Revision of the birch-associated genus Massalongia (Diptera, Cecidomyiidae), with description of a new species from Japan and a taxonomic key to worldwide species.

Betula (Betulaceae), or birch, is a Holarctic genus of trees and shrubs whose species have ornamental, industrial, and medical importance. Gall midges of the genus Massalongia (Diptera: Cecidomyiidae: Cecidomyiidi) are exclusively associated with birches in the Palearctic region. In 2018, an undescribed Massalongia species was discovered forming leaf galls on the midveins of B. grossa on Mount Tara, Saga Prefecture, Kyushu, Japan. In this study the species is described as M. nakamuratetsui Elsayed & Tokuda, sp. nov., and a DNA barcode provided for it. The other known species of Massalongia are redescribed because the original descriptions are outdated and insufficient. A lectotype is designated for M. bachmaieri. In addition, the monotypic genus Apagodiplosis, containing A. papyriferae associated with B. papyrifera in the Nearctic region, is synonymized here under Massalongia, resulting in M. papyriferae comb. nov., rendering Massalongia a Holarctic genus with six species. Comparing the sequence data of M. nakamuratetsui with all sequences available in The Barcode of Life Data (BOLD) system supports the occurrence of Massalongia in the Nearctic region and suggest that more species could be discovered there. Massalongia species form leaf or bud galls, and their mature larvae drop to the ground in autumn and overwinter in characteristic waterproof bottle-like cocoons, which is possibly a protective adaptation for pupation in wet and snowy lands. A taxonomic key to all Massalongia species is provided.