"So Moses and Aaron came in to Pharaoh and said to him, 'Thus says the LORD God of the Hebrews: "How long will you refuse to humble yourself before Me? Let My people go, that they may serve Me. Or else, if you refuse to let My people go, behold, tomorrow I will bring locusts into your territory. And they shall cover the face of the earth, so that no one will be able to see the earth; and they shall eat the residue of what is left, which remains to you from the hail, and they shall eat every tree which grows up for you out of the field. They shall fill your houses, the houses of all your servants, and the houses of the Egyptians--which neither your fathers nor your fathers' fathers have seen, since the day that they were on the earth to this day"'" (Exo. 10:3-6).
Pharaoh's true problem is identified here: pride! God promises another destructive plague--locusts--if Pharaoh will not humble himself and let the Israelites go.
After Moses and Aaron leave Pharaoh's presence, "Pharaoh's servants said to him, 'How long shall this man be a snare to us? Let the men go, that they may serve the LORD their God. Do you not yet know that Egypt is destroyed?'" (10:7). Pharaoh's servants appear more reasonable than he is.
Pharaoh softens for a moment and calls Moses and Aaron back before him. He tells them - "Go, serve the LORD your God. Who are the ones that are going?" (10:8). Moses informs him that every one of them would be going, along with all their animals to "hold a feast to the LORD" (10:9).
Pharaoh's soft side vanishes again and he replies in anger - "'The LORD had better be with you when I let you and your little ones go! Beware, for evil is ahead of you. Not so! Go now, you who are men, and serve the LORD, for that is what you desired.' And they were driven out from Pharaoh's presence" (10:10,11). Pharaoh thinks they are plotting evil; namely, to leave Egypt permanently. He will not let them implement the "evil" they are planning.
At God's command, "Moses stretched out his rod over the land of Egypt, and the LORD brought an east wind on the land all that day and all that night. When it was morning, the east wind brought the locusts. And the locusts went up over all the land of Egypt and rested on all the territory of Egypt. They were very severe; previously there had been no such locusts as they, nor shall there be such after them. For they covered the face of the whole earth, so that the land was darkened; and they ate every herb of the land and all the fruit of the trees which the hail had left. So there remained nothing green on the trees or on the plants of the field throughout all the land of Egypt" (10:13-15). Clearly, God's power extends well beyond the borders of Egypt! The locusts came upon the land like an invading army (cf. Joel 1:6,15; 2:25)! The Egyptian god of grain (Nepri) was powerless against Jehovah!
It didn't take long for Pharaoh to summon Moses and Aaron. "I have sinned against the LORD your God and against you. Now therefore, please forgive my sin only this once, and entreat the LORD your God, that He may take away from me this death only" (Exo. 10:16,17). Moses prayed to God that the locusts would be removed, and the LORD used a very strong west wind to blow all the locusts into the Red Sea. After relief came, Pharaoh's heart hardened again and he "did not let the children of Israel go" (10:20). He implied that he wouldn't do it again, but that was not true.