Desert locusts enter outbreak stage when small bands of wingless hoppers crowded in an area trigger the formation (via pheromone) of large swarms of winged locusts. This transformation is both behavioral and physiological and may progress into an upsurge, which occurs over a period of months through successive generations of high-density breeding. If the upsurge continues, it may progress into a plague. A plague develops from multiple outbreaks and an upsurge, takes at least a year develop, and causes simultaneous damage to many regions. Shown here is an outbreak of desert locusts in Morocco in 2004. (Photo by Magnus Ullman, via Wikimedia Commons, CC BY-SA 3.0)