Galls, also known as cecidia, are abnormal swellings that form on the outer tissues of plants, resembling benign tumors or warts in animals. These growths can be triggered by a wide range of parasites, including viruses, fungi, bacteria, other plants, insects, and mites. The scientific study of these structures is called cecidology.

Galls come in many different sizes, shapes and colours; I’m always looking out for them. Our native arroyo willows display at least two different insect-triggered galls. The sawfly/midge lays its eggs under the surface of the leaf/stem. Cecidologists are still trying to figure out how the plant’s genetic instructions cause the responding structures, which provide shelter and food for the insect’s larvae when they hatch.