Nocturnal grasshopper migration in West Africa: transport and concentration by the wind, and the implications for air-to-air control

Abstract

During radar observations of the migratory flight ofOedaleus senegalensisand of other grasshoppers in West Africa, we have observed that nocturnally flying insects are sometimes concentrated by mesoscale zones of wind convergence. The concentrations were typically 1.2-2.0 km wide, often more than 20 km long, and were similar to those observed elsewhere. The convergence zones appeared to be usually caused by atmospheric gravity currents. Some of these currents were cold air outflows from rain storms, and others were possibly of katabatic origin. Occasionally zones may also have been caused by bores and gravity waves set off by these currents. In this paper, we investigate the practicability of controlling populations of sahelian grasshoppers by the air-to-air spraying of insecticide onto such concentrations of insects. Using our data on concentration in convergence zones and a rudimentary model of zone distribution and behaviour, we estimate that less than 30% of the flying population of grasshoppers would be entrained in convergence zones, and that effective search for the concentrations might require the simultaneous use of at least two aircraft per 500 km square. These results imply that strategic control by air-to-air spraying is unlikely to be practicable. It is necessary to emphasize, however, that the evidence on which this deduction is based is fragmentary. A much more definitive conclusion could be expected from the results of further research with an aircraft equipped with a windfinding system and a radar able to measure and delineate insect concentrations.