Galls are growths that occur when the chemicals from certain insects and trees interact to form swollen areas. These galls can occur on any part of a quaking aspen tree, but they most often occur on the twigs, leaves and leaf buds. Many insects cause galls on a quaking aspen tree. And 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.
Inspect your quaking aspen's leaves--the place you are most likely to find galls. Galls that appear rounded and yellow from the top surface of the leaf, and felt-like (due to the many pale hairs that house the mites) on the underside likely belong to a common quaking aspen mite (Phyllocoptes populi). Spherical galls that protrude from the underside of the leaf or grow through it belong to a species of gall midge. Galls that appear conical on the top side of the leaf and spherical on the bottom side belong to a different species of midge (Lasioptera populnea) also common to quaking aspens.
Inspect the petiole (the small stem that connects the leaf to the twig) for galls. One species of midge (Contarinia petioli) causes pear-shaped galls to develop on the petioles of aspen leaves. Once in a while, several of these galls are located near one another and will merge together. Galls located at the base of petioles are likely caused by yet another species of gall mite.
Inspect budding leaves for signs of galls. Aspen gall mites feed on leaf buds before they are fully formed. The galls that they create start out quite small and are often hard to spot in their beginning stages. For example, one species of gall mite (Aceria populi) creates a gall that begins as a soft, down-like mass on a quaking aspen bud. And as the twig develops, the gall turns into a yellow or red lumpy wart-like growth that eventually envelops the twig.
Inspect the twigs of your quaking aspen for galls. Galls on a quaking aspens twigs may be caused by a species of beetle (Saperda populnea) that creates elongated galls that can reach up to 20 mm in length. Another common quaking aspen pest is the poplar twiggal fly, which creates small spherical galls on this season's twigs. These galls are small and often go unnoticed. But as the twig grows around it, the gall will appear as a large swollen band and eventually turn into a knot on trunks and large branches.