Aceria caulis

So-called Walnut Petiole Galls produced by the Black Walnut Petiole Mite may be found on black walnut trees in Ohio. The eriophyid gall mite is specific to black walnut and occurs throughout its native range.

While taking pictures of Walnut Caterpillars, I noticed that the sometimes caterpillar-like Walnut Petiole Galls produced by an eriophyid mite are reaching their maximum size and becoming very evident on their namesake host. The galls are specific to black walnut and may occur on the petiole, rachis, and petiolules of the compound leaf.