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Strains of Schistocerca gregaria (Forskal 1775) originating from Nigeria, Mauretania, and the Canary Islands were studied for the identification of solitary and gregarious behaviour. Under laboratory conditions, nymphs and adults were reared isolated and crowded for 4-6 generations. Various reproductive parameters were registered and compared. In gregarious strains, 4-5 generations can be bred per year under laboratory conditions. For the solitary population, only 2 generations were obtained. Differences were found also in the survival rate up to the adult stage, which was higher in gregarious than in solitary populations, independent from the consecutive generations. For isolated individuals of Nigeria, 5-6 moults were observed in both, males and females. For the offspring, the nymphal period varied between 31-42 days and the complition of 6 moults took up to 52 days. In crowded FF the pre-oviposition period was shorter than under isolated conditions, averaging between 28-32.5 days; it was much longer and more variable for the solitary than for the gregarious populations. In isolated FF of the gregarious strains the pre-oviposition period varied from 30-85 days. In gregarious strains reared crowded, means of 41 to 45 eggs/pod were deposited, while in the solitary one 52-58 eggs/pod were laid depending on the generations. In gregarious FF reared isolated, the mean number of eggs per pod Varied between 44.6 and 53.5. The solitary female collected on the Canary Islands produced a mean of 75.7 eggs per pod. Under crowded and isolated conditions, the period of embryogenesis ranged between 28-34 days at 30 degrees C, for all generations and strains.

The newly hatched hoppers were dark in all populations and generations, except for some greenish individuals found in the strain from Nigeria. Reaching the 2nd instar all hoppers showed the black pattern under crowded conditions and became grey-green or light brown after isolation. During the maturation of the adults only crowd-reared males of the gregarious strains became bright yellow coloured, while crowd-grown solitary males did not drastically change their colour. In isolation, no marked colour change was observed.

The results were discussed in relation to UVAROV’s phase theory. Inherent solitary and gregarious populations of S gregaria do really exist in nature, besides scattered and group-building populations of the gregarious form.