Gregory the Great — on Joel 1:4 (Morals on the Book of Job, Book XXXIII)

“We certainly know that in clouds of smoke, when some are fading away above, others rise up from below. So too in carnal thoughts, though some evil desires pass away, yet others succeed. But frequently the wretched mind beholds what has already passed but does not behold where it is still detained. It rejoices in being no longer subject to some sins but neglects to be careful and to lament because others have succeeded in their place, to which perhaps it yields more sinfully. And so it is that while some sins pass away and others succeed, the heart of the reprobate is possessed without intermission by this serpent. Therefore it is well said by the prophet Joel: "That which the palmer-worm has left, the locust has eaten; and that which the locust has left, the canker-worm has eaten; and that which the canker-worm has left, the mildew has eaten."”