Exodus 8:18 - Ellicott's Commentary On The Whole Bible

The magicians did so — i.e., tried to do so — took moist earth, and dried it, and pulverised it, and tried the effect of their magic charms upon it, but. failed to produce mosquitoes, as Aaron had done. Mosquitoes were things too delicate to be caught, and manipulated, and produced at a given moment by sleight-of-hand. The magicians tried to produce a counterfeit of the miracle, but could not. Then they excused themselves to their master with the words, “This is the finger of a god.”