22 He made their rains hail. It is a similar expression to the former, “He made their land frogs;” except that the whole land was not actually turned into frogs, though the whole of the rain may have been turned into hail. “A burning fire in their land:” understand, “He sent.”
23. “He smote their vines also and fig-trees; and broke every tree of their coasts”. This was done by the violence of the hail, and by lightnings; whence he spoke of the fire as “burning.”
24. “He spoke the word, and the locust came, and the caterpillar, of which there was no number”. The locusts and the caterpillars are one plague: of which the one is the parent, the other the offspring.
25. “And did eat up all the grass in their land, and devoured the fruit of the ground”. Even grass is fruit, as Scripture is wont to speak, which calls even the ripe grain grass; but it wished these two things to harmonize in number with the two which it had spoken of before, that is, the locust and the caterpillar. But the whole of this does belong to the variety of speech, which is a remedy for weariness, not to any difference of senses.
26. “He smote every first-born in their land: even the first-fruits of all their strength”. This is the last plague, excepting the death in the Red Sea. “The first-fruits of all their strength,” I imagine to be an expression derived from the first-born of cattle. These plagues are ten in number, but they are not all mentioned, nor in the same order in which they are there read to have happened. For praise-giving is free from the law which binds one who is relating or composing a history. And since the Holy Spirit is the Author and Dictator, through the Prophet, of this praise; by the very same authority with which He guided him who wrote that history, he does both mention something to have taken place which is not there read, and passes over what is there read.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)