Jumping oak galls
Calls are coming in from Crow Wing and Morrison Counties about bur and white oak leaves with ?tiny balls? on the underside of the leaves. These ?balls? are caused by the jumping oak gall, a tiny non-stinging wasp whose larvae feed on plant juices. It?s the larvae that produce the galls made up of leaf tissue, through hormones in their saliva. They get their name because the galls drop into the grass once the larvae are mature and bounce around like a Mexican-jumping beans until they work their way deep into the thatch layer. Here they spend the winter. In the spring, the adults emerge to lay eggs on the newly expanding oak leaves.
The wasps produce two types of galls during alternate years. One year, they produce the tiny balls on the undersides of the leaves. These galls can cause noticeable damage as the leaves brown and die around the galls. Trees can be defoliated and whole stands can be impacted. Damage tends to be more of a problem in forests dominated by white and bur oak adjacent to open pasture. The grass provides protection for the over wintering insect and allows population numbers to climb higher than they would elsewhere. However, unless there is a fair amount of grass on the forest floor, the leaf damage declines dramatically as you move into the woods further from the pasture.
During alternate years, the insect forms vein-galls, swellings along the veins, that cause little if any damage. In Minnesota, long-term damage by either gall is rare, even during outbreak years. Natural enemies and weather conditions limit population numbers and the alternating life cycle allows trees to recover between attacks. Control is rarely warranted.