Exodus 8:16
And the LORD said unto Moses, Say unto Aaron, Stretch out thy rod, and smite the dust of the land, that it may become lice throughout all the land of Egypt. And the LORD said unto Moses, Say unto Aaron, Stretch out thy rod, and smite the dust of the land, that it may become lice throughout all the land of Egypt.
The command to produce lice came after Pharaoh saw relief from the frogs, hardened his heart, and again stubbornly refused to let Israel go. At God's command Aaron stretched out his hand, and the dust of the land became lice throughout all Egypt. This was counted as the fourth miraculous sign, spreading over the land and afflicting both people and animals. The turning of dust into lice was one of the signs and wonders meant to convince Pharaoh that the great I AM had sent Moses and that it was Pharaoh's duty to let Israel go to serve the living God.
The miracle was worked by God's power through Moses and Aaron, not by any human power, influence, or skill of their own. Moses and Aaron acted as servants of God under His command, while Pharaoh's magicians acted as servants of Satan under his command. The magicians worked not by human skill or science alone but by the power of the devil, their god. Satan aided the counterfeits to destroy the influence of God's work, to harden Pharaoh's heart against God's power, to stagger the faith of Moses and Aaron in their divine mission, and to undermine faith in Christ's miracles in all later ages.
At Pharaoh's call the magicians tried by their enchantments to produce lice in the same way and could not copy it. Their failure proved the work of God superior to the power of Satan and showed that God's work surpasses Satan's counterfeit signs and wonders. The magicians could imitate only a few of the miracles God wrought through Moses, as seen when their rods became serpents but Aaron's rod swallowed them up. Their failure with the lice marked the end of their imitative power, because God would not suffer Satan to proceed further and cut off the channel through which he worked.
Compelled by God's power, the magicians acknowledged to Pharaoh that the plague was the direct action of God, the finger of God. Even with that testimony Pharaoh's heart was hardened and he remained unmoved, would not listen, and stubbornly refused to let Israel go, as the Lord had said. The plague also struck at Egyptian idolatry, because priests had to avoid even slight impurity to approach their altars, so the lice prevented acts of worship and exposed the failure of Egypt's gods to protect their worshipers.