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Adults: Small black sawflies 3.7 mm long. All black except for small amounts of white on each femur, tibia and tarsus. Wings with light brown banding and darker near the body. Antennae shorter than thorax (longer in Fenusa dohrnii). All sawflies of this group posses an ovipositor modified to be used as a saw to facilitate the deposition of eggs under the surface of leaves. Teeth (serula) of the lancet (saw) are low and flattened with 5 or 6 sub-basal teeth (only 3 in F. dohrnii). Males and females generally the same except males without ovipositor. Larvae: Creamy white with distinctive banding pattern on ventral side of thorax and abdomen. Final feeding stage 5.5 mm long. One of only three species in Alberta to feed inside birch leaves. Dorso-ventrally flattened with a prognathous (forward-facing) head. A small black stripe runs from the thorax to the 1st abdominal segment visible on the ventral surface in all instars but more easily distinguished in larger larvae. This stripe consists of four dark brown to black plates in the centre of thoracic segments 1-3 and abdominal segment one. Each plate is separate, but appears to be one contiguous peice. This stripe can be used to distinguish between F. pumila and two other birch leafmining species, Profenusa thomsoni and Heterarthrus nemoratus. In neither P. thomsoni nor H. nemoratus does the stripe continue on to the 1st abdominal segment. In F. pumila the 1st thoracic segment width is constant when viewed from the ventral or dorsal aspect. The three North American Fenusa species complete development inside leaves of the host plant. Therefore, the larval host is an important diagnostic character. Fenusa pumila is widely distributed in North America on Birch, F. dohrnii feeds on Alder and F. ulmi occurs in eastern North America on Elm. Adapted from Smith 1971, Goulet 1992 and Lindquist 1959.

Adults emerge in late May to early June and fly to the leaf surface. Females use the saw-like ovipositor to deposit eggs near the mid-rib in the upper surface of newly expanded or expanding birch leaves (Drouin and Wong 1984). Eggs hatch 4 – 10 days later and larvae feed inside the birch leaf, often disrupting the host leaf's development. Infested leaves therefore have a distinctive 'crinkled' appearance. Larvae complete development within the leaf and create a blotch shaped mine adjacent to the mid-rib. When development is completed larvae drop to the ground to pupate. A second generation will emerge approximately 2 weeks later. In Alberta the offspring of the second generation overwinter, but elsewhere there may be multiple generations per year, the offspring of the last overwintering. In Alaska their appears to be only 1 generation per year (pers. obs). Leafminer adults appear to prefer the upper and outer leaves of birch trees which corresponds to new growth (Drouin and Wong 1984). Large populations are capable of totally defoliating whole forests. Where Profenusa thomsoni co-occurs feeding by both species can totally defoliate trees by mid to late July. An introduced species, F. pumila has been the target of multiple biological control efforts. Two ichneumonid parasitoid wasps Lathrolestes nigricollis (Thomson) and Grypocentrus albipes Ruthe and one Eulophid wasp Chrysocharis nitetis (Walker) have been introduced together or separately to Newfoundland (Raske and Jones 1975),the Eastern United States (Driesche van et al. 1997), Quebec (Guèvremont and Quednau 1977) and Alberta (Langor et al. 2000) with varying degrees of success.