Eriophyid gall mites are so small, about 1/100 of an inch long, that it is nearly impossible to see them without the aid of a powerful hand lens or microscope. They are blown by the wind and carried by birds and insects. Their small size, which makes detection difficult before they have caused galls on Linden trees, is why controlling them is a challenge.
Identification and Life Cycle
Viewed under a microscope, eriophyid mites have long, sausage-shaped bodies with two pairs of legs. Their relatives, scorpions, spiders and spidermites, have eight legs. Adult female eriophyid mites spend the winter in bud scales and crevices and cracks of twigs. They lay their eggs in the spring. A female eriophyid mite can lay 80 eggs in a month; each egg can develop into an adult in one to two weeks. Mites do not mate. The female is fertilized by the sac left by the departing male.
Eriophyid mites cause galls by sucking up the contents of plant cells. To protect itself, plants form a barrier of deformed tissue that is a gall.
Galls on Linden
Eriophyid mites form two kinds of galls on Linden trees. Erium or velvet galls are usually in the form of a reddish, velvet patch on the bottoms of leaves. Horticulturalists at the University of Minnesota describe the eriophyid mites that cause these galls as looking like "tiny, pale carrots" when viewed under a microscope. Eriophyid mites can also form spindle galls on leaves. These galls are about 1/5 of an inch long and are smaller at the ends than at the middle, suggesting the spindle of a spinning wheel, hence the name.
Horticulturalists at the University of Minnesota, Washington State University, the University of Saskatchewan and the University of California, Davis, all recommended against attempting to control eriophyid mites with miticides because the damage they cause is only cosmetic and attempting to eliminate them can be frustrating if not impossible. The primary hurdle to controlling eriophyid mites is that they are so small that it is ordinarily impossible to identify them until after they have caused galls.
There are miticides that can control eriophyid mites. Horticulturalists at the Missouri Botanical Gardens recommend spraying trees in the early spring when buds are breaking. They suggest using miticides containing the active ingredients avermectin, bicofol, bifenthrin, carbaryl, chlorfenbapyr, endosufan, fenpyroximate or pyridabern. Entomologists at the University of Minnesota recommend miticides containing bifenthrin or carbaryl.