The locusts are meant to destroy all of the plants left after the hail. This includes the wheat and emmer described in verse 31. This is a complete decimation of plant life. It’s at this point that Pharaoh’s servants start to question why Pharaoh will not give in to God. Verse 2 indicates that God has not only sent these plagues to decimate the Egyptians who hold His people in captivity, but also to show His people that He is Lord, so that they can share with future generations what they had seen. It is amazing that even after the signs God provides the people, they still will one day grumble about returning to this captivity.