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Adults: Small black sawflies, 4mm long. Females only, males not recorded. Species is likely parthenogenic. Tibia and tarsus white to light yellow, wings brown tinged and darker near the body. Adults may often co-occur with Fenusa pusilla on birch and appear similar but can be distinguished by the ovipositor. Both species posess a saw-like ovipositor used to deposit eggs under the surface of leaves. Profenusa thomsoni's saw is well developed and posses wide, closely spaced serrulae (teeth). Larvae: Creamy white to light brown as the larva ages. Six instars. One of only three species in Alberta to feed inside birch leaves. Dorso-ventrally flattened with a forward facing head except for the final instar where the characteristic eruciform shape is retained and the mouthparts face ventrally. 7.0 mm long in the final (5th ) instar. Medium sized dark brown to black plates on the thorax, visible in all instars but more easily distinguished in larger larvae. These plates can be used to distinguish between P. thomsoni and two other birch leafmining species Fenusa pumila and Heterarthrus nemoratus. In F. pumila the plates appear as a continuous dark black stripe running onto the 1st abdominal segment, while in H. nemoratus a single plate is visible on the 1st thoracic segment only (occasionally with light plates on the 2nd or 3rd thoracic segments). The larval mine of P. thomsoni can be used to separate it from F. pumila. Since P. thomsoni attacks mature leaves, feeding does not disrupt the shape of the leaf whereas F. pusilla attacks expanding leaves and larval feeding tends to result in crinkled leaves. This character is more usefull early in the season second generation F. pumila may attack fully expanded leaves. Adapted from Smith 1971 and Goulet 1992.

Drouin and Wong (1984) detailed the life history of this species in Alberta. Adults emerge in late May to early June, becoming abundant by July. Parthenogenic females lay eggs inside birch leaves using the sawlike ovipositor to open small slits in the upper surface of leaves. Larvae feed inside the leaf creating a distinctive blotch shaped mine but drop to the ground to pupate. There are 6 larval instars, 5 spent feeding inside the leaf and the 6th is spent looking for a suitable location to pupate. In North America there is only one generation per year. Leafminer adults appear to prefer fully expanded leaves, but will lay eggs in any acceptable host. Where P. thomsoni and Fenusa pumila co-occur they are capable of defoliating whole trees and can be significant forest and horticultural pests. Profenusa thomsoni has occasionally occurred at outbreak levels in urban areas of Alberta, the Northwest Territories and Alaska since the 1960's but can be controlled by the Ichneumonid parasitoid wasp Lathrolestes luteolator. This wasp lays its eggs inside developing P. thomsoni larvae that hatch after the leafminer pupates and consumes the host (Digweed et al. 2003).