Albert Barnes Commentary Isaiah 33:4

"And your spoil shall be gathered as the caterpillar gathereth: as locusts leap shall men leap upon it." — Isaiah 33:4 (ASV)

And your spoil – The booty that the Assyrian army had gathered in their march toward Jerusalem, and which would now be left by them to be collected by the Jews.

Shall be gathered like the gathering of the caterpillar – The grammatical construction here allows for two interpretations. It may either mean, as the caterpillar or the locust is gathered, or it may mean, as the caterpillar gathers its spoil. It often happened that in countries where the locust was an article of food, they were scraped together in large quantities, thrown into ditches or reservoirs, and kept to be eaten. This is the custom in some parts of Africa. But the meaning here is undoubtedly that the plunder of the Assyrian army would be collected by the Jews, as the locust gathered its food.

The sense is that as locusts spread out over a land, as they go to and fro without rule and without molestation, gathering whatever is in their way and consuming everything, so the Jews in great numbers, and without regular military array, would run to and fro collecting the spoils of the Assyrian army. In a country where such devastation was made by the caterpillar and locust as in Palestine, this was a very striking figure. The word rendered ‘caterpillar’ here, חסיל (châseyl, from חסל, châsal, to cut off, consume), properly denotes the devourer and is usually applied to a species of locust. So it is understood here by most of the versions. The Septuagint renders it, ‘As if one were gathering locusts, so will they insult you.’