The gall is found on the underside of a leaf of the host plant. It is globular and up to 5 mm in diameter. Descriptions vary according to the source. According to the Plant Parasites of Europe website, the gall can be hairy and sometimes warty. The colour varies from bright red to a red-tinged green. Redfernet al. describe the gall as having brown warts and either no hairs or slightly hairy. The galls can be found on the leaves of creeping willow and rosemary-leaved willow.