Vein pocket galls on pin oak
These galls are caused by the larval (maggot) stages of very small flies called midges. Vein pocket galls are elongate swellings that occur along mid- and lateral leaf veins of scrub and pin oaks. The process begins when the unfolding leaves begin to flatten out. At this time, the small midge lays its eggs. The tiny maggots move to the veins and begin to feed. In a few days they are covered by gall tissue and complete development by mid-spring. The mature larvae drop to the ground and remain there over the winter. A related midge species causes the marginal folds or leaf pockets that can be found on red oak or pin oak. Several small larvae may be found inside the swollen folds of the leaf.