25 What then those things do signify, let the interpreter say as he can, let reader and hearer judge as is just. The water turned into blood seems to me to signify a carnal view of the causes of things. Dog-fly, are the manners of dogs, who see not even their parents when first they are born. The frog is very talkative vanity. Mildew does hurt secretly, which also some have interpreted by rust, others black mould: which evil thing to what vice is it more appropriately compared, than to what does show itself least readily, like the trusting much in one's self?
For it is a blighting air which does work this secretly among fruits: just like in morals, secret pride, when a man thinks himself to be something, though he is nothing. The locust is malice hurting with the mouth, that is, with unfaithful testimony. The hail is iniquity taking away the goods of others; whence theft, robberies, and depredations do spring: but more by his wickedness the plunderer himself is plundered. The hoar-frost does signify the fault wherein the love of one's neighbour by the darkness of foolishness, like as it were by the cold of night, is frozen up.
But the fire, if here it is not that which is mentioned which was in the hail out of the lightning clouds, forasmuch as he has said here, “He gave over their possession to the fire,” where he implies that a thing was burned, which by that fire we read not to have been done—it seems to me, I say, to signify the savageness of wrath, whereby even man-slaying may be committed. But by the death of beasts was figured, as far as I judge, the loss of chastity. For concupiscence, whereby offspring do arise, we have in common with beasts.
To have this therefore tamed and ordered, is the virtue of chastity. The death of the first-born things, is the putting off of the very justice whereby a man does associate with mankind. But whether the figurative significations of these things be so, or whether they are better understood in another way, whom would it not move, that with ten plagues the Egyptians are smitten, and with ten commandments the tables are inscribed, that thereby the people of God should be ruled? Concerning the comparing of which one with the other, inasmuch as we have spoken elsewhere, there is no need to load the exposition of this Psalm therewith: thus much we remind you, that here too, though not in the same order, yet ten plagues of the Egyptians are commemorated, forasmuch as in the place of three which are in Exodus and are not here, to wit, lice, boils, darkness; other three are commemorated, which are not there, that is to say, mildew, hoarfrost, and fire; not of lightning, but that where-unto their possession was given over, which is not read of in that place.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)