Like a very fair heifer. (The insertion of "like" weakens the passage.) The well nourished heifer reminds of the prosperity of the fruitful Nile valley. But destruction cometh; it cometh out of the north; rather, a gadfly from the north hath come upon her (not, "hath come, hath come," as the received text has—a very slight change in one letter is required, supported by the versions).
The figure is precisely analogous to that of the "bee in the land of Assyria" (Isaiah 7:18). St. Chrysostom renders "a gadfly" (see Field, 'Origen's Hexapla,' 2.708); and so virtually Aquila and Symmachus.