Hello Susan,

Those bumps are galls caused by an aphid—Phylloxera caryaecaulis. Gall-forming insects that attack trees usually do not cause serious damage to the tree. They have complex life cycles but are only susceptible to control before they form the protective shells. Once they are visible on the leaves, no control can be carried out. Read more here: https://extension.umn.edu/yard-and-garden-insects/insect-and-mite-galls

Thank you for contacting Ask Extension