Sakagami and Tadauchi (1995) describe this species and present tables comparing morphometric measurements of it with other Japanese and European Lasioglossum species. Lasioglossum pumilum is black, yellow, and pale brown, and non-metallic, and 4.2-5.2 mm long. This species is found in Japan on Hokkaido and Honshu islands. No information about social behavior or nesting biology is presented.
