A genus of over 900 Eriophyid mites that cause galls and blistering on a wide range of plants. Some are agricutral pests, and others are useful biological controls of invasive weeds.

The species in this case is almost certainly Aceria erinea, the Walnut Felt Gall Mite - such excessive hair growth on the underside of the leaf is called a felt gall or erineum. Chemicals injected by the mites as they feed on the leaf cause excessive cell growth, enlarging the blister and producing the hairs that protect the arachnids from predators and dehydration.