A Long-Range Migration of Grasshoppers Observed in the Sahelian Zone of Mali by Two Radars

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Abstract
On the night of Oct. 21st-22nd, 1978, a dense concentration of grasshoppers overflew 1 radar in the Tilemsi Valley in Mali, an 2 h later, was detected passing over the 2nd radar site 100 km to the south. It was very probable that Oedaleus senegalensis formed the major component of the overflying concentration. A larger species (probably Catantops axillaris), and a group of smaller species (probably including Acrotylus longipes, Aiolopus simulatrix and Ochrilidia, spp.) also contributed. This flight was an example, albeit a spectacular 1, of the southward migrations from the northern Sahel undertaken by O. senegalensis and other grasshoppers at the end of the rainy season on the north-easterly or northerly winds in the wake of the retreating Intertropical Convergence Zone (ITCZ). The present observations provide direct evidence of the long-range nature of some of these flights. The O. senegalensis studied engaged in a post-teneral migration. In females, flight activity diminished with the onset of vitellogenesis. The grasshopper concentration was associated with a line of wind-velocity change of unknown origin. The horizontal convergence associated with this wind-shift would produce a concentrating effect on the flying insects. The altitude and timing of flight of the migrating grasshoppers resulted in a displacement largely determined by the wind. The insects in the overflying concentration showed a mean orientation towards the downwind direction.

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