A migration of grasshoppers, particularly Diabolocatantops axillaris (Thunberg) (Orthoptera: Acrididae), in the West African Sahel

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Abstract
A dense layer of large insects in windborne, migratory flight was observed by radar in the Tilemsi Valley in Mali between about 23.00 and 02.00 h on 10–11 October 1978. The volume density, height of flight, area density, displacement speed and direction, orientation and migration rate were determined for the overflying insects. Light-trap catches and radar signatures provided strong evidence that the pest grasshopper Diabolocatantops axillaris (Thunberg) contributed to the overflying layer. Other species contributing probably included Oedaleus senegalensis (Krauss) and possibly Ochrilidia spp. For D. axillaris, the migration can be regarded as a search for overwintering sites by adults in reproductive diapause, and thus is an example of C. G. Johnson's Class III migration. Estimated trajectories placed the probable source areas of the overflying grasshoppers in the Gourma, about 150 km west-south-west of the radar site. Migration direction was approximately downwind, but the grasshoppers showed a degree of common orientation towards the east-south-east, which added a southwards component to their displacement. At 02.30 h on the same night, a very dense line-concentration associated with a wind-shift moved across the radar site, and insects still in flight probably became entrained in this wind convergence zone and added to the line-concentration. Other evidence of long-distance, windborne migration in D. axillaris was adduced from records of captures on ships at sea, mainly off the West African coast. The consequences of downwind displacement and concentration for grasshopper ecology and pest management are discussed. The migration behaviour of D. axillaris and other grasshopper species probably reduces migration losses by the efficient location of new habitats and forms an essential part of life-history strategies for survival in a sahelian environment.