Galls on a Witch-hazel

The leaves on the witch-hazel tree in the yard are covered with cone-shaped galls. Some leaves bear two or three of these things. Most galls are light green, but some have turned red; many have an opening on the underside of the leaf.

The galls provide shelter for the reproductive process of the witch-hazel cone gall aphid (Hormaphis hamamelidis). These insects use both sexual and asexual reproduction in their cycle. In spring, a female hatches and creates a gall around herself on a witch-hazel tree. She then reproduces asexually and fills the gall with her young. That generation, all of which are female, disperses to other leaves and other plants and then repeats the cycle, forming a gall and reproducing asexually. Their young, in turn, include both males and females, which reproduce sexually and lay eggs for the next year on the witch-hazel's branches. The eggs pass the winter there and then hatch the following spring.