Eriophyes cerasicrumena, the black cherry leaf gall mite, is a species of gall mite in the family Eriophyidae. This species was formerly a member of the genus Phytoptus. They produce galls on black cherry plants. Caterpillars of cherry gall azure feed on these galls, and apparently also on the mites themselves
Mite pouch galls

Judy Gallagher
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Flickr