In our study of Joel chapter two, we will look at the coming Day of the Lord and its purpose for mankind. In chapter one, we saw the prophet warn the people of a coming local plague of judgment by locusts and call for them to repent and turn back to God. Now, his message speaks of the coming judgment on all the earth.
"Blow the trumpet in Zion; sound the alarm on my holy hill. Let all who live in the land tremble, for the day of the Lord is coming." NIV translation
In Numbers 10, Moses was instructed to make two silver trumpets for signaling God's people. These trumpets were used to call the people to assemble, to tell them to move out, and for the people to be remembered by God in their battles. They were also used in worship as they were blown over the sacrifices. In this case, we see that the trumpet is a call for God's people to listen for there is danger approaching.
"It is close at hand - a day of darkness and gloom, a day of clouds and blackness. Like dawn spreading across the mountains a large mighty army comes, such as never was of old nor ever will be in ages to come." NIV translation
A Jewish day begins at sundown and it is the same here as this day is described by the prophet. The day is described as a day of blackness speaking about the sin and rebellion that is widespread. We see the warning that a massive army is approaching and we can see that this is the army that John saw in Revelation 19 & 20.
"Before them fire devours, behind them a flame blazes. Before them the land is like the garden of Eden, behind them, a desert waste - nothing escapes them. They have the appearance of horses; they gallop along like cavalry. With a noise like that of chariots they leap over the mountaintops, like a crackling fire consuming stubble, like a mighty army drawn up for battle." NIV translation
This passage describes the effects of the plague of locusts during the days of the prophet as well as the coming army of locusts (see Revelation 9). The army of locusts that is to come attacks people and not the crops.