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Adults: Small black sawflies. Females only. 4.5 mm long. Black thorax with yellow pronotum and tegula, white legs but with coxa and basal half of femur black. Forewing with slight brown band. Larvae: Creamy white with distinct brown head capsule about 8 mm long at the last feeding stage. One of only three species in Alberta to feed inside birch leaves. Dorso-ventrally flattened with a prognathous (forward-facing) head. Ventral side of thorax with brown or black banding, only easily visible on 1st segment. This banding can be used to distinguish between two other birch leafmining species, Profenusa thomsoni and Fenusa pusilla in which the bands are larger and more distinct on the 2nd and 3rd thoracic and 1st abdominal segments. The 1st thoracic segment of H. nemoratus larva is wider in the middle than at the union with the head and 2nd thoracic segments when viewed from the ventral or dorsal aspect. The position and character of the larval mine can be occasionally used to identify larvae if found alone in birch leaves. Heterarthrus nemoratus mine near the edge of the leaf forming a blotch shaped mine that rarely reaches the mid-rib. Larval mines appear reddish brown and readily crack and break when handled. When opened, mines are usually free of frass. By comparison, P. thomsoni and F. pusilla tend to mine near the bottom or centre of the leaf, occasionally reaching the edge as later instar larvae and have mines filled with frass. Pupae: Pupation occurs in the leaf, larvae spin a silk cocoon between the upper and lower surface of the leaf within the larval mine. Pupal cells appear lens-like when viewed through the leaf. Heterarthrus nemoratus is the only birch leafmining sawfly to pupate in the leaf. Adapted from Smith 1971, Glasgow 1932, Becker 1938 and Lindquist 1959.

Females emerge in mid May to late July and fly to mature birch leaves where they lay 3-5 eggs per leaf (Drouin and Wong 1984, Lindquist 1959). The species is believed to be parthenogenic (Smith 1971). Eggs are usually deposited in a slit cut at the edge of the leaf. Larvae hatch approximately 20 days later and feed inside the leaf, and are generally restricted to the outer 2/3rds of the blade (Becker 1938). Pupation occurs inside the leaf, the insect overwintering in this stage, either on the ground or in the tree (Arru 1988). There is one generation per year in North America. Severe damage can result from multiple larvae feeding inside one leaf, resulting in a burned appearance to the whole tree. Occasionally found feeding inside leaves with either or both Profenusa thomsoni and Fenusa pusilla.