“Oak Apple Experiment,” Friend, Oct. 1996, 43
If you live in an area where oak trees grow, find some oak apples—the large, hard, ball-like galls found on the limbs and twigs of most oak trees. Break open a few oak apples with a hammer, and locate the hard kernels, which are near the center. Then use a pair of pliers to tear the tough kernels apart.
What Happens and Why?
The kernel in each oak apple will have a hollow place or—if you are lucky enough to find one that grew on a tree during the previous summer—even live insect larvae or pupae.
Oak apples are not edible for people. They are caused by a small, stubby, harmless wasp that lives most of its life inside the gall (oak apple). A female lays her eggs in a fast-growing part of the tree during the spring, and a growth soon appears. As the growth becomes larger, the eggs hatch and the larvae feed on the soft, nutritious tissue inside. Then, when the sap goes down in the tree, the gall becomes extremely hard and the larvae become pupae. In most climates, the adult wasps emerge in the spring from the hard oak apples and promptly seek mates. They seldom live more than a few weeks more.
If some large oak apples are stored in glass jars with breathing holes, would the adult wasps emerge so that you could see them flying around in the jar? Why not find out?