John Calvin Commentary Psalms 105:34

"He spake, and the locust came, And the grasshopper, and that without number," — Psalms 105:34 (ASV)

He spake, and the grasshopper came. This calamity, which was brought upon the fields, could not be attributed to Fortune, for the grasshoppers made their appearance suddenly and in countless multitudes, so that they covered all the land of Egypt. The miracle was very evident from the word spoken, by which it was introduced.

Its prior announcement removed all doubt that it was the work of the Most High. Accordingly, it is expressly said that grasshoppers and caterpillars rushed in at the commandment of God, as if soldiers should run to battle at the sound of the trumpet. Whenever these insects trouble us and destroy the fruits of the earth, they are assuredly the scourges of God, but it is here intended to point out an extraordinary work of his hand.

Finally, the prophet recites the last miracle, which was performed by the angel on the night previous to the departure of the people, when he slew all the first-born throughout Egypt. I only take a hasty and passing glance at this history, just as I have done with the other preceding facts, because they have been treated more extensively elsewhere, and for now, it is sufficient for us to know the sacred writer's purpose.

He, however, amplifies this display of divine power by a repetition, declaring that the first-born and the flower of their strength were destroyed. Some translate, but unfortunately, The beginning of their sorrow. As a man’s strength shows itself in procreation, the Hebrews call the first-begotten the beginning of strength, as we have explained on Genesis 49:3:

“Reuben, thou art my first-born, my might,
and the beginning of my strength.”