eggs on oaks, stimulating the plant tissue to grow at a rapid rate. The eggs become encapsulated, and the ensuing gall providing food and shelter for the emerging larvae. Different kinds of cynipids wasps will lay their eggs on bark, leaves, branches root, leaves and acorns, each one causing a different kind of gall to form in a very specific location and at a specific time. They range from the tiny jumping gall, to the large apple gall, and from white to red to brown to green. These galls for the most part do not harm the plant, although one leaf gall will cause leaves to turn brown and fall of. The trees are not harmed in the process, and as the wasps are so tiny with such complex lifecycles, preventing their appearance is not practical or possible. You can knock them off of the tree if they really bother you, but know that they will be back next year. If you would like to see the wasps, collect the galls in the late summer/fall and place them in sealed bags. The wasps will emerge in the early spring."