Wis 16:9

“But the mention of locusts in the eighth place, I think, through this kind of plague or detractor, or always by its own discord, is intended to signify the inconsistency of the human race. For the locust, although it has no king, as Scripture says, leads its army in order with one line (Prov. 30:27); but men, although made rational by God, have neither been able to govern themselves in an orderly manner, nor have they patiently endured the moderation of God the King.”

“A distinction must be noted, because just as punishment sometimes tests and admonishes the elect, so that they may avoid sins and persist in works of righteousness, and diligently observe the faith of God and the keeping of His commandments, so also correction applied to the depraved casts them down and destroys them, since no correction or worthy change is found in them; therefore, by the will of God, small animals killed the idolaters, and conversely, the Israelites, worshippers of the true God, could not be exterminated by the venom of serpents, because the Lord knows how to rescue the virtuous from temptation, but to keep the wicked for torment on the day of judgment.”

“For those, namely the Egyptians who did not worship you, the bites of locusts and flies killed: of locusts, Exodus 10, and of flies, Exodus 8: killed, literally some of them, although it is not read there expressly. And no healing was found for their life, namely of the Egyptians, as for the children of Israel from the serpents; in the Psalm: "There is no health in my flesh because of your wrath," etc.; Jeremiah 30: "Your wound is incurable, your affliction is grievous." Because they were worthy, namely as their merits demanded, by such things, namely by vile animals, to be destroyed: whence Augustine on John: "The Lord could have subdued the proud people of Pharaoh with bears and lions, but he sent flies and the vilest creatures, so that their pride might be tamed." For the same reason men are also afflicted by fleas: whence Augustine: "Why do you puff yourself up, human pride? Resist the fleas, so that you may sleep."”