![]() The extent of infestations can vary greatly, with large concentrations of galls on some trees or portions of trees, and very little on adjacent trees or the other part of a single tree. Heavy infestations may cause leaf deformity. There may be several generations during a growing season. At this time they head for the trunk and branches to overwinter in roughened areas on the bark, bud scales, or other protected places. Once the tree has stopped new growth, usually in early summer, the mites are unable to stimulate the abnormal growth to produce the galls. They then exit through an opening on the underside of the leaf to crawl to newly developing leaves and begin the cycle again if the tree still has new leaf growth. They remain within the galls, feeding on the gall tissue until they have matured into adults, usually within a few weeks. The eggs hatch into whitish-orange, carrot-shaped nymphs with two pairs of legs. Once the leaf has expanded completely and reached its full size, the adult mites lay numerous eggs within the galls and die. Mite feeding causes the abnormal cell growth that causes gall formation. The mites get nutrients from the inner gall tissue and are protected from natural enemies (and also pesticides). Each mite species produces slightly different substances, resulting in the different characteristic gall shapes. This abnormal growth forms the galls that gradually enclose the mites. ![]() As they feed on the developing leaf tissue, growth regulating chemicals produced by the mites interact with plant hormones causing an abnormal stimulation of cell growth. The adult mites become active very early in spring, just as buds begin to open. The mites, less than 2mm long, are too small to be seen without magnification. These strange swellings are plant galls caused by the feeding of tiny eriophyid mites on newly developing foliage. ![]() If you have a maple tree, you may have noticed weird growths on the leaves in some years. Bumpy growths caused by maple gall mites cover silver maple leaves. ![]()
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