Q: I noticed these bumps on the branches of my aspens. What is it and will it damage my tree?
A: Those bumps are called galls. They are caused by the poplar twiggall fly which lays eggs in developing twigs. The twigs essentially grow over the eggs and begin to swell. As the larvae grows, so does the bump. Eventually the larvae matures and gets out of gall, but the bump and distorted growth is permanent.
Even though the bumps can look bad, they are purely aesthetic and do not damage the health of the tree. They are often noticed in fall as the leaves fall off and expose the branches of the aspen.