Galls are growths that occur when the chemicals from certain insects interact with an aspen tree to form swollen areas. These galls can occur on any part of a quaking aspen tree, but they most often show up on the twigs, leaves and leaf buds. Many insects cause galls on a quaking aspen tree, but the aspen twiggall flies are often a culprit. While the insect itself may be too small to see or long gone, you can identify the type of insect that made the gall by its shape and location on the tree.