John Calvin on Isaiah 7:18

18. And it shall be in that day. The Jews thought that the Assyrians were bound by their league with them; but the Prophet ridicules this folly, and declares that they will be ready at God’s bidding to drive them in any direction that he thinks fit. Yet instead of command he employs the metaphor hiss, in allusion to the climate of those kingdoms of which he speaks; for Egypt abounds in flies, because the country is hot and marshy; and when the air is both hot and moist, there must be produced a great abundance of flies. Assyria, on the other hand, abounded in bees; and when he says that he will bring them by a hiss, he alludes to the natural habits of bees and flies, but he means that he will find no difficulty in sending them. As if he had said, “There will be no need of great exertion; for as soon as I shall give the sign, they will instantly run.” In this manner he shows what efficacy belongs to his secret operation or design, that by a hiss he compels the most powerful nations to yield obedience.