1. Either the axil of the scale is galled, or the galled fruit is distinctly coalescent with the scale; window-pit absent..........skuhravae sp. n.
— Galling of the fruit, the gall is in ripe situation never coalescent with the scale..........2
2. Window-pit absent or indistinct, gall subrotund and glabrous, wings of fruit often completely reduced..........tarda sp. n.
— Window-pit(s) distinct..........[all other Semudobia spp.]
...
Gall...galling of the fruit; the fruit is button-like swollen, subrotund, wings are completely or nearly completely reduced; window-pit rather indistinct (B. pubescens).
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[Record from B. papyrifera was from a cultivated tree in Quebec where "an introduction of infected fruits of Palaearctic birch species is supposed". Only otherwise recorded from North America on introduced B. pendula in Montana.]

- J. C. Roskam: (1977) Biosystematics of insects living in female birch catkins. I. Gall midges of the genus Semudobia Kieffer (Diptera, Cecidornyiidae)^©