Jumping oak galls are caused by a very tiny, native, stingless wasp (Neuroterus sp.) which lays eggs in leaf buds. As the leaf develops, pinhead-size galls, also referred to as abnormal plant growths, form on the undersides of the leaves.
Each round, button-like gall contains one wasp larva. Below is a photo of the dropped galls on and around a penny. They are very small.
As the galls drop from the leaves, they bounce into the leaf litter or soil where the larvae mature into pupae and spend the winter in the gall. In early spring the following year, the female wasps chew their way out of their galls to complete the life cycle. Fallen galls are sometimes observed to “jump” due to vigorous movements of larvae within each gall, much like moth larvae of “Mexican jumping beans.” It is speculated that this behavior allows the galls to fall deeper into leaf litter or into the soil where they are sheltered throughout the winter.
Here is a link to a quick video that shows the movement of the gall from the UW - Madison, Department of Entomology: https://www.facebook.com/watch/?v=619747861516548
Heavy infestations of the jumping oak gall can make trees look like they are scorched or even dying at first glance. Fortunately, nearly all trees will recover, even if leaves are brown. Controls are typically not warranted. By the time the damage is observed, any opportunity to treat has already passed for that year, and populations are likely to decline naturally. However, severe leaf damage can stress trees, particularly if most leaves on a tree are brown which can cause a second flush of leaves to be pushed in summer. The best management strategy is to practice good tree care to reduce tree stress which includes: mulching, avoiding soil compaction, watering during drought periods, and avoid wounding trees.
It would be interesting to see how many readers have seen the jumping oak galls this season. If you would like to report your observations, please send Amy Stone an email message at stone.91@osu.edu