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

(1) On the night of 21-22 October 1978, a dense concentration of grasshoppers overflew one radar in the Tilemsi Valley in Mali, and 2 hours later, was detected passing over the second radar site 100 km to the south. (2) 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. (3) This flight was an example, albeit a spectacular one, of the southward migrations from the northern Sahel undertaken by Oedaleus 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. (4) The 0. senegalensis studied engaged in a post-teneral migration. In females, flight activity diminished with the onset of vitellogenesis. (5) 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. (6) The altitude and timing of flight of the migrating grasshoppers resulted in a displacement largely determined by the wind. (7) The insects in the overflying concentration showed a mean orientation towards the downwind direction.