The palmerworm, locust, etc., are thought to be different forms, at different stages of development, of one insect. The essential fact is that, according to the usual method of the Spirit in prophecy, some local circumstance is shown to be of spiritual significance, and is made the occasion of a far-reaching prophecy (e.g.) Isaiah 7:1-14 where the Syrian invasion and the unbelief of Ahaz give occasion to the great prophecy of Joel 1:14. Here in Joel a plague of devouring insects is shown to have spiritual significance Joel 1:13; Joel 1:14 and is made the occasion of the prophecy of the day of the Lord, not yet fulfilled. (See Scofield "Isaiah 2:12"). This is more developed in Joel 2, where the literal locusts are left behind, and the future day of Jehovah fills the scene.
The whole picture is of the end-time of this present age, of the "times of the Gentiles" Luke 21:24; Revelation 16:14 of the battle of Armageddon; Revelation 16:14; Revelation 19:11-21 of the regathering of Israel. (See Scofield "Romans 11:26") and of kingdom blessing. It is remarkable that Joel, coming at the very beginning of written prophecy (B.C. 836), gives the fullest view of the consummation of all written prophecy.
The order of events is:
(1) The invasion of Palestine from the north by Gentile world-powers headed up under the Beast and false prophet Joel 2:1-10 "Armageddon," (See Scofield "Revelation 16:14").
(2) the Lord's army and destruction of the invaders Joel 2:11; Revelation 19:11-21.
(3) the repentance of Judah in the land Joel 2:12-17 (See Scofield "Deuteronomy 30:3").
(4) the answer of Jehovah Joel 2:18-27
(5) the effusion of the Spirit in the (Jewish) "last days" Joel 2:28; Joel 2:29.
(6) the return of the Lord in glory and the setting up of the kingdom Joel 2:30-32; Acts 15:15-17 by the regathering of the nation and judgment of the nations Joel 3:1-16
(7) full and permanent kingdom blessing Joel 3:17-21; Zechariah 14:1-21. (See Scofield "Matthew 25:32").