Bugs

Then G-d ordered Aaron to smite the dust of the earth with his staff, and no sooner did he do so than all over Egypt bugs crawled forth from the dust to cover the land. Man and beast suffered untold misery from this terrible plague.

Although his counsellors pointed out that this surely was G-d's punishment, Pharaoh steeled his heart and remained relentless in his determination to keep the children of Israel in bondage.