An abnormal growth (gall) on the stems, leaves, or buds of a plant can be formed by many insects, mites, and fungi. Willows are hosts to many parasitic insects, several of which form galls. Only willow pinecone gall midge (Rabdophaga strobiloides) forms a pinecone-shaped gall at the tip of a willow stem.
The adult midge is a small fly about 3 ⁄16″ long. It is most often identified by the large distinctive gall that houses the growing larva. The gall appears at the end of a willow stem. It consists of numerous, stunted, overlapping, loosely appressed, scale-like leaves. In the summer it is green, more or less globular, and densely covered with long, white, matted and tangled, woolly hairs. In the fall the cone turns brown and the shape resembles a pine cone.