Boxelder Gall Mites

Identification

• The boxelder erineum gall mite (Cosetacus negundi) is a member of the Eriophyidae family and a thin, white microscopic organism that inhabits the foliage of a boxelder tree (Acer negundo). Gall mites overwinter as adults under the bark of the tree or safely tucked away in crevices, before emerging in the spring to begin feeding on tender new foliage. As the mite satisfies its hunger, it secretes a solution that demands the attention of the tree.

Gall Significance

• The boxelder gall mites have developed a unique plan to provide for their shelter and food supply. The leaf responds to the chemicals the mite has secreted by developing an abnormal growth through mutation of the leaf tissue. The mite enters this new structure that forms on the leaf and is provided protection from predators and the elements, as well as a safe haven for easy feeding. Rather than being a structure built by the mites, the gall is actually part of the living plant.

Gall Formation

• All species of gall mites produce galls with varying features, sizes and colors. The boxelder gall mite prompts the leaves of the tree to produce a large, rounded gall that develops on the underside of the leaves and sports a collection of thick, white hairs that also jut out from the top of the boxelder leaf. The galls never form on the veins of the leaf and, aside from damaging the aesthetic value of the tree, no serious injury is likely.

Control

• Only in extreme cases of infestation can the boxelder gall mite injure the health of the tree. Occasionally, a collection of numerous, heavy galls may cause a branch or limb to droop. Chemical control is rarely warranted against these mites, not only because they do little damage, but also because the mites are typically well-protected inside their galls and shielded from insecticide. In some instance, a gardener may wish to prune away highly-infected twigs or limbs.