During the locust control operation held in South Morocco in 1995, a quantitative study was carried out on morphologic measures of 586 adults of both sexes of the desert locust Schistocerca gregaria. The results of the study reveal that these populations present a mosaïc structure with the predominance of the gregarious phase at the beginning of the outbreak in the localities of Foum-Bousseroual and M'Rikha in the south east of Errachidia (Z₁). The gregarisation degree diminishes in the regions situated to the north of this zone (Z₁). The gregarious phase prevalence in the south (of Z₁) may be related to favorable conditions which have contributed to maintaining the allochthonous gregarious populations.

The solitary populations localised in the north of this zone and in the Tata zone (Z₂) are indigenous. The bad environmental conditions combined with timely control operations stopped the departure of a new invasion.