He sent messengers to Bilam son of Beor, to Petor which is on the river, where his people lives, to say to him: "Behold, this people which left Egypt has covered the eye of the earth, and it is now camped opposite me." (Bamidbar 22:5)
What is the meaning of the "eye of the earth" in this verse? Shemot 10:4-5 uses the phrase in a similar way:
"For if you [Pharaoh] refuse to let my people go, I shall bring tomorrow locusts in your borders. They will cover the eye of the earth, and it will be impossible to see the earth..."
In both places, Onkelus translates the phrase as "the eye of the sun of the earth", meaning the bright disc formed by the sun. In Shemot, the meaning of this is clear: the swarm of locusts will be so thick that it will cover the sun and there will be darkness. (Or perhaps, the locusts will both cover the sun and form a continuous layer on the surface of the ground.)
In Bamidbar, it seems that Israel too is being compared to a swarm of locusts, which devours the lands whose path it crosses, and looks ready to devour Balak's land and people too.
Interestingly, this is not the first place in Bamidbar where Jews are compared to locusts. When the spies returned from their trip, they reported: "There we saw the Nefilim sons of Anak (from the Nefilim), and we were in our eyes like locusts, and so were we in their eyes."
The spies saw themselves as small insects, incapable of conquering the mighty inhabitants of Canaan. But while the spies saw the locust as a weak animal, Balak saw the locust as an unstoppable and overpowering force! How could the same animal be used as a metaphor for completely opposite concepts?
The difference, of course, is that the spies referred to individual locusts, while Balak referred to an entire swarm. Indeed, while an individual locust is capable of little damage, a swarm of them is extremely formidable. Similarly, a single Israelite might be weaker than a single Anakite, but when the Israelites were united nobody could stop them from conquering the land.
How do we know that the spies should have realized this point, and didn't? After the spies spread their bad report and God decreed forty years of desert wandering upon them, the people rebelled and tried to invade Canaan immediately. When Moshe describes the results of this invasion in Devarim, he makes reference to a different kind of insect:
"The Amorite who lived on that mountain went out against you, and chased you as do the bees, and struck you down in Se'ir, as far as Hormah." (1:44)
Bees, like locusts, are individually weak but together form a fearsome swarm. Just like the Israelites' enemies knew the power that comes from unity, so should the Israelites have known.
(Based on a dvar torah from R' Shlomo Glicksberg)