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The effects of three plant species, Calotropis procera, Zygophyllum gaetulum and Peganum harmala on survival, feeding behaviour and reproduction in the desert locust ( Schistocerca gregaria) were studied under laboratory conditions. The species originate in Moroccan arid areas and were seen to be avoided by locusts during the 1995 upsurge in Morocco. Alkaloids were extracted from leaves of the plants and applied topically to lettuce for leaves which were then offered with no other food to young adults of the desert locust for 3 days. The locust were then fed on untreated lettuce for 15 days. Controls were fed with lettuce leaves treated with ethanol. The results indicate that the alkaloids extracted from each of the three plant species cause a significant mortality compared to untreated controls and reduced food intake, as well as causing weight loss. Alkaloids extracted from C. procera and Z. gaetulum prevent sexual maturity both in males and females. However, alkaloids extracted from P. harmala merely delay sexual maturity by at least 8 days and engender a reduction in female fecundity and hatching rate compared to untreated controls. The presence and the concentration of alkaloids and other secondary substances in the investigated plants may explain the avoidance behaviour manifested by locusts under natural conditions.