When do grasshoppers become locusts?

Grasshoppers are solitary, but under certain circumstances, when food supplies become rare, they interact with other grasshoppers, changing their behaviour and becoming gregarious. Under suitable conditions of drought followed by rapid vegetation growth, serotonin in their brains triggers dramatic changes: they start to breed abundantly, becoming gregarious and nomadic when their populations become dense enough. They form bands of wingless nymphs that later become swarms of winged adults.