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

(Betulaceae), or birch, is a Holarctic genus of trees and shrubs whose species have ornamental, industrial, and medical importance. Gall midges of the genus (Diptera: Cecidomyiidae: Cecidomyiidi) are exclusively associated with birches in the Palearctic region. In 2018, an undescribed species was discovered forming leaf galls on the midveins of on Mount Tara, Saga Prefecture, Kyushu, Japan. In this study the species is described as Elsayed & Tokuda, , 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 The other known species of Massalongia are redescribed because the original descriptions are outdated and insufficient.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 . In addition, the monotypic genus , containing associated with in the Nearctic region, is synonymized here under , resulting in , rendering a Holarctic genus with six species. Comparing the sequence data of with all sequences available in The Barcode of Life Data (BOLD) system supports the occurrence of in the Nearctic region and suggest that more species could be discovered there. 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 species is provided.