Swarms

Swarms

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 year.

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.

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