Jamieson, Fausset and Brown on Joel 1:6

6. nation--applied to the locusts, rather than "people" (Pr 30:25, 26), to mark not only their numbers, but also their savage hostility; and also to prepare the mind of the hearer for the transition to the figurative locusts in the second chapter, namely, the "nation" or Gentile foe coming against Judea (compare Joe 2:2).

my land--that is, Jehovah's; which never would have been so devastated were I not pleased to inflict punishment (Joe 2:18; Is 14:25; Jr 16:18; Eze 36:5; 38:16).

strong--as irresistibly sweeping away before its compact body the fruits of man's industry.

without number--so Jud 6:5; 7:12, "like grasshoppers (or "locusts") for multitude" (Jr 46:23; Na 3:15).

teeth ... lion--that is, the locusts are as destructive as a lion; there is no vegetation that can resist their bite (compare Re 9:8). Pliny says "they gnaw even the doors of houses."