This comparison with grasshoppers is frequently used in the Scriptures, as if to say, “Grasshoppers are easily shooed away.” All inhabitants of the earth are like this in the sight of God. They cannot bear God’s least judgment but are scattered by one word and breath of God. So today we observe that the pope and all the most holy servants of the Mass are in God’s sight nothing but grasshoppers that are scattered. The nations, again, are not to be understood metaphysically. The reference is to every undertaking and righteousness on their part that aims at appeasing God. Their works are nothing but grasshoppers that must be dispersed. And they hang on very weakly.
Luther’s Works Vol. 17: Lectures on Isaiah Chapters 40-66