In reality, what we call locusts are really grasshoppers during a certain phase of their life cycles called the swarming phase. The polymorphic grasshopper (a grasshopper that has the ability to change its physical form) may go through a swarming phase. This phase can be brought on by chemicals or overcrowding. It is during that time when they are called locusts.
As locusts, grasshoppers become much stronger, more active, and interact more with each other. Perhaps that is why locusts are associated with huge swarms more so than grasshoppers. As swarms, their superior numbers give them a great advantage when compared with grasshoppers that live as individuals.
It is because of their activity as swarms that locusts have gained such a bad reputation. They have been known to cause millions of dollars in damage to crops. In the 19th century, Rocky Mountain locusts caused trouble across the Western U.S. When 12 trillion locusts devastated nearly 200,000 square miles of agriculture. It was possibly the largest concentration of animals in the history of the world!
