Some of the most dramatic plant deformities are caused by the tiniest of creatures: the eriophyid mite. Diminutive to the point of microscopic, these small creatures are unique, sporting two pairs of legs where all other mites have four.
Eriophyid mites are wedge shaped; their front end is slightly larger than the sucker-tipped rear. These suckers hold onto the plant while the mite stabs leaves with its stylet-like mouth to feed. As the mite draws fluid from the plant, it transfers growth-altering compounds that can result in deformities such as: leaf bronzing, galls, blisters, witches brooming, stunted growth, enlarged tissues, or dense patches plant hair called erineum.
Females are prolific, producing up to 100 spherical eggs during their lifespan. The young hatch in roughly two weeks and mature into adults two to four weeks later. There are several generations per year.
Once damage is noticed, mite colonies are already established. Because eriophyid mite damage can mimic other problems caused by disease or herbicides, a microscopic exam is needed to confirm the mites. Homeowners should check with an expert like those in the County Extension system.
A key component to management is to eliminate plants/plant parts showing symptoms. Applications of dormant oil on over-wintering females hiding in bud scales or bark crevasses can help. Control with pesticides is challenging because the problem is not well studied. Some control during the growing season could be achieved with application of carbaryl (Sevin®). Because this insecticide kills off predaceous mites and insects, it can backfire and cause an eriophyid mite population explosion.