Albert Barnes Commentary Hosea 5:12

"Therefore am I unto Ephraim as a moth, and to the house of Judah as rottenness." — Hosea 5:12 (ASV)

Therefore I will be unto Ephraim a moth - Literally, and I as a moth. This form of speaking expresses what God was doing, while Ephraim was “willingly following” sin. “And I” was all the while “as a moth.” The moth in a garment, and the decay in wood, corrode and prey upon the substance, in which they lie hidden, slowly, imperceptibly, but, in the end, effectually. Such were God’s first judgments on Israel and Judah; such they are now commonly upon sinners. He tried, and now also tries at first, gentle measures and mild chastisements, uneasy indeed and troublesome and painful; yet slow in their working; each stage of loss and decay, a little beyond what preceded it; but leaving long respite and time for repentance, before they finally wear out and destroy the impenitent.

The two images that He uses may describe different kinds of decay, both slow, yet the one slower than the other, as Judah was, in fact, destroyed more slowly than Ephraim. For the “rottenness,” or caries in wood, preys more slowly upon wood that is hard, than the moth on the wool.

So God visits the soul with different distresses, bodily or spiritual. He impairs, little by little, health of body, or sharpness of understanding; or He withdraws grace or spiritual strength; or allows lukewarmness and distaste for the things of God to creep over the soul. These are the gnawing of the moth, overlooked by the sinner if he perseveres in carelessness regarding his conscience, yet in the end bringing entire decay of health, of understanding, of heart, of mind, unless God intervenes by the mightier mercy of some heavy chastisement to awaken him: “A moth does mischief, and makes no sound.

So the minds of the wicked, because they neglect to take account of their losses, lose their soundness, as it were, without knowing it. For they lose innocence from the heart, truth from the lips, continence from the flesh, and, as time goes on, life from their age.” To Israel and Judah the moth and rottenness denoted the slow decay, by which they were gradually weakened, until they were carried away captive.