The invasion of the locusts is an event that Bible scholars have debated for centuries. Is it a real locust invasion, or do the locusts represent an army hoard? Is it a current event, or is this a prediction? There is really not much that points to an invading army in the rest of the text, so most likely this is truly a swarm of insects. The text reads not as a future event, but as current one.
The devastation caused to the agrarian society of the nation of Israel at that time was horrible. Note verse 4, and the four waves of locusts that devour everything in their path. Also note the response of the people, from the drunkard, to the bride, to the priests, to the farmers. Everyone mourns because this affects everyone. The devastation is widespread and complete, something that will be remembered for generations to come.