It seems that for a particular breed of grasshopper, the same stuff that regulates human sleep, aggression or even depression can make them all "Hulk smash!" Serotonin is a neurotransmitter chemical; when levels of it are low, clinical depression and a number of other psychological issues may result. Apparently, when levels of it are high, good ol' Mr. Grasshopper likes to gather up a few million friends who are also feelin' the buzz and go eat everything everywhere. Scientists are studying other breeds of grasshopper to see if the same kind of effect shows up -- even though only ten or so breeds of the bug have a tendency to swarm, others may have the same reaction to the chemical.
They also found that if serotonin blockers were administered, then grasshoppers stayed grasshoppers, even if other swarming conditions were present. So it may be possible to spray a swarm with some of these chemicals to keep them from swarming and destroying crops.
But that could lead to a large number of depressed grasshoppers. Scientists are not yet sure how to deal with that, although teaching therapists to communicate by rubbing their legs together has not yet been ruled out.