Sweat bees in the subgenus Dialictus of the genus Lasioglossum are extremely abundant, diverse, rapidly evolving, and poorly known in many regions, factors that make them a significant taxonomic impediment to research. In this work I break down the taxonomic impediment in the western Nearctic region by generating a phylogeny based on ultraconserved elements (UCE) and re-evaluating the limits of Dialictus, revising two common species groups using integrative taxonomy, and updating the 2010 revision of the Canadian Dialictus with new taxonomic information. The UCE phylogeny largely corroborates previous studies of Lasioglossum while resolving the status of three problematic or uncertain taxa: Biennilaeus and Pyghalictus are confirmed as valid subgenera and Eickwortia is expanded to include 18 species formerly placed in Dialictus (including the problematic species L. figueresi). Five new synonymies are established, leaving 25 total subgenera of Lasioglossum I recognise as valid. The first species group revised in depth is the so-called “red-tailed” Dialictus, an artificial assemblage of species recognised by orange-red colouration on the metasoma. I describe 20 new species, establish two new synonymies, and designate a lectotype for the poorly-known species L. mesillense. The second species group is the L. gemmatum species complex, a monophyletic group united primarily by an enlarged tegula. I describe 10 new species and establish 1 new synonymy. For both groups, keys to species, figures, distribution maps, available DNA barcodes, and contextual phylogenies are provided. In the update to the Dialictus of Canada, I describe two new species (one of which is likely endemic to Canada and one likely adventive), report three new species distribution records in Canada, resurrect two species from synonymy, establish five new synonymies, and provide a key to species updated with all taxonomic and nomenclatural changes since 2010.