major swarm movements take place towards zones of convergence of air- masses which 

 are often associated with precipitation. The result is that locusts and rain are likely 

 to arrive in an area together, an occurrence which has been frequently noticed, before 

 its mechanism was understood. The importance of this for the maintenance and multi- 

 plication of the Desert Locust is obvious and appearances of large locust populations 

 after an area has received a shower of rain loses its element of mystery. 



The coveregence hypothesis accounts also for the regular seasonal movements of 

 locust swarms between the areas receiving winter- spring rainfall and those subject to ■ 

 monsoon rains (Waloff, 1946; Donnelly, 1947; Davies, 1952; Fortescue- Foulkes, 

 1952). The value of such movements for the species is obvious. 



If we remember the ability of the adult Desert Locust either to mature and lay 

 eggs soon after becoming adult, or, in the absence of suitable conditions, to delay the 

 maturation for several months, the risk of losses of its popiilation through unfavourable 

 climatic conditions appears less serious than one might conclude from its physiologi- 

 cal requirements. There is no doubt that the instability of the environment and, in par- 

 ticular, the unreliability of rainfall in desert regions make the life of the Locust very 

 precarious, but its mobility, linked up as it is with weather dynamics, helps it to over- 

 come its physiological handicaps. 



While these points are of general biological interest, they also have an important 

 practical bearing. The Desert Locust is unquestionably one of the most important in- 

 sect pests and its periodical swarming and invasions of fertile lands have always been 

 associated with the neighbouring deserts, which were blamed as the source of swarms. 

 Investigations of the last 20 years have not yet solved the problem of where and how 

 exactly swarms arise from scattered locust populations, but there is enough evidence 

 to state, in a general way, that swarm formation cannot occur in areas with persistent 

 extreme desert conditions. The maintenance of locust populations in the desert de- 

 pends on the existence of favourable ecological islands, be they permanent or season- 

 al, and such areas are dependent either on fairly regular seasonal rainfall or on run- 

 off of rain water from highlands along seasonal river-beds, or, finally, on artificial 

 irrigation. It is the latter which deserves our particular attention. There are already 

 some observations suggesting the importance of irrigation and cultivation for creating 

 or expanding habitats favourable for the Desert Locust. The land development scheme 

 at Abyan, Aden Protectorate, has lately become an area with a fairly persistent popu- 

 lation of Desert Locust and repeated efforts are required to keep it under control. Ex- 

 tensive cultivation areas in the Tokar delta on the Red Sea coast of the Sudan form a 

 classical locality for the Desert Locust and it has to be kept under regular observation 

 and control. In Tripolitania, the breeding by invading swarms in 1946 was mainly con- 

 centrated on reclaimed sand-dunes immediately adjoining cultivation (Brown, 1947). 

 On the Red Sea coastal plains of Eritrea, Saudi Arabia and Yemen, locusts are normal- 

 ly found in the sandy deltas of seasonal rivers where native cultivation is extensive, 

 if sporadic. Even in the heart of the desert, in the Fezzan, considerable breeding 

 populations of locust, were found in spring 1952 in alfalfa cultivated on run- off water 

 from the hills (K. Guichard, unpublished); and a similar observation was made in 

 Mauretania (Bruneau de Mire, 1952). 



87 



