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Elm finger galls, Eriophyes paraulmi, stand up a half inch from the upper leaf surface. They are frequently curled and deformed, unlike many other leaf galls.
Most plant galls are produced by insects, but these are made by Eriophyid mites, eight legged relatives of ticks, spiders and other arachnids. These mites produce finger-like galls on the leaves of maples, elms, black gum, black cherry, button bush, maple, viburnum, birch, linden, and even poison ivy. More from Wikipedia:
Eriophyidae is a family of more than 200 genera of mites, which live as plant parasites, commonly causing galls or other damage to the plant tissues and hence known as gall mites. About 3,600 species have been described, but this is probably less than 10% of the actual number existing in this poorly-researched family. They are tiny, microscopic mites and are yellow to pinkish white to purplish in color. The mites are worm like, and have only two pairs of legs. Their primary method of population spread is by wind. They affect a wide range of plants, and several are major pest species causing substantial economic damage to crops.
These mites have their place in nature. Most leaf galls do not cause substantial plant damage although a severe infestation may weaken the plant. Some of the Eriophyid mites have been used as a biological control of selected invasive or unwanted species. The bindweed gall mite, Aceria malherbae is used as a biological control against field bindweed.
|Cherry Leaf Gall- click to enlarge|