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A new, gall-forming eriophyoid mite species is described from Australia. Aculus crassulae sp. nov. was found causing significant leaf deformation in Crassula helmsii (Kirk) Cockayne (Crassulaceae), a semi-aquatic, succulent plant. Native to Australia and New Zealand, this plant is now a highly invasive weed in the United Kingdom and Western Europe. The host specificity of the new mite species, and damage caused to the host plant, infer its potential to be a valuable biological control agent in countries where Australian swamp stonecrop is threatening native flora. The species description provided here, which also includes a revised diagnosis for the genus Aculus, incorporates line drawings and scanning electron micrographs (SEM). This is supplemented by a partial mitochondrial gene sequence of cytochrome c oxidase subunit I (mtCOI) and the sequence was compared with Aculus amygdali Xue Hong and Aculus ichnocarpi (Ghosh Chakrabarati) available in the NCBI database. Pairwise comparison of mtCOI sequences between A. crassulae sp. nov. and two congeneric species revealed 22.6% and 23.1% genetic divergence, respectively.

Hazelnut big bud mite, Phytoptus avellanae Nalepa, is one of the most harmful pests of Corylus spp. (Corylaceae) worldwide. Herein, we show that this species represents a complex of two cryptic species: one that lives and reproduces in buds causing their enlargement ('big buds') and drying, whereas the other is a vagrant living on leaves, under bud scales and in catkins, based on phylogenetic analyzes of mitochondrial cytochrome c oxidase subunit I (COI) DNA and the nuclear D2 region of 28S rDNA sequences. A molecular assessment based on mtCOI DNA and nuclear D2 28S rDNA revealed consistent differences of 16.8 and 3.5% between the two species, respectively. Molecular analysis also revealed that atypical flattened nymphs (Tegonotus-like nymphs sensu Keifer in Mites Injurious to Economic Plants, University of California Press, Berkeley, pp 327-562, 1975) with differently annulated opisthosoma, which appear in the life cycle of P. avellanae s.l., belong to the 'vagrant' lineage, i.e. vagrant cryptic species. Light microscopy images of Tegonotus-like nymphs molting into males and females are presented for the first time. Our results suggest that the name P. avellanae comprise two species. Big bud mite should keep the name P. avellanae, and the vagrant cryptic species should be re-named after a proper morphological description is made.