Walking around the yard the other day, I noticed these little bumps all over the leaves of my witch hazel tree which, otherwise, looks quite happy and healthy. Some of the bumps are green, others are red.
These galls are caused by the witch hazel gall aphid, Hormaphis hamamelidis. The growth on the leaf is a response of the plant to an injection of salivary secretions, and possibly an infecting virus, by the aphid. The aphid’s offspring then develop inside the gall where they are protected from predators as they develop, getting the nutrients they need from the sap of the plant.
This species of aphid feeds on witch hazel and birch trees. This paper is a very thorough study of the witch hazel gall aphids:
Lewis, I.F. and L. Walton. 1958. Gall-formation on Hamamelis virginiana resulting from material injected by the aphid Hormaphis hamamelidis. Transactions of the American Microscopical Society 77(2): 146-200.