Walnut blister mite or walnut gall mite is a form of eriophyid mite found on walnut tree leaves. The mites are barely visible to the naked eye, and live on the undersides of leaves. Their activity causes unsightly swellings on the upper surface of the leaves, but
Although the infection can appear severe, the actual damage to the tree is minimal and there is no point in using insecticides as they are likely to cause more harm than good. The mites do not weaken the tree, nor do they provide an entry point for other infections.
The infections are more prevalent on younger trees.