Becoming Closer
Locusts - the theories
nLiteral
•Lindsey’s mutant demon possessed locusts.
•Helicopters and nerve gas
•real locusts (Barclay)
nFigurative (locusts eat grass)
As I mentioned, everyone but the historicists has a lot of trouble with this one.  A representative thought can be found in Hal Lindsey’s work.  Ever the literalist, he states that these must be some form of mutant locust -- but demon possessed.
A comment here on the method of the futurist, particularly the more literal breed:  the books must be kept constantly up to date, for the technology changes too.  The passage is made obscure by the insistence upon literalism.
Lindsey recognizes this.  In a rare display of the possibility that his interpretation may not be exactly right on, he tells the story of a friend who suggested that these locusts are in fact Huey Cobra helicopters -- spraying nerve gas out of their tails.
There is a point in there:  if -- from the same author and interpretation -- these locusts can be either mutant, demon-possessed insects or helicopters with nerve gas, what else might they be????
Barclay (the quintessential liberal interpreter) also thinks they are literal locusts -- because the life cycle of the locust is about five months.
It seems more probable to me that we have not yet reached the end of the symbolic possibilities here.  If these are not literal locusts, we can at least say:
•  they are some sort of physical being -- this is not intended to be a “spiritual only” symbol.
•  there is a great deal of description given them, and whatever theory we adopt must match up to this description.
•  they are called locusts, and it is for that word that we must seek an explanation.  We will find it in the prophet Joel.