Therefore will I be unto Ephraim as a moth, and to the house of Judah as rottenness. This verse is well explained by Calvin as follows: "The meaning of the prophet is by no means obscure, and that is, that the Lord would by a slow corrosion consume both the people; and that, though he would not by one onset destroy them, yet they would pine away until they became wholly rotten."
The two agents of destruction here named—the moth which eats away clothes, and the woodworm which gnaws away wood—figuratively represent slow but sure destruction. They are found together in Job 13:28.
Kimchi explains the sense in like manner: "Like the moth which eats away garments, and like the woodworm which consumes bones and wood, so shall I consume you." The pronoun at the beginning of the verse is emphatic: "I your God, who would have been your protector and preserver, whom you have sinfully forsaken, and whose commandments you have arbitrarily set aside—even I am to you as the source of rottenness, and of slow but sure ruin."