Plant galls have fascinated botanists for over 1000 years and have been used in medicine, industry, and as human food. They are found on all plant parts and resemble spectacular spiny balls, grotesque horns, exquisite vases, clubs, elongated polyps, or other impressive shapes, often in vivid yellow, magenta, or scarlet colors. Galls are induced by many types of organisms, including viruses, bacteria, fungi, nematodes, insects, and mites. Many North American galls are found on trees in the Fagaceae family, which includes oaks, chestnuts, beeches, etc. In contrast, legumes host most galls in South America, Africa, and India, while more than half of the known galls are on eucalyptus trees in Australia. In some cases galls are harmless, while others, others cause loss of flowering, fruiting, or plant death.
Perhaps some of the least studied, but most interesting galls are caused by Eriophyid mites (Figure 1). While the galls are conspicuous, mites that induce altered plant growth are easily overlooked. Eriophyids are pale yellowish to nearly translucent and small (0.1 to 0.3 mm-long), with only four segmented fore legs on their elongated body. Overwintering females (deutogynes) deposit 50 to 100 eggs each in the spring or early summer. Two types of nymphs develop before adults (protogynes) breed. Galls are induced by mite feeding. Leaf-feeding mites attach their anal sucker to the plant tissue and arch their body before inserting their mouthpart (chelicerae) into the leaf with forward movements. After feeding for as little as 10 seconds, a foliar mite can remain motionless in a sucking position for a few hours to two days before moving to another feeding site.
The plant response to feeding can occur within ten minutes of feeding on leaves. Callose thickenings (composed of glucose) form along the plant cell wall at the puncture site. Within 20 minutes, nucleus enlargement and other changes occur in the punctured cell, followed by denaturation of the nuclear DNA within 45 minutes of feeding. Surrounding cells divide and form a nutritive layer which supplies food for the actively breeding mite colony. Some eriophyid mites induce brilliantly-colored, densely matted hairs called erinea, which provide food and shelter for the colony.
Eriophyid mites can also transmit plant viruses. For example, Cecidophyopsis ribis, a currant bud mite found in the Pacific Northwest a virus causes currant reversion, which eliminates fruit production. If galls are bothersome in ornamental plantings, they can be removed and destroyed during the growing season to eliminate mites on that specific tissue. Some eriophyid mite species can be suppressed with ultrafine horticultural oil applied in the fall before leaf drop to reduce adults seeking overwintering sites. However, these products will not remove galls. For fruit trees, miticides are available with recommendations for application listed on the label.
REVISED: September 29, 2015