While taking pictures of Walnut Caterpillars (Datana ministra), I noticed that the sometimes caterpillar-like Walnut Petiole Galls produced by the eriophyid mite, Aceria caulis (family Eriophyidae), are reaching their maximum size and becoming very evident on their namesake host. The galls are specific to black walnut (Juglans nigra) and may occur on the petiole, rachis, and petiolules of the compound leaf.
The galls are covered in densely compacted hairs that resemble the erineum patches produced by other eriophyid mites. As the galls "mature," they change color from pinkish-white to pink to deep red and finally to dark brown. They may be very obvious causing tree owners to fear for the health of their trees
Although the galls may cause the compound leaves to become twisted and malformed, they do not disrupt the vascular flow. The affected food-producing leaflets remain functional even though they may be pointed in an array of directions. Thus, the eriophyid galls do not appear to cause significant harm to the overall health of their walnut hosts.
There are online recommendations to remove and destroy the galls on small trees to reduce the mite population. However, this could mean removing a considerable number of food-contributing leaves making the treatment more damaging than the condition. Besides, as with many plant gall-makers, populations of this eriophyid appear to rise and fall dramatically from year-to-year. I seldom see heavy galling two years in a row on the same walnut tree.