THE DESERT LOCUST AND ITS ENVIRONMENT 



Dr B.P. Uvarov, C.M.G., F.R.S. 

 (Anti- Locust Research Centre, London) 



The distribution area of the Desert Locust (Schistocerca gregaria Forsk.) coin- 

 cides with that of the hot Palaearctic deserts, although its swarm migrations may tem- 

 porarily extend beyond the limits of the latter, both southwards and northwards. From 

 the broad biogeographical point of view, therefore, the Desert Locust can be regarded 

 as a typical inhabitant of these deserts and it is of general interest to see to what an 

 extent the biology of the species is adapted to the desert conditions and, in particular, 

 whether such changes in the latter as may be induced by man, are likely to bring us 

 nearer to a solution of the Desert Locust problem which has a reasonable claim to be 

 regarded as the oldest problem of applied entomology. 



The genus Schistocerca has a somewhat unusual geographical distribution, since 

 it includes some 80 species of South and Central America, with a few penetrating to 

 North America, while S. gregaria is the only species occurring in the eastern hemi- 

 sphere. It is fairly closely related to a South American swarming species, but cer- 

 tainly quite distinct from it. 



Another feature of the genus Schistocerca is that, as far as is known, its Ameri- 

 can members are arboreal in their habits, and this, again, makes S. gregaria an apparent 

 exception and its partiality for deserts may suggest a great ecological and physiologi- 

 cal divergence from its congeners. However, when speaking of deserts, one should 

 bear in mind that perennial shrubs are an essential feature of many desert areas and, 

 in particular, of some sandy tracts where even something approaching sparse woods 

 can be found, where they have not been destroyed by man. Observations on the Des - 

 ert Locust, particularly in the adult stage, indicate its tendency for sitting on shrubs 

 or tall plants. This is particularly noticeable in swarms which normally roost for the 

 night on shrubs and trees. In this respect, therefore, S. gregaria has retained the 

 generic habit. 



With regard to its food, while the Desert Locust feeds readily on a large variety 

 of plants including grasses (Bhatia, 1940), the latter play only a minor part in its diet 

 and the vast majority of its food -plants are either annual and perennial herbs, or 

 shrubs. In this respect, the Desert Locust offers a sharp contrast to the Migratory Lo- 

 cust (Locusta migratoria L.) which is predominantly a grass feeder (Kozhanchikov, 

 1950). 



We see, therefore, that, ecologically, the Desert Locust cannot be regarded as a 

 typical insect of open desert, or of arid grasslands such as occur in the marginal des- 

 ert areas. In fact, leaving aside for the moment the occurrence of swarms which ex- 

 tend well beyond ecological barriers, the species is normally not encountered every- 

 where in the desert regions, but mainly in certain areas such as well overgrown sand 

 dunes on coastal plains, scrub belts along the beds of seasonal rivers and similar 

 habitats which represent 'ecological islands' in the desert. 



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