Any maturity of Desert Locust adults that form a swarm. When the hoppers become
gregarious, their colouration changes from largely green to yellow and black, and the
adults change from brown to pink (immature) or yellow (mature). Their bodies become
shorter, and they give off a pheromone that causes them to be attracted to each other,
enhancing hopper band and subsequently swarm formation.
Swarms can occur as low-flying sheets (stratiform) or may pile high in
the air (cumuliform), with the top level as much as 1 500 m above the ground. Stratiform
swarms are flat, usually tens of metres deep, and often occur during cool, overcast
weather or in the late afternoon. Cumuliform swarms are associated with convective
updrafts on hot afternoons, especially common during the warmer and drier months of the
Swarms may fly up to nine or 10 hours in a day, moving downwind,
although mature swarms may sometimes move a short distance upwind if the wind is light.
Swarms start to settle about an hour before sunset as convection dies away. Very high
airborne densities can occur during this period.
Swarm densities vary considerably. The generally accepted figure for an
average medium-density settled (resting on the ground) swarm is about 50 million
locusts/km2 (50 locusts/m2) across a range varying from 20 million km2 to 150
million/km2. Swarms generally spread out when flying, typically covering between two and
three times the area they occupy when roosting in the sun or feeding. Volume densities
of flying swarms can be as high as 10 locusts per m3. About half the swarms exceed 50
km2 in size.