Acalitus calycophthirus is an eriophyid mite which causes big bud galls on birch (Betula species) twigs.

It is found in Europe and was first described by the Austrian zoologist, Alfred Nalepa in 1891.

The gall consists of clusters of swollen leaves and, at first in the spring, are silvery-green.

The gall shelters a large number of mites which spend most of the year there, passing the winter in a dormant state.

In the following spring and early summer they emerge to invade the new buds.