The palmer worm is generally regarded to be a hairy caterpillar that travels in large numbers, although some researchers believe that the word refers to an actual locust. Palmer worms are referenced in the Bible in Joel 1:4, which reads (King James Version) "That which the palmerworm hath left hath the locust eaten."
The worm is described as a voracious caterpillar that was a scourge on life during Joel's time. In the ancient world, this caterpillar was super destructive before reaching its winged state and was then even more destructive afterwards. It is thought that the word palmer is a reference to its propensity to wander like a pilgrim.