Eriophyid Mite Galls of Lama (Diospyros spp.) and Broussaisia arguta

Among the most conspicuous apparent plant diseases are the galls and similar growth abnormalities caused by feeding toxins of eriophyid mites.  Members of this specialized group of plant-feeding mites are distinct from other types of plant feeding and predatory mites more familiar to most observers.  Whereas the typical mite is usually considered to resemble a miniature spider, with a round or oval-shaped body, eight legs, and rapid movement, eriophyid mites are smaller than most other mites such that they are barely visible with a hand lens.  Furthermore, they are sedentary, remaining motionless much of the time while feeding, and are slow moving when they change position.   Their body is light-colored and elongate, shaped like a drawn-out water droplet.   Their small legs are four or six in number, rather than eight, and are all located at the broadened end of the body.  Like other mites, feeding action is by drawing out plant juices, during which a toxin is also injected that affects the physiology and development of the host.  This abnormal physiological effect results in a disease-like condition as distinct from the direct mechanical feeding injury caused by chewing insects, for example.

Whereas there is evidence of eriophyid mite damage on several species of plants in Hawai‘i, the abnormal growths on two unrelated endemic hosts, lama (Diospyros spp.) (family Ebenaceae), and Broussaisia arguta (family Hydrangeaceae) appear to attract the most attention among casual observers.  Often, these abnormalities resemble, and are mistaken for, rust fungi, or the witches' broom-like symptoms associated with rust fungi (see the "rust fungi" page).  Mites are present only as long as the tissue remains fresh, and since the galls, or abnormal growths themselves tend to persist after the mites are no longer present, the cause of the abnormality is often not apparent and cannot be demonstrated in older tissue.

   Leaves of Broussaisia arguta with fresh eriophyid mite galls.

  A fresh eriophyid mite gall on a branch of lama.

  A lama tree with numerous, conspicuous old mite galls, some of which are indicated by arrows.