I know Ephraim, And Israel is not hidden from me; For now, Ephraim, you have played the prostitute. Israel is defiled.
Note that in this passage, as frequently in Hosea (some 37 times) Ephraim is the name used for Israel.  Ephraim was almost as large as Judah, the largest of the twelve tribes; and it was always jealous and envious of Judah.  The tribe of Ephraim led the defection of the ten tribes to form the Northern kingdom; and it also enthusiastically supported the calf-worship instituted by Jeroboam I at Dan and at Bethel.  All of the children of Israel eventually were corrupted through Ephraim's leadership; and it was thus fitting that his name should finally become that of the whole northern Israel.  Given summed it up this way: