While exploring along the Oregon coast on the last day of March, I noticed that the willow bushes which grew on the lower edge of the hillside were just starting to bare their catkins. One bush in particular caught my eye. Almost every branch had a large deformed mass about halfway between the base and the tip.
This willow, possibly Hooker’s willow (Salix hookeriana), was infected with galls. Stem galls are unusual growths that are formed by the feeding or egg laying activities of insects or other small organisms. Willows in general seem to be susceptible to galls, and this genera has one of the highest number of infecting species. This particular gall is probably the result of a developing fly or wasp that has bored into the branch and has secreted a chemical which causes swelling as a result of the irritation. In some cases, the chemical is injected by the adult female as she lays eggs within the stem. As the gall grows, it provides protection from predators, food and shelter for the parasite.
Although galls may look bad, they usually don’t threaten the health of the plants they occupy. Besides stem galls, there are also galls that develop on leaves, buds and roots. In many cases, experts are able to tell the species within the gall just by its shape. There are over 1,500 species that produce galls.