Knobby growths on aspens are symptoms of twig gall, something particularly common in Colorado’s Front Range area. A gall is basically a growth that is generally not harmful to the tree. This particular gall is caused by the appropriately named poplar twiggall fly, which in the spring, deposits its eggs into the tender new growth on aspen trees. The gall is a developing growth in response to the feeding of the newly hatched larvae inside. The larvae live and feed within the gall all season long, exiting during late winter or early spring to pupate and develop into mature dark-bodied flies--about 1/6-inch long. It is not necessary or effective to remove galls by pruning. They will remain with the tree and become larger as the trees mature, but they do not stunt or limit tree growth. Pruning often requires substantial branch destruction and creates wounds that can allow pathogens to enter. Furthermore, this practice can be counter-productive if it is done after flies emerge in late winter. Spraying to prevent further infestation is not recommended since it is difficult to predict when flies will be laying eggs. For more information see the fact sheet, Poplar Twiggall Fly.