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..........3
3. Often a window-pit on both sides of the gall, one of them large; gall glabrous, wings of fruit often completely reduced; East-Nearctic species..........brevipalpis sp. n.
— Only one window-pit developed; gall more or less hairy, wings present..........4
4. Gall usually concave on ad-axial side, window-pit irregularly formed; West-Nearctic species..........steenisi sp. n.
— Gall usually convex on both sides, window-pit subrotund; mainly Palaearctic species..........betulae Wtz.
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Gall...galling of the fruit; the gall ovoid, more or less hairy, wings present, although not so large as in healthy fruits; window-pit distinct (B. pubescens).
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[Record from B. populifolia was from a cultivated tree in Ontario where "an introduction of infected fruits of Palaearctic birch species is supposed". Only otherwise recorded from North America on introduced B. pendula in Pennsylvania. Host records on other Betula species were all from outside North America (sometimes on North American species in cultivation elsewhere).]

Betula (birch)
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Swollen bract or ovary of female catkin...Semudobia spp.

Five species of Semudobia are known from North America, three native and two originally introduced from Europe in seeds of Betula pendula. One of the three native species is Holoarctic, the two others are Nearctic only. The biology of all five species is generally similar. Eggs are deposited in spring between the bracts and flowers. Usually only one larva infests a bract or ovary. Pupation occurs in the gall the following spring.
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S. betulae (Winnertz), an immigrant from Europe. Host: B. pendula. Distr.: Ontario and Pennsylvania.