Outbreaks and recession populations of the desert locust Schistocerca gregaria (Forsk.)

New evidence on the nature of recession populations of the desert locust, Schistocerca gregaria (Forsk.), and on the nature and role of its outbreaks is presented. It is shown that the hopper bands and swarms reported during the 1963–67 recession differed qualitatively and quantitatively from those characteristic of plague periods and were numerically much smaller than some low density non-swarming populations. Gregarisation frequently occurs in the latter and may be on a scale sufficient to give rise to an outbreak; in turn outbreaks may lead to plague upsurges.