Unmitigated Gall

When is a pinecone not a pinecone? When it grows on a willow.

Now that the leaves have fallen from the trees we can see details in the landscape that we might miss during the growing season. Sometimes the little things call our attention or capture our imagination. I think of willow thickets and the distinctive appearance of the willow pinecone gall, an elegant appendage seen at the tips of willow twigs. These may be prominently visible now that the vegetation has gone dormant for the winter months.

As a budding botanist, I puzzled over these vegetative structures. I knew that willows, in the genus Salix and the family Salicaceae, do not produce cones (like conifers, such as pine and spruce) nor strobiles (like birch and alder). Yet I often noticed conical growths on willow wands, with imbricate bracts, leaf-like and layered, arranged in a perfect spiral toward a pointed tip, looking for all the world like a gray, leafy pinecone. And I saw them only on willows. Specifically on three native species:  pussy willow (Salix discolor), heart-leaved willow (S. eriocephala), and sandbar willow (S. interior).

A willow pinecone gall in winter shows hardening of the scales that protect the larva of the gall midge within.

A willow pinecone gall in winter shows hardening of the scales that protect the larva of the gall midge within.

The leafy, cone-like appendages in question are galls, produced by a little gall gnat midge named Rabdophaga strobiloides. The insect’s scientific name is telling:  Rabdo means rod (or branch, twig); phaga refers to the feeding habit of the organism; a strobile is a structure composed of woody scales in the form of a cone; the suffix, oides, says that something “looks like” the preceding. So the scientific name translates into “twig-feeder” that produces a gall that “looks like a strobile.”

While we are familiar with the knowledge that many insects and other animals feed upon the leaves, stems, and flowers of plants, consider how striking it is that some of these insects also induce plant growth in such a way as to provide shelter, as well. Yet, here in the United States, there are more than 2,000 types of insect galls, of which some 700 are produced by gall midges related to Rabdophaga strobiloides. Some 39 of them focus exclusively on willows.

In the case of our pinecone willow gall midge, females deposit eggs in the new growth of willow twigs during the spring. When larvae hatch from the eggs they produce a chemical that causes the willow to produce aberrant growth and thus the gall begins to form. (In some insects, the deposition of the egg includes a chemical that initiates the gall-formation process.) The midge larvae undergo three instar phases, each of which contributes to the gradual formation of the gall. (Early in the season, the nascent gall is round; as the year progresses, the gall assumes its familiar cone shape.) Mature larvae overwinter in the galls and pupate in the early spring. We can expect the adult to emerge from the gall in late April or early May.

So, this winter, as you roam the margins of frozen wetlands, look for the “pinecones” that appear on willow wands—and know that you are seeing the winter palace of a tiny midge.