What is a "canker worm" anyway?
Cankerworm appears in the following verses in the King James Version:
Joel 1:4 That which the palmerworm hath left hath the locust eaten; and that which the locust hath left hath the cankerworm eaten; and that which the cankerworm hath left hath the caterpiller eaten.
Joel 2:25 And I will restore to you the years that the locust hath eaten, the cankerworm, and the caterpiller, and the palmerworm, my great army which I sent among you.
Nahum 3:15,16 There shall the fire devour thee; the sword shall cut thee off, it shall eat thee up like the cankerworm: make thyself many as the cankerworm, make thyself many as the locusts. 16 Thou hast multiplied thy merchants above the stars of heaven: the cankerworm spoileth, and flieth away.
The cankerworm is probably the larva of the locust which has hatched after the main swarm has moved on and it continues its parents legacy of destruction.