Phytoptus avellanae is an acarine gall-mite species inducing big bud galls of up to 10 millimetres (0.39 in) across, sometimes slightly open, on the buds of hazel (Corylus avellana) and on filbert (Corylus maxima). Synonyms include Phytocoptella avellanae, Eriophyes avellanae, Calycophthora avellanae, Phytoptus coryli, Phytoptus pseudogallarum, and Acarus pseudogallarum. The mites are white, about 0.3 mm long, with numerous tergites and sternites. Two forms of P. avellanae exist, a gall causer and a vagrant form that has a more complex life-cycle and does not form galls.