Locusts, warning, and promise — the day of the LORD and the outpoured Spirit.
"I will pour out my spirit upon all ______; and your sons and your daughters shall prophesy."
When the Spirit fell at Pentecost, Peter's explanation was simply 'this is that which was spoken by the prophet Joel' — and he quoted this book at length.
Joel begins with an actual locust plague — swarm after swarm stripping the land — and reads it as a rehearsal for the day of the LORD.
'Rend your heart, and not your garments' comes from Joel 2 — torn clothes were the ancient sign of grief; God asked for the tearing to happen deeper.
Joel promises that God will 'restore to you the years that the locust hath eaten' — one of Scripture's tenderest lines about wasted time.