"Locusts" is an image of the incalculability, the multiplicity of various manifestations of sin by the nations who have turned away from the fulfillment of the Will of God, who have rejected the fulfillment of God's Commandments.
"Locusts" is also a symbol of the multitude of wicked people, who with their countless iniquities, evil deeds "eat", destroy in people the true Faith in the goodness of the Will of God.