Your spoil... — The words are addressed to the invader. He who came to spoil should find himself spoiled. As caterpillars and locusts devour the green herbage, so should he (or they, the indefinite pronoun standing for the people of Jerusalem) strip his camp of all its treasures.
Related Commentaries of Isaiah 33:4
Isaiah 33:4
4 And your spoil shall be gathered like the gathering of the caterpiller: as the running to and fro of locusts shall he run upon them.