What swarming insects devastated the land in the book of Joel?

Answer: Locusts

Joel describes wave after wave of locusts eating what the previous swarm had left, a sign calling the people to repent.

“What the devouring locust has left, the swarming locust has eaten; what the swarming locust has left, the young locust has eaten; and what the young locust has left, the destroying locust has eaten.”— Joel 1:4 (BSB)