Exodus 10:4

Else, if thou refuse to let my people go, behold, to morrow will I bring the locusts into thy coast:

Through Moses and Aaron Pharaoh was warned that if he refused to let the people go, locusts would be brought into his coasts on the morrow. He was set a choice to humble himself before God and release Israel or to refuse and suffer the effects of the plague. The locusts would cover the face of the earth and eat the residue that had escaped the hail, consuming what remained after it.

The warning displayed great signs and wonders before Pharaoh and Israel to confirm Israel's faith in Him as the only true and living God and to give unmistakable evidence of the distinction He places between the Egyptians and His people. It was given so that Israel could tell their sons and grandsons what He wrought in Egypt and know that He is the LORD, and so that all nations would know that though Israel was despised and bound down by hard labor, He had chosen them as His peculiar people and would work for their deliverance in a wonderful manner. The threatened plague exposed the powerlessness of Serapis, worshiped as Egypt's protector from locusts, to protect the land, for the locusts came and left at Moses' command, showing the folly of trusting any protection other than Jehovah.