Which plague came just before the locusts?

Answer: Hail

The locusts devoured every plant and fruit "that the hail had left behind"; hail was the plague just before them.

“Then the LORD said to Moses, “Stretch out your hand over the land of Egypt, so that the locusts may swarm over it and devour every plant in the land—everything that the hail has left behind.””— Exodus 10:12 (BSB)