“And it was commanded them that they should not hurt the grass of the earth, neither any green thing, neither any tree; but only those men which have not the seal of God in their foreheads.”
They were told not to damage the grass of the earth or any green thing or any tree, but only those people who do not have the seal of God on their foreheads — the locusts are commanded (laleō) to spare vegetation (compare the first trumpet) and attack only the unsealed. This establishes that God's sealing (chapter 7) provides actual protection; the marked faithful are exempt from this woe.