Elm leafminer (Fenusa ulmi) is a very small common sawfly. It is native to eastern Europe and Scandinavia. It was brought to North America most likely on imported elms. It now occurs in the United States from New England to the upper Midwest, in the Pacific Northwest, and in southeast Canada. Based on the scarcity of reports, it is still relatively uncommon in Minnesota.
Due to the small size of the adult, elm leafminer is most often identified by the damage the larva causes to its host plant. Mines are seen from mid-May to early June on American elm and slippery elm. The larva feeds between the upper and lower surfaces of the leaf. It creates a serpentine mine at first. That soon develops into a small blotch between two lateral veins, later into a large blotch on one side if the midrib. The mines are clear and the flattened, whitish-green or yellowish-white larva can be seen when viewing the upper side of the leaf. The infected part of the leaf turns brown and eventually falls off. A heavy infestation may cause the entire tree to defoliate, but the infected tree flushes again and survives.